mdns_sd/

service_daemon.rs

//! Service daemon for mDNS Service Discovery.

// How DNS-based Service Discovery works in a nutshell:
//
// (excerpt from RFC 6763)
// .... that a particular service instance can be
//    described using a DNS SRV [RFC2782] and DNS TXT [RFC1035] record.
//    The SRV record has a name of the form "<Instance>.<Service>.<Domain>"
//    and gives the target host and port where the service instance can be
//    reached.  The DNS TXT record of the same name gives additional
//    information about this instance, in a structured form using key/value
//    pairs, described in Section 6.  A client discovers the list of
//    available instances of a given service type using a query for a DNS
//    PTR [RFC1035] record with a name of the form "<Service>.<Domain>",
//    which returns a set of zero or more names, which are the names of the
//    aforementioned DNS SRV/TXT record pairs.
//
// Some naming conventions in this source code:
//
// `ty_domain` refers to service type together with domain name, i.e. <service>.<domain>.
// Every <service> consists of two labels: service itself and "_udp." or "_tcp".
// See RFC 6763 section 7 Service Names.
//     for example: `_my-service._udp.local.`
//
// `fullname` refers to a full Service Instance Name, i.e. <instance>.<service>.<domain>
//     for example: `my_home._my-service._udp.local.`
//
// In mDNS and DNS, the basic data structure is "Resource Record" (RR), where
// in Service Discovery, the basic data structure is "Service Info". One Service Info
// corresponds to a set of DNS Resource Records.
#[cfg(feature = "logging")]
use crate::log::{debug, error, trace};
use crate::{
    current_time_millis,
    dns_cache::{DnsCache, IpType},
    dns_parser::{
        ip_address_rr_type, DnsAddress, DnsEntryExt, DnsIncoming, DnsOutgoing, DnsPointer,
        DnsRecordBox, DnsRecordExt, DnsSrv, DnsTxt, InterfaceId, RRType, ScopedIp,
        CLASS_CACHE_FLUSH, CLASS_IN, FLAGS_AA, FLAGS_QR_QUERY, FLAGS_QR_RESPONSE, MAX_MSG_ABSOLUTE,
    },
    error::{e_fmt, Error, Result},
    service_info::{valid_ip_on_intf, DnsRegistry, MyIntf, Probe, ServiceInfo, ServiceStatus},
    Receiver, ResolvedService, TxtProperties,
};
use flume::{bounded, Sender, TrySendError};
use if_addrs::{IfAddr, Interface};
use mio::{event::Source, net::UdpSocket as MioUdpSocket, Interest, Poll, Registry, Token};
use socket2::Domain;
use socket_pktinfo::PktInfoUdpSocket;
use std::{
    cmp::{self, Reverse},
    collections::{hash_map::Entry, BinaryHeap, HashMap, HashSet},
    fmt, io,
    net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr, SocketAddrV4, SocketAddrV6, UdpSocket},
    str, thread,
    time::Duration,
    vec,
};

/// The default max length of the service name without domain, not including the
/// leading underscore (`_`). It is set to 15 per
/// [RFC 6763 section 7.2](https://www.rfc-editor.org/rfc/rfc6763#section-7.2).
pub const SERVICE_NAME_LEN_MAX_DEFAULT: u8 = 15;

/// The default interval for checking IP changes automatically.
pub const IP_CHECK_INTERVAL_IN_SECS_DEFAULT: u32 = 5;

/// The default time out for [ServiceDaemon::verify] is 10 seconds, per
/// [RFC 6762 section 10.4](https://datatracker.ietf.org/doc/html/rfc6762#section-10.4)
pub const VERIFY_TIMEOUT_DEFAULT: Duration = Duration::from_secs(10);

/// The mDNS port number per RFC 6762.
pub const MDNS_PORT: u16 = 5353;

const GROUP_ADDR_V4: Ipv4Addr = Ipv4Addr::new(224, 0, 0, 251);
const GROUP_ADDR_V6: Ipv6Addr = Ipv6Addr::new(0xff02, 0, 0, 0, 0, 0, 0, 0xfb);
const LOOPBACK_V4: Ipv4Addr = Ipv4Addr::new(127, 0, 0, 1);

const RESOLVE_WAIT_IN_MILLIS: u64 = 500;

/// Response status code for the service `unregister` call.
#[derive(Debug)]
pub enum UnregisterStatus {
    /// Unregister was successful.
    OK,
    /// The service was not found in the registration.
    NotFound,
}

/// Status code for the service daemon.
#[derive(Debug, PartialEq, Clone, Eq)]
#[non_exhaustive]
pub enum DaemonStatus {
    /// The daemon is running as normal.
    Running,

    /// The daemon has been shutdown.
    Shutdown,
}

/// Different counters included in the metrics.
/// Currently all counters are for outgoing packets.
#[derive(Hash, Eq, PartialEq)]
enum Counter {
    Register,
    RegisterResend,
    Unregister,
    UnregisterResend,
    Browse,
    ResolveHostname,
    Respond,
    CacheRefreshPTR,
    CacheRefreshSrvTxt,
    CacheRefreshAddr,
    KnownAnswerSuppression,
    CachedPTR,
    CachedSRV,
    CachedAddr,
    CachedTxt,
    CachedNSec,
    CachedSubtype,
    DnsRegistryProbe,
    DnsRegistryActive,
    DnsRegistryTimer,
    DnsRegistryNameChange,
    Timer,
}

impl fmt::Display for Counter {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            Self::Register => write!(f, "register"),
            Self::RegisterResend => write!(f, "register-resend"),
            Self::Unregister => write!(f, "unregister"),
            Self::UnregisterResend => write!(f, "unregister-resend"),
            Self::Browse => write!(f, "browse"),
            Self::ResolveHostname => write!(f, "resolve-hostname"),
            Self::Respond => write!(f, "respond"),
            Self::CacheRefreshPTR => write!(f, "cache-refresh-ptr"),
            Self::CacheRefreshSrvTxt => write!(f, "cache-refresh-srv-txt"),
            Self::CacheRefreshAddr => write!(f, "cache-refresh-addr"),
            Self::KnownAnswerSuppression => write!(f, "known-answer-suppression"),
            Self::CachedPTR => write!(f, "cached-ptr"),
            Self::CachedSRV => write!(f, "cached-srv"),
            Self::CachedAddr => write!(f, "cached-addr"),
            Self::CachedTxt => write!(f, "cached-txt"),
            Self::CachedNSec => write!(f, "cached-nsec"),
            Self::CachedSubtype => write!(f, "cached-subtype"),
            Self::DnsRegistryProbe => write!(f, "dns-registry-probe"),
            Self::DnsRegistryActive => write!(f, "dns-registry-active"),
            Self::DnsRegistryTimer => write!(f, "dns-registry-timer"),
            Self::DnsRegistryNameChange => write!(f, "dns-registry-name-change"),
            Self::Timer => write!(f, "timer"),
        }
    }
}

#[derive(Debug)]
enum InternalError {
    IntfAddrInvalid(Interface),
}

impl fmt::Display for InternalError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            InternalError::IntfAddrInvalid(iface) => write!(f, "interface addr invalid: {iface:?}"),
        }
    }
}

type MyResult<T> = core::result::Result<T, InternalError>;

/// A wrapper around UDP socket used by the mDNS daemon.
///
/// We do this because `mio` does not support PKTINFO and
/// does not provide a way to implement `Source` trait directly and safely.
struct MyUdpSocket {
    /// The underlying socket that supports control messages like
    /// `IP_PKTINFO` for IPv4 and `IPV6_PKTINFO` for IPv6.
    pktinfo: PktInfoUdpSocket,

    /// The mio UDP socket that is a clone of `pktinfo` and
    /// is used for event polling.
    mio: MioUdpSocket,
}

impl MyUdpSocket {
    pub fn new(pktinfo: PktInfoUdpSocket) -> io::Result<Self> {
        let std_sock = pktinfo.try_clone_std()?;
        let mio = MioUdpSocket::from_std(std_sock);

        Ok(Self { pktinfo, mio })
    }
}

/// Implements the mio `Source` trait so that we can use `MyUdpSocket` with `Poll`.
impl Source for MyUdpSocket {
    fn register(
        &mut self,
        registry: &Registry,
        token: Token,
        interests: Interest,
    ) -> io::Result<()> {
        self.mio.register(registry, token, interests)
    }

    fn reregister(
        &mut self,
        registry: &Registry,
        token: Token,
        interests: Interest,
    ) -> io::Result<()> {
        self.mio.reregister(registry, token, interests)
    }

    fn deregister(&mut self, registry: &Registry) -> std::io::Result<()> {
        self.mio.deregister(registry)
    }
}

/// The metrics is a HashMap of (name_key, i64_value).
/// The main purpose is to help monitoring the mDNS packet traffic.
pub type Metrics = HashMap<String, i64>;

const IPV4_SOCK_EVENT_KEY: usize = 4; // Pick a key just to indicate IPv4.
const IPV6_SOCK_EVENT_KEY: usize = 6; // Pick a key just to indicate IPv6.
const SIGNAL_SOCK_EVENT_KEY: usize = usize::MAX - 1; // avoid to overlap with zc.poll_ids

/// A daemon thread for mDNS
///
/// This struct provides a handle and an API to the daemon. It is cloneable.
#[derive(Clone)]
pub struct ServiceDaemon {
    /// Sender handle of the channel to the daemon.
    sender: Sender<Command>,

    /// Send to this addr to signal that a `Command` is coming.
    ///
    /// The daemon listens on this addr together with other mDNS sockets,
    /// to avoid busy polling the flume channel. If there is a way to poll
    /// the channel and mDNS sockets together, then this can be removed.
    signal_addr: SocketAddr,
}

impl ServiceDaemon {
    /// Creates a new daemon and spawns a thread to run the daemon.
    ///
    /// Creates a new mDNS service daemon using the default port (5353).
    ///
    /// For development/testing with custom ports, use [`ServiceDaemon::new_with_port`].
    ///
    /// # Errors
    ///
    /// Returns [`Error::Msg`] if the daemon cannot be initialized. This wraps an
    /// underlying OS-level failure.
    ///
    /// Note that this constructor does not open the mDNS multicast sockets — those
    /// are opened lazily by the daemon thread once it starts, so platform issues
    /// such as "multicast not permitted" are surfaced later via [`DaemonEvent`] from
    /// [`monitor`](Self::monitor) rather than here.
    pub fn new() -> Result<Self> {
        Self::new_with_port(MDNS_PORT)
    }

    /// Creates a new mDNS service daemon using a custom port.
    ///
    /// # Arguments
    ///
    /// * `port` - The UDP port to bind for mDNS communication.
    ///   - In production, this should be `MDNS_PORT` (5353) per RFC 6762.
    ///   - For development/testing, you can use a non-standard port (e.g., 5454)
    ///     to avoid conflicts with system mDNS services.
    ///   - Both publisher and browser must use the same port to communicate.
    ///
    /// # Example
    ///
    /// ```no_run
    /// use mdns_sd::ServiceDaemon;
    ///
    /// // Use standard mDNS port (production)
    /// let daemon = ServiceDaemon::new_with_port(5353)?;
    ///
    /// // Use custom port for development (avoids macOS Bonjour conflict)
    /// let daemon_dev = ServiceDaemon::new_with_port(5454)?;
    /// # Ok::<(), mdns_sd::Error>(())
    /// ```
    ///
    /// # Errors
    ///
    /// See [`new`](Self::new) for the set of OS-level failures that may surface
    /// here. Note that `port` is *not* validated against the kernel until the
    /// daemon thread tries to bind the mDNS sockets, so an unusable `port`
    /// (e.g., already in use, requires elevated privileges) will not be
    /// reported by this constructor — listen for such failures via
    /// [`monitor`](Self::monitor).
    pub fn new_with_port(port: u16) -> Result<Self> {
        // Use port 0 to allow the system assign a random available port,
        // no need for a pre-defined port number.
        let signal_addr = SocketAddrV4::new(LOOPBACK_V4, 0);

        let signal_sock = UdpSocket::bind(signal_addr)
            .map_err(|e| e_fmt!("failed to create signal_sock for daemon: {}", e))?;

        // Get the socket with the OS chosen port
        let signal_addr = signal_sock
            .local_addr()
            .map_err(|e| e_fmt!("failed to get signal sock addr: {}", e))?;

        // Must be nonblocking so we can listen to it together with mDNS sockets.
        signal_sock
            .set_nonblocking(true)
            .map_err(|e| e_fmt!("failed to set nonblocking for signal socket: {}", e))?;

        let poller = Poll::new().map_err(|e| e_fmt!("failed to create mio Poll: {e}"))?;

        let (sender, receiver) = bounded(100);

        // Spawn the daemon thread
        let mio_sock = MioUdpSocket::from_std(signal_sock);
        let cmd_sender = sender.clone();
        thread::Builder::new()
            .name("mDNS_daemon".to_string())
            .spawn(move || {
                Self::daemon_thread(mio_sock, poller, receiver, port, cmd_sender, signal_addr)
            })
            .map_err(|e| e_fmt!("thread builder failed to spawn: {}", e))?;

        Ok(Self {
            sender,
            signal_addr,
        })
    }

    /// Sends `cmd` to the daemon via its channel, and sends a signal
    /// to its sock addr to notify.
    fn send_cmd(&self, cmd: Command) -> Result<()> {
        let cmd_name = cmd.to_string();

        // First, send to the flume channel.
        self.sender.try_send(cmd).map_err(|e| match e {
            TrySendError::Full(_) => Error::Again,
            TrySendError::Disconnected(_) => Error::DaemonShutdown,
        })?;

        // Second, send a signal to notify the daemon.
        let addr = SocketAddrV4::new(LOOPBACK_V4, 0);
        let socket = UdpSocket::bind(addr)
            .map_err(|e| e_fmt!("Failed to create socket to send signal: {}", e))?;
        socket
            .send_to(cmd_name.as_bytes(), self.signal_addr)
            .map_err(|e| {
                e_fmt!(
                    "signal socket send_to {} ({}) failed: {}",
                    self.signal_addr,
                    cmd_name,
                    e
                )
            })?;

        Ok(())
    }

    /// Starts browsing for a specific service type.
    ///
    /// `service_type` must end with a valid mDNS domain: '._tcp.local.' or '._udp.local.'
    ///
    /// Returns a channel `Receiver` to receive events about the service. The caller
    /// can call `.recv_async().await` on this receiver to handle events in an
    /// async environment or call `.recv()` in a sync environment.
    ///
    /// When a new instance is found, the daemon automatically tries to resolve, i.e.
    /// finding more details, i.e. SRV records and TXT records.
    ///
    /// # Errors
    ///
    /// Returns [`Error::Msg`] if `service_type` does not end with
    /// `._tcp.local.` or `._udp.local.`.
    ///
    /// Returns [`Error::Again`] if the daemon's command queue is full.
    ///
    /// Returns [`Error::DaemonShutdown`] if the daemon thread has already exited.
    pub fn browse(&self, service_type: &str) -> Result<Receiver<ServiceEvent>> {
        check_domain_suffix(service_type)?;

        let (resp_s, resp_r) = bounded(10);
        self.send_cmd(Command::Browse(service_type.to_string(), 1, false, resp_s))?;
        Ok(resp_r)
    }

    /// Preforms a "cache-only" browse.
    ///
    /// `service_type` must end with a valid mDNS domain: '._tcp.local.' or '._udp.local.'
    ///
    /// The functionality is identical to 'browse', but the service events are based solely on the contents
    /// of the daemon's cache. No actual mDNS query is sent to the network.
    ///
    /// See [accept_unsolicited](Self::accept_unsolicited) if you want to do cache-only browsing.
    ///
    /// # Errors
    ///
    /// Same error conditions as [`browse`](Self::browse).
    pub fn browse_cache(&self, service_type: &str) -> Result<Receiver<ServiceEvent>> {
        check_domain_suffix(service_type)?;

        let (resp_s, resp_r) = bounded(10);
        self.send_cmd(Command::Browse(service_type.to_string(), 1, true, resp_s))?;
        Ok(resp_r)
    }

    /// Stops searching for a specific service type.
    ///
    /// # Errors
    ///
    /// Returns [`Error::Again`] if the daemon's command queue is full.
    ///
    /// Returns [`Error::DaemonShutdown`] if the daemon thread has already exited.
    pub fn stop_browse(&self, ty_domain: &str) -> Result<()> {
        self.send_cmd(Command::StopBrowse(ty_domain.to_string()))
    }

    /// Starts querying for the ip addresses of a hostname.
    ///
    /// Returns a channel `Receiver` to receive events about the hostname.
    /// The caller can call `.recv_async().await` on this receiver to handle events in an
    /// async environment or call `.recv()` in a sync environment.
    ///
    /// The `timeout` is specified in milliseconds.
    ///
    /// # Errors
    ///
    /// Returns [`Error::Msg`] if:
    ///
    /// - `hostname` does not end with `.local.`;
    /// - `hostname` is exactly `.local.` (the label before `.local.` is empty);
    /// - `hostname` is longer than 255 bytes.
    ///
    /// Returns [`Error::Again`] if the daemon's command queue is full.
    ///
    /// Returns [`Error::DaemonShutdown`] if the daemon thread has already exited.
    pub fn resolve_hostname(
        &self,
        hostname: &str,
        timeout: Option<u64>,
    ) -> Result<Receiver<HostnameResolutionEvent>> {
        check_hostname(hostname)?;
        let (resp_s, resp_r) = bounded(10);
        self.send_cmd(Command::ResolveHostname(
            hostname.to_string(),
            1,
            resp_s,
            timeout,
        ))?;
        Ok(resp_r)
    }

    /// Stops querying for the ip addresses of a hostname.
    ///
    /// # Errors
    ///
    /// Same error conditions as [`stop_browse`](Self::stop_browse).
    pub fn stop_resolve_hostname(&self, hostname: &str) -> Result<()> {
        self.send_cmd(Command::StopResolveHostname(hostname.to_string()))
    }

    /// Registers a service provided by this host.
    ///
    /// If `service_info` has no addresses yet and its `addr_auto` is enabled,
    /// this method will automatically fill in addresses from the host.
    ///
    /// To re-announce a service with an updated `service_info`, just call
    /// this `register` function again. No need to call `unregister` first.
    ///
    /// # Errors
    ///
    /// Returns [`Error::Msg`] if the [`ServiceInfo`] is malformed, for example:
    ///
    /// - the fullname does not end with `._tcp.local.` or `._udp.local.`;
    /// - the hostname does not end with `.local.`, is exactly `.local.`, or
    ///   is longer than 255 bytes.
    ///
    /// Returns [`Error::Again`] if the daemon's command queue is full.
    ///
    /// Returns [`Error::DaemonShutdown`] if the daemon thread has already exited.
    pub fn register(&self, service_info: ServiceInfo) -> Result<()> {
        check_service_name(service_info.get_fullname())?;
        check_hostname(service_info.get_hostname())?;

        self.send_cmd(Command::Register(service_info.into()))
    }

    /// Unregisters a service. This is a graceful shutdown of a service.
    ///
    /// Returns a channel receiver that is used to receive the status code
    /// of the unregister.
    ///
    /// # Errors
    ///
    /// Returns [`Error::Again`] if the daemon's command queue is full.
    ///
    /// Returns [`Error::DaemonShutdown`] if the daemon thread has already exited.
    pub fn unregister(&self, fullname: &str) -> Result<Receiver<UnregisterStatus>> {
        let (resp_s, resp_r) = bounded(1);
        self.send_cmd(Command::Unregister(fullname.to_lowercase(), resp_s))?;
        Ok(resp_r)
    }

    /// Starts to monitor events from the daemon.
    ///
    /// Returns a channel [`Receiver`] of [`DaemonEvent`].
    ///
    /// # Errors
    ///
    /// Returns [`Error::Again`] if the daemon's command queue is full.
    ///
    /// Returns [`Error::DaemonShutdown`] if the daemon thread has already exited.
    pub fn monitor(&self) -> Result<Receiver<DaemonEvent>> {
        let (resp_s, resp_r) = bounded(100);
        self.send_cmd(Command::Monitor(resp_s))?;
        Ok(resp_r)
    }

    /// Shuts down the daemon thread and returns a channel to receive the status.
    ///
    /// # Errors
    ///
    /// Returns [`Error::Again`] if the daemon's command queue is full.
    ///
    /// Returns [`Error::DaemonShutdown`] if the daemon thread has already exited.
    pub fn shutdown(&self) -> Result<Receiver<DaemonStatus>> {
        let (resp_s, resp_r) = bounded(1);
        self.send_cmd(Command::Exit(resp_s))?;
        Ok(resp_r)
    }

    /// Returns the status of the daemon.
    ///
    /// # Errors
    ///
    /// Returns [`Error::Again`] if the daemon's command queue is full.
    ///
    /// Returns [`Error::DaemonShutdown`] if the daemon thread has already exited.
    pub fn status(&self) -> Result<Receiver<DaemonStatus>> {
        let (resp_s, resp_r) = bounded(1);

        if self.sender.is_disconnected() {
            resp_s
                .send(DaemonStatus::Shutdown)
                .map_err(|e| e_fmt!("failed to send daemon status to the client: {}", e))?;
        } else {
            self.send_cmd(Command::GetStatus(resp_s))?;
        }

        Ok(resp_r)
    }

    /// Returns a channel receiver for the metrics, e.g. input/output counters.
    ///
    /// The metrics returned is a snapshot. Hence the caller should call
    /// this method repeatedly if they want to monitor the metrics continuously.
    ///
    /// # Errors
    ///
    /// Returns [`Error::Again`] if the daemon's command queue is full.
    ///
    /// Returns [`Error::DaemonShutdown`] if the daemon thread has already exited.
    pub fn get_metrics(&self) -> Result<Receiver<Metrics>> {
        let (resp_s, resp_r) = bounded(1);
        self.send_cmd(Command::GetMetrics(resp_s))?;
        Ok(resp_r)
    }

    /// Change the max length allowed for a service name.
    ///
    /// As RFC 6763 defines a length max for a service name, a user should not call
    /// this method unless they have to. See [`SERVICE_NAME_LEN_MAX_DEFAULT`].
    ///
    /// `len_max` is capped at an internal limit, which is currently 30.
    ///
    /// # Errors
    ///
    /// Returns [`Error::Msg`] if `len_max` exceeds the internal cap (30).
    ///
    /// Returns [`Error::Again`] if the daemon's command queue is full.
    ///
    /// Returns [`Error::DaemonShutdown`] if the daemon thread has already exited.
    pub fn set_service_name_len_max(&self, len_max: u8) -> Result<()> {
        const SERVICE_NAME_LEN_MAX_LIMIT: u8 = 30; // Double the default length max.

        if len_max > SERVICE_NAME_LEN_MAX_LIMIT {
            return Err(Error::Msg(format!(
                "service name length max {len_max} is too large"
            )));
        }

        self.send_cmd(Command::SetOption(DaemonOption::ServiceNameLenMax(len_max)))
    }

    /// Change the interval for checking IP changes automatically.
    ///
    /// Setting the interval to 0 disables the IP check.
    ///
    /// See [`IP_CHECK_INTERVAL_IN_SECS_DEFAULT`] for the default interval.
    pub fn set_ip_check_interval(&self, interval_in_secs: u32) -> Result<()> {
        let interval_in_millis = interval_in_secs as u64 * 1000;
        self.send_cmd(Command::SetOption(DaemonOption::IpCheckInterval(
            interval_in_millis,
        )))
    }

    /// Get the current interval in seconds for checking IP changes automatically.
    pub fn get_ip_check_interval(&self) -> Result<u32> {
        let (resp_s, resp_r) = bounded(1);
        self.send_cmd(Command::GetOption(resp_s))?;

        let option = resp_r
            .recv_timeout(Duration::from_secs(10))
            .map_err(|e| e_fmt!("failed to receive ip check interval: {}", e))?;
        let ip_check_interval_in_secs = option.ip_check_interval / 1000;
        Ok(ip_check_interval_in_secs as u32)
    }

    /// Include interfaces that match `if_kind` for this service daemon.
    ///
    /// For example:
    /// ```ignore
    ///     daemon.enable_interface("en0")?;
    /// ```
    pub fn enable_interface(&self, if_kind: impl IntoIfKindVec) -> Result<()> {
        let if_kind_vec = if_kind.into_vec();
        self.send_cmd(Command::SetOption(DaemonOption::EnableInterface(
            if_kind_vec.kinds,
        )))
    }

    /// Ignore/exclude interfaces that match `if_kind` for this daemon.
    ///
    /// For example:
    /// ```ignore
    ///     daemon.disable_interface(IfKind::IPv6)?;
    /// ```
    pub fn disable_interface(&self, if_kind: impl IntoIfKindVec) -> Result<()> {
        let if_kind_vec = if_kind.into_vec();
        self.send_cmd(Command::SetOption(DaemonOption::DisableInterface(
            if_kind_vec.kinds,
        )))
    }

    /// If `accept` is true, accept and cache all responses, even if there is no active querier
    /// for a given service type. This is useful / necessary when doing cache-only browsing. See
    /// [browse_cache](Self::browse_cache).
    ///
    /// If `accept` is false (default), accept only responses matching queries that we have initiated.
    ///
    /// For example:
    /// ```ignore
    ///     daemon.accept_unsolicited(true)?;
    /// ```
    pub fn accept_unsolicited(&self, accept: bool) -> Result<()> {
        self.send_cmd(Command::SetOption(DaemonOption::AcceptUnsolicited(accept)))
    }

    /// Include or exclude Apple P2P interfaces, e.g. "awdl0", "llw0".
    /// By default, they are excluded.
    pub fn include_apple_p2p(&self, include: bool) -> Result<()> {
        self.send_cmd(Command::SetOption(DaemonOption::IncludeAppleP2P(include)))
    }

    #[cfg(test)]
    pub fn test_down_interface(&self, ifname: &str) -> Result<()> {
        self.send_cmd(Command::SetOption(DaemonOption::TestDownInterface(
            ifname.to_string(),
        )))
    }

    #[cfg(test)]
    pub fn test_up_interface(&self, ifname: &str) -> Result<()> {
        self.send_cmd(Command::SetOption(DaemonOption::TestUpInterface(
            ifname.to_string(),
        )))
    }

    /// Enable or disable the loopback for locally sent multicast packets in IPv4.
    ///
    /// By default, multicast loop is enabled for IPv4. When disabled, a querier will not
    /// receive announcements from a responder on the same host.
    ///
    /// Reference: <https://learn.microsoft.com/en-us/windows/win32/winsock/ip-multicast-2>
    ///
    /// "The Winsock version of the IP_MULTICAST_LOOP option is semantically different than
    /// the UNIX version of the IP_MULTICAST_LOOP option:
    ///
    /// In Winsock, the IP_MULTICAST_LOOP option applies only to the receive path.
    /// In the UNIX version, the IP_MULTICAST_LOOP option applies to the send path."
    ///
    /// Which means, in order NOT to receive localhost announcements, you want to call
    /// this API on the querier side on Windows, but on the responder side on Unix.
    pub fn set_multicast_loop_v4(&self, on: bool) -> Result<()> {
        self.send_cmd(Command::SetOption(DaemonOption::MulticastLoopV4(on)))
    }

    /// Enable or disable the loopback for locally sent multicast packets in IPv6.
    ///
    /// By default, multicast loop is enabled for IPv6. When disabled, a querier will not
    /// receive announcements from a responder on the same host.
    ///
    /// Reference: <https://learn.microsoft.com/en-us/windows/win32/winsock/ip-multicast-2>
    ///
    /// "The Winsock version of the IP_MULTICAST_LOOP option is semantically different than
    /// the UNIX version of the IP_MULTICAST_LOOP option:
    ///
    /// In Winsock, the IP_MULTICAST_LOOP option applies only to the receive path.
    /// In the UNIX version, the IP_MULTICAST_LOOP option applies to the send path."
    ///
    /// Which means, in order NOT to receive localhost announcements, you want to call
    /// this API on the querier side on Windows, but on the responder side on Unix.
    pub fn set_multicast_loop_v6(&self, on: bool) -> Result<()> {
        self.send_cmd(Command::SetOption(DaemonOption::MulticastLoopV6(on)))
    }

    /// Proactively confirms whether a service instance still valid.
    ///
    /// This call will issue queries for a service instance's SRV record and Address records.
    ///
    /// For `timeout`, most users should use [VERIFY_TIMEOUT_DEFAULT]
    /// unless there is a reason not to follow RFC.
    ///
    /// If no response is received within `timeout`, the current resource
    /// records will be flushed, and if needed, `ServiceRemoved` event will be
    /// sent to active queriers.
    ///
    /// Reference: [RFC 6762](https://datatracker.ietf.org/doc/html/rfc6762#section-10.4)
    ///
    /// # Errors
    ///
    /// Returns [`Error::Again`] if the daemon's command queue is full.
    ///
    /// Returns [`Error::DaemonShutdown`] if the daemon thread has already exited.
    pub fn verify(&self, instance_fullname: String, timeout: Duration) -> Result<()> {
        self.send_cmd(Command::Verify(instance_fullname, timeout))
    }

    fn daemon_thread(
        signal_sock: MioUdpSocket,
        poller: Poll,
        receiver: Receiver<Command>,
        port: u16,
        cmd_sender: Sender<Command>,
        signal_addr: SocketAddr,
    ) {
        let mut zc = Zeroconf::new(signal_sock, poller, port, cmd_sender, signal_addr);

        if let Some(cmd) = zc.run(receiver) {
            match cmd {
                Command::Exit(resp_s) => {
                    // It is guaranteed that the receiver already dropped,
                    // i.e. the daemon command channel closed.
                    if let Err(e) = resp_s.send(DaemonStatus::Shutdown) {
                        debug!("exit: failed to send response of shutdown: {}", e);
                    }
                }
                _ => {
                    debug!("Unexpected command: {:?}", cmd);
                }
            }
        }
    }
}

/// Creates a new UDP socket that uses `intf` to send and recv multicast.
fn _new_socket_bind(intf: &Interface, should_loop: bool) -> Result<MyUdpSocket> {
    // Use the same socket for receiving and sending multicast packets.
    // Such socket has to bind to INADDR_ANY or IN6ADDR_ANY.
    let intf_ip = &intf.ip();
    match intf_ip {
        IpAddr::V4(ip) => {
            let addr = SocketAddrV4::new(Ipv4Addr::new(0, 0, 0, 0), MDNS_PORT);
            let sock = new_socket(addr.into(), true)?;

            // Join mDNS group to receive packets.
            sock.join_multicast_v4(&GROUP_ADDR_V4, ip)
                .map_err(|e| e_fmt!("join multicast group on addr {}: {}", intf_ip, e))?;

            // Set IP_MULTICAST_IF to send packets.
            sock.set_multicast_if_v4(ip)
                .map_err(|e| e_fmt!("set multicast_if on addr {}: {}", ip, e))?;

            // Per RFC 6762 section 11:
            // "All Multicast DNS responses (including responses sent via unicast) SHOULD
            // be sent with IP TTL set to 255."
            // Here we set the TTL to 255 for multicast as we don't support unicast yet.
            sock.set_multicast_ttl_v4(255)
                .map_err(|e| e_fmt!("set set_multicast_ttl_v4 on addr {}: {}", ip, e))?;

            if !should_loop {
                sock.set_multicast_loop_v4(false)
                    .map_err(|e| e_fmt!("failed to set multicast loop v4 for {ip}: {e}"))?;
            }

            // Test if we can send packets successfully.
            let multicast_addr = SocketAddrV4::new(GROUP_ADDR_V4, MDNS_PORT).into();
            let test_packets = DnsOutgoing::new(0).to_data_on_wire();
            for packet in test_packets {
                sock.send_to(&packet, &multicast_addr)
                    .map_err(|e| e_fmt!("send multicast packet on addr {}: {}", ip, e))?;
            }
            MyUdpSocket::new(sock)
                .map_err(|e| e_fmt!("failed to create MySocket for interface {}: {e}", intf.name))
        }
        IpAddr::V6(ip) => {
            let addr = SocketAddrV6::new(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 0), MDNS_PORT, 0, 0);
            let sock = new_socket(addr.into(), true)?;

            let if_index = intf.index.unwrap_or(0);

            // Join mDNS group to receive packets.
            sock.join_multicast_v6(&GROUP_ADDR_V6, if_index)
                .map_err(|e| e_fmt!("join multicast group on addr {}: {}", ip, e))?;

            // Set IPV6_MULTICAST_IF to send packets.
            sock.set_multicast_if_v6(if_index)
                .map_err(|e| e_fmt!("set multicast_if on addr {}: {}", ip, e))?;

            // We are not sending multicast packets to test this socket as there might
            // be many IPv6 interfaces on a host and could cause such send error:
            // "No buffer space available (os error 55)".

            MyUdpSocket::new(sock)
                .map_err(|e| e_fmt!("failed to create MySocket for interface {}: {e}", intf.name))
        }
    }
}

/// Creates a new UDP socket to bind to `port` with REUSEPORT option.
/// `non_block` indicates whether to set O_NONBLOCK for the socket.
fn new_socket(addr: SocketAddr, non_block: bool) -> Result<PktInfoUdpSocket> {
    let domain = match addr {
        SocketAddr::V4(_) => socket2::Domain::IPV4,
        SocketAddr::V6(_) => socket2::Domain::IPV6,
    };

    let fd = PktInfoUdpSocket::new(domain).map_err(|e| e_fmt!("create socket failed: {}", e))?;

    fd.set_reuse_address(true)
        .map_err(|e| e_fmt!("set ReuseAddr failed: {}", e))?;
    #[cfg(unix)]
    if let Err(e) = fd.set_reuse_port(true) {
        debug!(
            "SO_REUSEPORT is not supported, continuing without it: {}",
            e
        );
    }

    if non_block {
        fd.set_nonblocking(true)
            .map_err(|e| e_fmt!("set O_NONBLOCK: {}", e))?;
    }

    fd.bind(&addr.into())
        .map_err(|e| e_fmt!("socket bind to {} failed: {}", &addr, e))?;

    trace!("new socket bind to {}", &addr);
    Ok(fd)
}

/// Specify a UNIX timestamp in millis to run `command` for the next time.
struct ReRun {
    /// UNIX timestamp in millis.
    next_time: u64,
    command: Command,
}

/// Specify kinds of interfaces. It is used to enable or to disable interfaces in the daemon.
///
/// Note that for ergonomic reasons, `From<&str>` and `From<IpAddr>` are implemented.
#[derive(Debug, Clone)]
#[non_exhaustive]
pub enum IfKind {
    /// All interfaces.
    All,

    /// All IPv4 interfaces.
    IPv4,

    /// All IPv6 interfaces.
    IPv6,

    /// By the interface name, for example "en0"
    Name(String),

    /// By an IPv4 or IPv6 address.
    /// This is used to look up the interface. The semantics is to identify an interface of
    /// IPv4 or IPv6, not a specific address on the interface.
    Addr(IpAddr),

    /// 127.0.0.1 (or anything in 127.0.0.0/8), enabled by default.
    ///
    /// Loopback interfaces are required by some use cases (e.g., OSCQuery) for publishing.
    LoopbackV4,

    /// ::1/128, enabled by default.
    LoopbackV6,

    /// By interface index, IPv4 only.
    IndexV4(u32),

    /// By interface index, IPv6 only.
    IndexV6(u32),
}

impl IfKind {
    /// Checks if `intf` matches with this interface kind.
    fn matches(&self, intf: &Interface) -> bool {
        match self {
            Self::All => true,
            Self::IPv4 => intf.ip().is_ipv4(),
            Self::IPv6 => intf.ip().is_ipv6(),
            Self::Name(ifname) => ifname == &intf.name,
            Self::Addr(addr) => addr == &intf.ip(),
            Self::LoopbackV4 => intf.is_loopback() && intf.ip().is_ipv4(),
            Self::LoopbackV6 => intf.is_loopback() && intf.ip().is_ipv6(),
            Self::IndexV4(idx) => intf.index == Some(*idx) && intf.ip().is_ipv4(),
            Self::IndexV6(idx) => intf.index == Some(*idx) && intf.ip().is_ipv6(),
        }
    }
}

/// The first use case of specifying an interface was to
/// use an interface name. Hence adding this for ergonomic reasons.
impl From<&str> for IfKind {
    fn from(val: &str) -> Self {
        Self::Name(val.to_string())
    }
}

impl From<&String> for IfKind {
    fn from(val: &String) -> Self {
        Self::Name(val.to_string())
    }
}

/// Still for ergonomic reasons.
impl From<IpAddr> for IfKind {
    fn from(val: IpAddr) -> Self {
        Self::Addr(val)
    }
}

/// A list of `IfKind` that can be used to match interfaces.
pub struct IfKindVec {
    kinds: Vec<IfKind>,
}

/// A trait that converts a type into a Vec of `IfKind`.
pub trait IntoIfKindVec {
    fn into_vec(self) -> IfKindVec;
}

impl<T: Into<IfKind>> IntoIfKindVec for T {
    fn into_vec(self) -> IfKindVec {
        let if_kind: IfKind = self.into();
        IfKindVec {
            kinds: vec![if_kind],
        }
    }
}

impl<T: Into<IfKind>> IntoIfKindVec for Vec<T> {
    fn into_vec(self) -> IfKindVec {
        let kinds: Vec<IfKind> = self.into_iter().map(|x| x.into()).collect();
        IfKindVec { kinds }
    }
}

/// Selection of interfaces.
struct IfSelection {
    /// The interfaces to be selected.
    if_kind: IfKind,

    /// Whether the `if_kind` should be enabled or not.
    selected: bool,
}

/// A struct holding the state. It was inspired by `zeroconf` package in Python.
struct Zeroconf {
    /// The mDNS port number to use for socket binding.
    /// Typically MDNS_PORT (5353), but can be customized for development/testing.
    port: u16,

    /// Local interfaces keyed by interface index.
    my_intfs: HashMap<u32, MyIntf>,

    /// A common socket for IPv4 interfaces. It's None if IPv4 is disabled in OS kernel.
    ipv4_sock: Option<MyUdpSocket>,

    /// A common socket for IPv6 interfaces. It's None if IPv6 is disabled in OS kernel.
    ipv6_sock: Option<MyUdpSocket>,

    /// Local registered services， keyed by service full names.
    my_services: HashMap<String, ServiceInfo>,

    /// Received DNS records.
    cache: DnsCache,

    /// Registered service records, keyed by interface index.
    dns_registry_map: HashMap<u32, DnsRegistry>,

    /// Active "Browse" commands.
    service_queriers: HashMap<String, Sender<ServiceEvent>>, // <ty_domain, channel::sender>

    /// Active "ResolveHostname" commands.
    ///
    /// The timestamps are set at the future timestamp when the command should timeout.
    /// `hostname` is case-insensitive and stored in lowercase.
    hostname_resolvers: HashMap<String, (Sender<HostnameResolutionEvent>, Option<u64>)>, // <hostname, (channel::sender, UNIX timestamp in millis)>

    /// All repeating transmissions.
    retransmissions: Vec<ReRun>,

    counters: Metrics,

    /// Waits for incoming packets.
    poller: Poll,

    /// Channels to notify events.
    monitors: Vec<Sender<DaemonEvent>>,

    /// Options
    service_name_len_max: u8,

    /// Interval in millis to check IP address changes.
    ip_check_interval: u64,

    /// All interface selections called to the daemon.
    if_selections: Vec<IfSelection>,

    /// Socket for signaling.
    signal_sock: MioUdpSocket,

    /// Timestamps marking where we need another iteration of the run loop,
    /// to react to events like retransmissions, cache refreshes, interface IP address changes, etc.
    ///
    /// When the run loop goes through a single iteration, it will
    /// set its timeout to the earliest timer in this list.
    timers: BinaryHeap<Reverse<u64>>,

    status: DaemonStatus,

    /// Service instances that are pending for resolving SRV and TXT.
    pending_resolves: HashSet<String>,

    /// Service instances that are already resolved.
    resolved: HashSet<String>,

    multicast_loop_v4: bool,

    multicast_loop_v6: bool,

    accept_unsolicited: bool,

    include_apple_p2p: bool,

    cmd_sender: Sender<Command>,

    signal_addr: SocketAddr,

    #[cfg(test)]
    test_down_interfaces: HashSet<String>,
}

/// Join the multicast group for the given interface.
fn join_multicast_group(my_sock: &PktInfoUdpSocket, intf: &Interface) -> Result<()> {
    let intf_ip = &intf.ip();
    match intf_ip {
        IpAddr::V4(ip) => {
            // Join mDNS group to receive packets.
            debug!("join multicast group V4 on {} addr {ip}", intf.name);
            my_sock
                .join_multicast_v4(&GROUP_ADDR_V4, ip)
                .map_err(|e| e_fmt!("PKT join multicast group on addr {}: {}", intf_ip, e))?;
        }
        IpAddr::V6(ip) => {
            let if_index = intf.index.unwrap_or(0);
            // Join mDNS group to receive packets.
            debug!(
                "join multicast group V6 on {} addr {ip} with index {if_index}",
                intf.name
            );
            my_sock
                .join_multicast_v6(&GROUP_ADDR_V6, if_index)
                .map_err(|e| e_fmt!("PKT join multicast group on addr {}: {}", ip, e))?;
        }
    }
    Ok(())
}

impl Zeroconf {
    fn new(
        signal_sock: MioUdpSocket,
        poller: Poll,
        port: u16,
        cmd_sender: Sender<Command>,
        signal_addr: SocketAddr,
    ) -> Self {
        // Get interfaces.
        let my_ifaddrs = my_ip_interfaces(true);

        // Create a socket for every IP addr.
        // Note: it is possible that `my_ifaddrs` contains the same IP addr with different interface names,
        // or the same interface name with different IP addrs.
        let mut my_intfs = HashMap::new();
        let mut dns_registry_map = HashMap::new();

        // Use the same socket for receiving and sending multicast packets.
        // Such socket has to bind to INADDR_ANY or IN6ADDR_ANY.
        let mut ipv4_sock = None;
        let addr = SocketAddrV4::new(Ipv4Addr::new(0, 0, 0, 0), port);
        match new_socket(addr.into(), true) {
            Ok(sock) => {
                // Per RFC 6762 section 11:
                // "All Multicast DNS responses (including responses sent via unicast) SHOULD
                // be sent with IP TTL set to 255."
                // Here we set the TTL to 255 for multicast as we don't support unicast yet.
                sock.set_multicast_ttl_v4(255)
                    .map_err(|e| e_fmt!("set set_multicast_ttl_v4 on addr: {}", e))
                    .ok();

                // This clones a socket.
                ipv4_sock = match MyUdpSocket::new(sock) {
                    Ok(s) => Some(s),
                    Err(e) => {
                        debug!("failed to create IPv4 MyUdpSocket: {e}");
                        None
                    }
                };
            }
            // Per RFC 6762 section 11:}
            Err(e) => debug!("failed to create IPv4 socket: {e}"),
        }

        let mut ipv6_sock = None;
        let addr = SocketAddrV6::new(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 0), port, 0, 0);
        match new_socket(addr.into(), true) {
            Ok(sock) => {
                // Per RFC 6762 section 11:
                // "All Multicast DNS responses (including responses sent via unicast) SHOULD
                // be sent with IP TTL set to 255."
                sock.set_multicast_hops_v6(255)
                    .map_err(|e| e_fmt!("set set_multicast_hops_v6: {}", e))
                    .ok();

                // This clones the ipv6 socket.
                ipv6_sock = match MyUdpSocket::new(sock) {
                    Ok(s) => Some(s),
                    Err(e) => {
                        debug!("failed to create IPv6 MyUdpSocket: {e}");
                        None
                    }
                };
            }
            Err(e) => debug!("failed to create IPv6 socket: {e}"),
        }

        // Configure sockets to join multicast groups.
        for intf in my_ifaddrs {
            let sock_opt = if intf.ip().is_ipv4() {
                &ipv4_sock
            } else {
                &ipv6_sock
            };
            let Some(sock) = sock_opt else {
                debug!(
                    "no socket available for interface {} with addr {}. Skipped.",
                    intf.name,
                    intf.ip()
                );
                continue;
            };

            if let Err(e) = join_multicast_group(&sock.pktinfo, &intf) {
                debug!("failed to join multicast: {}: {e}. Skipped.", &intf.ip());
            }

            let if_index = intf.index.unwrap_or(0);

            // Add this interface address if not already present.
            dns_registry_map
                .entry(if_index)
                .or_insert_with(DnsRegistry::new);

            my_intfs
                .entry(if_index)
                .and_modify(|v: &mut MyIntf| {
                    v.addrs.insert(intf.addr.clone());
                })
                .or_insert(MyIntf {
                    name: intf.name.clone(),
                    index: if_index,
                    addrs: HashSet::from([intf.addr]),
                });
        }

        let monitors = Vec::new();
        let service_name_len_max = SERVICE_NAME_LEN_MAX_DEFAULT;
        let ip_check_interval = IP_CHECK_INTERVAL_IN_SECS_DEFAULT as u64 * 1000;

        let timers = BinaryHeap::new();

        // Enable everything, including loopback interfaces.
        let if_selections = vec![];

        let status = DaemonStatus::Running;

        Self {
            port,
            my_intfs,
            ipv4_sock,
            ipv6_sock,
            my_services: HashMap::new(),
            cache: DnsCache::new(),
            dns_registry_map,
            hostname_resolvers: HashMap::new(),
            service_queriers: HashMap::new(),
            retransmissions: Vec::new(),
            counters: HashMap::new(),
            poller,
            monitors,
            service_name_len_max,
            ip_check_interval,
            if_selections,
            signal_sock,
            timers,
            status,
            pending_resolves: HashSet::new(),
            resolved: HashSet::new(),
            multicast_loop_v4: true,
            multicast_loop_v6: true,
            accept_unsolicited: false,
            include_apple_p2p: false,
            cmd_sender,
            signal_addr,

            #[cfg(test)]
            test_down_interfaces: HashSet::new(),
        }
    }

    /// Send a Command into the daemon channel and poke the signal socket to wake the poll loop.
    fn send_cmd_to_self(&self, cmd: Command) -> Result<()> {
        let cmd_name = cmd.to_string();

        self.cmd_sender.try_send(cmd).map_err(|e| match e {
            TrySendError::Full(_) => Error::Again,
            TrySendError::Disconnected(_) => Error::DaemonShutdown,
        })?;

        let addr = SocketAddrV4::new(LOOPBACK_V4, 0);
        let socket = UdpSocket::bind(addr)
            .map_err(|e| e_fmt!("Failed to create socket to send signal: {}", e))?;
        socket
            .send_to(cmd_name.as_bytes(), self.signal_addr)
            .map_err(|e| {
                e_fmt!(
                    "signal socket send_to {} ({}) failed: {}",
                    self.signal_addr,
                    cmd_name,
                    e
                )
            })?;

        Ok(())
    }

    /// Clean up all resources before shutdown.
    ///
    /// This method:
    /// 1. Unregisters all registered services (sends goodbye packets)
    /// 2. Stops all active browse operations
    /// 3. Stops all active hostname resolution operations
    /// 4. Clears all retransmissions
    fn cleanup(&mut self) {
        debug!("Starting cleanup for shutdown");

        // 1. Unregister all services - send goodbye packets
        let service_names: Vec<String> = self.my_services.keys().cloned().collect();
        for fullname in service_names {
            if let Some(info) = self.my_services.get(&fullname) {
                debug!("Unregistering service during shutdown: {}", &fullname);

                for intf in self.my_intfs.values() {
                    if let Some(sock) = self.ipv4_sock.as_ref() {
                        self.unregister_service(info, intf, &sock.pktinfo);
                    }

                    if let Some(sock) = self.ipv6_sock.as_ref() {
                        self.unregister_service(info, intf, &sock.pktinfo);
                    }
                }
            }
        }
        self.my_services.clear();

        // 2. Stop all browse operations
        let browse_types: Vec<String> = self.service_queriers.keys().cloned().collect();
        for ty_domain in browse_types {
            debug!("Stopping browse during shutdown: {}", &ty_domain);
            if let Some(sender) = self.service_queriers.remove(&ty_domain) {
                // Notify the client
                if let Err(e) = sender.send(ServiceEvent::SearchStopped(ty_domain.clone())) {
                    debug!("Failed to send SearchStopped during shutdown: {}", e);
                }
            }
        }

        // 3. Stop all hostname resolution operations
        let hostnames: Vec<String> = self.hostname_resolvers.keys().cloned().collect();
        for hostname in hostnames {
            debug!(
                "Stopping hostname resolution during shutdown: {}",
                &hostname
            );
            if let Some((sender, _timeout)) = self.hostname_resolvers.remove(&hostname) {
                // Notify the client
                if let Err(e) =
                    sender.send(HostnameResolutionEvent::SearchStopped(hostname.clone()))
                {
                    debug!(
                        "Failed to send HostnameResolutionEvent::SearchStopped during shutdown: {}",
                        e
                    );
                }
            }
        }

        // 4. Clear all retransmissions
        self.retransmissions.clear();

        debug!("Cleanup completed");
    }

    /// The main event loop of the daemon thread
    ///
    /// In each round, it will:
    /// 1. select the listening sockets with a timeout.
    /// 2. process the incoming packets if any.
    /// 3. try_recv on its channel and execute commands.
    /// 4. announce its registered services.
    /// 5. process retransmissions if any.
    fn run(&mut self, receiver: Receiver<Command>) -> Option<Command> {
        // Add the daemon's signal socket to the poller.
        if let Err(e) = self.poller.registry().register(
            &mut self.signal_sock,
            mio::Token(SIGNAL_SOCK_EVENT_KEY),
            mio::Interest::READABLE,
        ) {
            debug!("failed to add signal socket to the poller: {}", e);
            return None;
        }

        if let Some(sock) = self.ipv4_sock.as_mut() {
            if let Err(e) = self.poller.registry().register(
                sock,
                mio::Token(IPV4_SOCK_EVENT_KEY),
                mio::Interest::READABLE,
            ) {
                debug!("failed to register ipv4 socket: {}", e);
                return None;
            }
        }

        if let Some(sock) = self.ipv6_sock.as_mut() {
            if let Err(e) = self.poller.registry().register(
                sock,
                mio::Token(IPV6_SOCK_EVENT_KEY),
                mio::Interest::READABLE,
            ) {
                debug!("failed to register ipv6 socket: {}", e);
                return None;
            }
        }

        // Setup timer for IP checks.
        let mut next_ip_check = if self.ip_check_interval > 0 {
            current_time_millis() + self.ip_check_interval
        } else {
            0
        };

        if next_ip_check > 0 {
            self.add_timer(next_ip_check);
        }

        // Start the run loop.

        let mut events = mio::Events::with_capacity(1024);
        loop {
            let now = current_time_millis();

            let earliest_timer = self.peek_earliest_timer();
            let timeout = earliest_timer.map(|timer| {
                // If `timer` already passed, set `timeout` to be 1ms.
                let millis = if timer > now { timer - now } else { 1 };
                Duration::from_millis(millis)
            });

            // Process incoming packets, command events and optional timeout.
            events.clear();
            match self.poller.poll(&mut events, timeout) {
                Ok(_) => self.handle_poller_events(&events),
                Err(e) => debug!("failed to select from sockets: {}", e),
            }

            let now = current_time_millis();

            // Remove the timers if already passed.
            self.pop_timers_till(now);

            // Remove hostname resolvers with expired timeouts.
            for hostname in self
                .hostname_resolvers
                .clone()
                .into_iter()
                .filter(|(_, (_, timeout))| timeout.map(|t| now >= t).unwrap_or(false))
                .map(|(hostname, _)| hostname)
            {
                trace!("hostname resolver timeout for {}", &hostname);
                call_hostname_resolution_listener(
                    &self.hostname_resolvers,
                    &hostname,
                    HostnameResolutionEvent::SearchTimeout(hostname.to_owned()),
                );
                call_hostname_resolution_listener(
                    &self.hostname_resolvers,
                    &hostname,
                    HostnameResolutionEvent::SearchStopped(hostname.to_owned()),
                );
                self.hostname_resolvers.remove(&hostname);
            }

            // process commands from the command channel
            while let Ok(command) = receiver.try_recv() {
                if matches!(command, Command::Exit(_)) {
                    debug!("Exit command received, performing cleanup");
                    self.cleanup();
                    self.status = DaemonStatus::Shutdown;
                    return Some(command);
                }
                self.exec_command(command, false);
            }

            // check for repeated commands and run them if their time is up.
            let mut i = 0;
            while i < self.retransmissions.len() {
                if now >= self.retransmissions[i].next_time {
                    let rerun = self.retransmissions.remove(i);
                    self.exec_command(rerun.command, true);
                } else {
                    i += 1;
                }
            }

            // Refresh cached service records with active queriers
            self.refresh_active_services();

            // Refresh cached A/AAAA records with active queriers
            let mut query_count = 0;
            for (hostname, _sender) in self.hostname_resolvers.iter() {
                for (hostname, ip_addr) in
                    self.cache.refresh_due_hostname_resolutions(hostname).iter()
                {
                    self.send_query(hostname, ip_address_rr_type(&ip_addr.to_ip_addr()));
                    query_count += 1;
                }
            }

            self.increase_counter(Counter::CacheRefreshAddr, query_count);

            // check and evict expired records in our cache
            let now = current_time_millis();

            // Notify service listeners about the expired records.
            let expired_services = self.cache.evict_expired_services(now);
            if !expired_services.is_empty() {
                debug!(
                    "run: send {} service removal to listeners",
                    expired_services.len()
                );
                self.notify_service_removal(expired_services);
            }

            // Notify hostname listeners about the expired records.
            let expired_addrs = self.cache.evict_expired_addr(now);
            for (hostname, addrs) in expired_addrs {
                call_hostname_resolution_listener(
                    &self.hostname_resolvers,
                    &hostname,
                    HostnameResolutionEvent::AddressesRemoved(hostname.clone(), addrs),
                );
                let instances = self.cache.get_instances_on_host(&hostname);
                let instance_set: HashSet<String> = instances.into_iter().collect();
                self.resolve_updated_instances(&instance_set);
            }

            // Send out probing queries.
            self.probing_handler();

            // check IP changes if next_ip_check is reached.
            if now >= next_ip_check && next_ip_check > 0 {
                next_ip_check = now + self.ip_check_interval;
                self.add_timer(next_ip_check);

                self.check_ip_changes();
            }
        }
    }

    fn process_set_option(&mut self, daemon_opt: DaemonOption) {
        match daemon_opt {
            DaemonOption::ServiceNameLenMax(length) => self.service_name_len_max = length,
            DaemonOption::IpCheckInterval(interval) => self.ip_check_interval = interval,
            DaemonOption::EnableInterface(if_kind) => self.enable_interface(if_kind),
            DaemonOption::DisableInterface(if_kind) => self.disable_interface(if_kind),
            DaemonOption::MulticastLoopV4(on) => self.set_multicast_loop_v4(on),
            DaemonOption::MulticastLoopV6(on) => self.set_multicast_loop_v6(on),
            DaemonOption::AcceptUnsolicited(accept) => self.set_accept_unsolicited(accept),
            DaemonOption::IncludeAppleP2P(enable) => self.set_apple_p2p(enable),
            #[cfg(test)]
            DaemonOption::TestDownInterface(ifname) => {
                self.test_down_interfaces.insert(ifname);
            }
            #[cfg(test)]
            DaemonOption::TestUpInterface(ifname) => {
                self.test_down_interfaces.remove(&ifname);
            }
        }
    }

    fn enable_interface(&mut self, kinds: Vec<IfKind>) {
        debug!("enable_interface: {:?}", kinds);
        let interfaces = my_ip_interfaces_inner(true, self.include_apple_p2p);

        for if_kind in kinds {
            self.if_selections.push(IfSelection {
                if_kind: resolve_addr_to_index(if_kind, &interfaces),
                selected: true,
            });
        }

        self.apply_intf_selections(interfaces);
    }

    fn disable_interface(&mut self, kinds: Vec<IfKind>) {
        debug!("disable_interface: {:?}", kinds);
        let interfaces = my_ip_interfaces_inner(true, self.include_apple_p2p);

        for if_kind in kinds {
            self.if_selections.push(IfSelection {
                if_kind: resolve_addr_to_index(if_kind, &interfaces),
                selected: false,
            });
        }

        self.apply_intf_selections(interfaces);
    }

    fn set_multicast_loop_v4(&mut self, on: bool) {
        let Some(sock) = self.ipv4_sock.as_mut() else {
            return;
        };
        self.multicast_loop_v4 = on;
        sock.pktinfo
            .set_multicast_loop_v4(on)
            .map_err(|e| e_fmt!("failed to set multicast loop v4: {}", e))
            .unwrap();
    }

    fn set_multicast_loop_v6(&mut self, on: bool) {
        let Some(sock) = self.ipv6_sock.as_mut() else {
            return;
        };
        self.multicast_loop_v6 = on;
        sock.pktinfo
            .set_multicast_loop_v6(on)
            .map_err(|e| e_fmt!("failed to set multicast loop v6: {}", e))
            .unwrap();
    }

    fn set_accept_unsolicited(&mut self, accept: bool) {
        self.accept_unsolicited = accept;
    }

    fn set_apple_p2p(&mut self, include: bool) {
        if self.include_apple_p2p != include {
            self.include_apple_p2p = include;
            self.apply_intf_selections(my_ip_interfaces_inner(true, self.include_apple_p2p));
        }
    }

    fn notify_monitors(&mut self, event: DaemonEvent) {
        // Only retain the monitors that are still connected.
        self.monitors.retain(|sender| {
            if let Err(e) = sender.try_send(event.clone()) {
                debug!("notify_monitors: try_send: {}", &e);
                if matches!(e, TrySendError::Disconnected(_)) {
                    return false; // This monitor is dropped.
                }
            }
            true
        });
    }

    /// Remove `addr` in my services that enabled `addr_auto`.
    fn del_addr_in_my_services(&mut self, addr: &IpAddr) {
        for (_, service_info) in self.my_services.iter_mut() {
            if service_info.is_addr_auto() {
                service_info.remove_ipaddr(addr);
            }
        }
    }

    fn add_timer(&mut self, next_time: u64) {
        self.timers.push(Reverse(next_time));
    }

    fn peek_earliest_timer(&self) -> Option<u64> {
        self.timers.peek().map(|Reverse(v)| *v)
    }

    fn _pop_earliest_timer(&mut self) -> Option<u64> {
        self.timers.pop().map(|Reverse(v)| v)
    }

    /// Pop all timers that are already passed till `now`.
    fn pop_timers_till(&mut self, now: u64) {
        while let Some(Reverse(v)) = self.timers.peek() {
            if *v > now {
                break;
            }
            self.timers.pop();
        }
    }

    /// Apply all selections to `interfaces` and return the selected addresses.
    fn selected_intfs(&self, interfaces: Vec<Interface>) -> HashSet<Interface> {
        let intf_count = interfaces.len();
        let mut intf_selections = vec![true; intf_count];

        // apply if_selections
        for selection in self.if_selections.iter() {
            // Mark the interfaces for this selection.
            for i in 0..intf_count {
                if selection.if_kind.matches(&interfaces[i]) {
                    intf_selections[i] = selection.selected;
                }
            }
        }

        let mut selected_addrs = HashSet::new();
        for i in 0..intf_count {
            if intf_selections[i] {
                selected_addrs.insert(interfaces[i].clone());
            }
        }

        selected_addrs
    }

    /// Apply all selections to `interfaces`.
    ///
    /// For any interface, add it if selected but not bound yet,
    /// delete it if not selected but still bound.
    fn apply_intf_selections(&mut self, interfaces: Vec<Interface>) {
        // By default, we enable all interfaces.
        let intf_count = interfaces.len();
        let mut intf_selections = vec![true; intf_count];

        // apply if_selections
        for selection in self.if_selections.iter() {
            // Mark the interfaces for this selection.
            for i in 0..intf_count {
                if selection.if_kind.matches(&interfaces[i]) {
                    intf_selections[i] = selection.selected;
                }
            }
        }

        // Update `my_intfs` based on the selections.
        for (idx, intf) in interfaces.into_iter().enumerate() {
            if intf_selections[idx] {
                // Add the interface
                self.add_interface(intf);
            } else {
                // Remove the interface
                self.del_interface_addr(&intf);
            }
        }
    }

    fn del_ip(&mut self, ip: IpAddr) {
        self.del_addr_in_my_services(&ip);
        self.notify_monitors(DaemonEvent::IpDel(ip));
    }

    /// Check for IP changes and update [my_intfs] as needed.
    fn check_ip_changes(&mut self) {
        // Get the current interfaces.
        let my_ifaddrs = my_ip_interfaces_inner(true, self.include_apple_p2p);

        #[cfg(test)]
        let my_ifaddrs: Vec<_> = my_ifaddrs
            .into_iter()
            .filter(|intf| !self.test_down_interfaces.contains(&intf.name))
            .collect();

        let ifaddrs_map: HashMap<u32, Vec<&IfAddr>> =
            my_ifaddrs.iter().fold(HashMap::new(), |mut acc, intf| {
                let if_index = intf.index.unwrap_or(0);
                acc.entry(if_index).or_default().push(&intf.addr);
                acc
            });

        let mut deleted_intfs = Vec::new();
        let mut deleted_ips = Vec::new();

        for (if_index, my_intf) in self.my_intfs.iter_mut() {
            let mut last_ipv4 = None;
            let mut last_ipv6 = None;

            if let Some(current_addrs) = ifaddrs_map.get(if_index) {
                my_intf.addrs.retain(|addr| {
                    if current_addrs.contains(&addr) {
                        true
                    } else {
                        match addr.ip() {
                            IpAddr::V4(ipv4) => last_ipv4 = Some(ipv4),
                            IpAddr::V6(ipv6) => last_ipv6 = Some(ipv6),
                        }
                        deleted_ips.push(addr.ip());
                        false
                    }
                });
                if my_intf.addrs.is_empty() {
                    deleted_intfs.push((*if_index, last_ipv4, last_ipv6))
                }
            } else {
                // If it does not exist, remove the interface.
                debug!(
                    "check_ip_changes: interface {} ({}) no longer exists, removing",
                    my_intf.name, if_index
                );
                for addr in my_intf.addrs.iter() {
                    match addr.ip() {
                        IpAddr::V4(ipv4) => last_ipv4 = Some(ipv4),
                        IpAddr::V6(ipv6) => last_ipv6 = Some(ipv6),
                    }
                    deleted_ips.push(addr.ip())
                }
                deleted_intfs.push((*if_index, last_ipv4, last_ipv6));
            }
        }

        if !deleted_ips.is_empty() || !deleted_intfs.is_empty() {
            debug!(
                "check_ip_changes: {} deleted ips {} deleted intfs",
                deleted_ips.len(),
                deleted_intfs.len()
            );
        }

        for ip in deleted_ips {
            self.del_ip(ip);
        }

        for (if_index, last_ipv4, last_ipv6) in deleted_intfs {
            let Some(my_intf) = self.my_intfs.remove(&if_index) else {
                continue;
            };

            if let Some(ipv4) = last_ipv4 {
                debug!("leave multicast for {ipv4}");
                if let Some(sock) = self.ipv4_sock.as_mut() {
                    if let Err(e) = sock.pktinfo.leave_multicast_v4(&GROUP_ADDR_V4, &ipv4) {
                        debug!("leave multicast group for addr {ipv4}: {e}");
                    }
                }
            }

            if let Some(ipv6) = last_ipv6 {
                debug!("leave multicast for {ipv6}");
                if let Some(sock) = self.ipv6_sock.as_mut() {
                    if let Err(e) = sock
                        .pktinfo
                        .leave_multicast_v6(&GROUP_ADDR_V6, my_intf.index)
                    {
                        debug!("leave multicast group for IPv6: {ipv6}: {e}");
                    }
                }
            }

            // Remove cache records for this interface.
            let intf_id = InterfaceId {
                name: my_intf.name.to_string(),
                index: my_intf.index,
            };
            let result = self.cache.remove_records_on_intf(intf_id);
            self.notify_service_removal(result.removed_instances);
            self.resolve_updated_instances(&result.modified_instances);
        }

        // Add newly found interfaces only if in our selections.
        self.apply_intf_selections(my_ifaddrs);
    }

    /// Remove an interface address when it was down, disabled or removed from the system.
    /// If no more addresses on the interface, remove the interface as well.
    fn del_interface_addr(&mut self, intf: &Interface) {
        let if_index = intf.index.unwrap_or(0);
        debug!(
            "del_interface_addr: {} ({if_index}) addr {}",
            intf.name,
            intf.ip()
        );

        let Some(my_intf) = self.my_intfs.get_mut(&if_index) else {
            debug!("del_interface_addr: interface {} not found", intf.name);
            return;
        };

        let mut ip_removed = false;

        if my_intf.addrs.remove(&intf.addr) {
            ip_removed = true;

            match intf.addr.ip() {
                IpAddr::V4(ipv4) => {
                    if my_intf.next_ifaddr_v4().is_none() {
                        if let Some(sock) = self.ipv4_sock.as_mut() {
                            if let Err(e) = sock.pktinfo.leave_multicast_v4(&GROUP_ADDR_V4, &ipv4) {
                                debug!("leave multicast group for addr {ipv4}: {e}");
                            } else {
                                debug!("leave multicast for {ipv4}");
                            }
                        }
                    }
                }

                IpAddr::V6(ipv6) => {
                    if my_intf.next_ifaddr_v6().is_none() {
                        if let Some(sock) = self.ipv6_sock.as_mut() {
                            if let Err(e) =
                                sock.pktinfo.leave_multicast_v6(&GROUP_ADDR_V6, if_index)
                            {
                                debug!("leave multicast group for addr {ipv6}: {e}");
                            }
                        }
                    }
                }
            }

            if my_intf.addrs.is_empty() {
                // If no more addresses, remove the interface.
                debug!("del_interface_addr: removing interface {}", intf.name);
                self.my_intfs.remove(&if_index);
                self.dns_registry_map.remove(&if_index);
                self.cache
                    .remove_addrs_on_disabled_intf(if_index, IpType::BOTH);
            } else {
                // Interface still has addresses of the other IP version.
                // Remove cached address records for the disabled IP version
                // only if no more addresses of that version remain.
                let is_v4 = intf.addr.ip().is_ipv4();
                let version_gone = if is_v4 {
                    my_intf.next_ifaddr_v4().is_none()
                } else {
                    my_intf.next_ifaddr_v6().is_none()
                };
                if version_gone {
                    let ip_type = if is_v4 { IpType::V4 } else { IpType::V6 };
                    self.cache.remove_addrs_on_disabled_intf(if_index, ip_type);
                }
            }
        }

        if ip_removed {
            // Notify the monitors.
            self.notify_monitors(DaemonEvent::IpDel(intf.ip()));
            // Remove the interface from my services that enabled `addr_auto`.
            self.del_addr_in_my_services(&intf.ip());
        }
    }

    fn add_interface(&mut self, intf: Interface) {
        let sock_opt = if intf.ip().is_ipv4() {
            &self.ipv4_sock
        } else {
            &self.ipv6_sock
        };

        let Some(sock) = sock_opt else {
            debug!(
                "add_interface: no socket available for interface {} with addr {}. Skipped.",
                intf.name,
                intf.ip()
            );
            return;
        };

        let if_index = intf.index.unwrap_or(0);
        let mut new_addr = false;

        match self.my_intfs.entry(if_index) {
            Entry::Occupied(mut entry) => {
                // If intf has a new address, add it to the existing interface.
                let my_intf = entry.get_mut();
                if !my_intf.addrs.contains(&intf.addr) {
                    if let Err(e) = join_multicast_group(&sock.pktinfo, &intf) {
                        debug!("add_interface: socket_config {}: {e}", &intf.name);
                    }
                    my_intf.addrs.insert(intf.addr.clone());
                    new_addr = true;
                }
            }
            Entry::Vacant(entry) => {
                if let Err(e) = join_multicast_group(&sock.pktinfo, &intf) {
                    debug!("add_interface: socket_config {}: {e}. Skipped.", &intf.name);
                    return;
                }

                new_addr = true;
                let new_intf = MyIntf {
                    name: intf.name.clone(),
                    index: if_index,
                    addrs: HashSet::from([intf.addr.clone()]),
                };
                entry.insert(new_intf);
            }
        }

        if !new_addr {
            trace!("add_interface: interface {} already exists", &intf.name);
            return;
        }

        debug!("add new interface {}: {}", intf.name, intf.ip());

        let Some(my_intf) = self.my_intfs.get(&if_index) else {
            debug!("add_interface: cannot find if_index {if_index}");
            return;
        };

        let dns_registry = match self.dns_registry_map.get_mut(&if_index) {
            Some(registry) => registry,
            None => self
                .dns_registry_map
                .entry(if_index)
                .or_insert_with(DnsRegistry::new),
        };

        for (_, service_info) in self.my_services.iter_mut() {
            if service_info.is_addr_auto() {
                service_info.insert_ipaddr(&intf);

                if let Ok(true) = announce_service_on_intf(
                    dns_registry,
                    service_info,
                    my_intf,
                    &sock.pktinfo,
                    self.port,
                ) {
                    debug!(
                        "Announce service {} on {}",
                        service_info.get_fullname(),
                        intf.ip()
                    );
                    service_info.set_status(if_index, ServiceStatus::Announced);
                } else {
                    for timer in dns_registry.new_timers.drain(..) {
                        self.timers.push(Reverse(timer));
                    }
                    service_info.set_status(if_index, ServiceStatus::Probing);
                }
            }
        }

        // Send browse queries on the new interface without known answers.
        // This avoids known-answer suppression (RFC 6762 Section 7.1) that
        // would cause the responder to suppress its response, preventing
        // address records from being attributed to the new interface.
        if let Some(my_intf) = self.my_intfs.get(&if_index) {
            for ty in self.service_queriers.keys() {
                self.send_query_on_intf(ty, RRType::PTR, my_intf);
            }
        }

        // Notify the monitors.
        self.notify_monitors(DaemonEvent::IpAdd(intf.ip()));
    }

    /// Registers a service.
    ///
    /// RFC 6762 section 8.3.
    /// ...the Multicast DNS responder MUST send
    ///    an unsolicited Multicast DNS response containing, in the Answer
    ///    Section, all of its newly registered resource records
    ///
    /// Zeroconf will then respond to requests for information about this service.
    fn register_service(&mut self, mut info: ServiceInfo) {
        // Check the service name length.
        if let Err(e) = check_service_name_length(info.get_type(), self.service_name_len_max) {
            error!("check_service_name_length: {}", &e);
            self.notify_monitors(DaemonEvent::Error(e));
            return;
        }

        if info.is_addr_auto() {
            let selected_intfs =
                self.selected_intfs(my_ip_interfaces_inner(true, self.include_apple_p2p));
            for intf in selected_intfs {
                info.insert_ipaddr(&intf);
            }
        }

        debug!("register service {:?}", &info);

        let outgoing_addrs = self.send_unsolicited_response(&mut info);
        if !outgoing_addrs.is_empty() {
            self.notify_monitors(DaemonEvent::Announce(
                info.get_fullname().to_string(),
                format!("{:?}", &outgoing_addrs),
            ));
        }

        // The key has to be lower case letter as DNS record name is case insensitive.
        // The info will have the original name.
        let service_fullname = info.get_fullname().to_lowercase();
        self.my_services.insert(service_fullname, info);
    }

    /// Sends out announcement of `info` on every valid interface.
    /// Returns the list of interface IPs that sent out the announcement.
    fn send_unsolicited_response(&mut self, info: &mut ServiceInfo) -> Vec<IpAddr> {
        let mut outgoing_addrs = Vec::new();
        let mut outgoing_intfs = HashSet::new();

        let mut invalid_intf_addrs = HashSet::new();

        for (if_index, intf) in self.my_intfs.iter() {
            let dns_registry = match self.dns_registry_map.get_mut(if_index) {
                Some(registry) => registry,
                None => self
                    .dns_registry_map
                    .entry(*if_index)
                    .or_insert_with(DnsRegistry::new),
            };

            let mut announced = false;

            // IPv4
            if let Some(sock) = self.ipv4_sock.as_mut() {
                match announce_service_on_intf(dns_registry, info, intf, &sock.pktinfo, self.port) {
                    Ok(true) => {
                        for addr in intf.addrs.iter().filter(|a| a.ip().is_ipv4()) {
                            outgoing_addrs.push(addr.ip());
                        }
                        outgoing_intfs.insert(intf.index);

                        debug!(
                            "Announce service IPv4 {} on {}",
                            info.get_fullname(),
                            intf.name
                        );
                        announced = true;
                    }
                    Ok(false) => {}
                    Err(InternalError::IntfAddrInvalid(intf_addr)) => {
                        invalid_intf_addrs.insert(intf_addr);
                    }
                }
            }

            if let Some(sock) = self.ipv6_sock.as_mut() {
                match announce_service_on_intf(dns_registry, info, intf, &sock.pktinfo, self.port) {
                    Ok(true) => {
                        for addr in intf.addrs.iter().filter(|a| a.ip().is_ipv6()) {
                            outgoing_addrs.push(addr.ip());
                        }
                        outgoing_intfs.insert(intf.index);

                        debug!(
                            "Announce service IPv6 {} on {}",
                            info.get_fullname(),
                            intf.name
                        );
                        announced = true;
                    }
                    Ok(false) => {}
                    Err(InternalError::IntfAddrInvalid(intf_addr)) => {
                        invalid_intf_addrs.insert(intf_addr);
                    }
                }
            }

            if announced {
                info.set_status(intf.index, ServiceStatus::Announced);
            } else {
                for timer in dns_registry.new_timers.drain(..) {
                    self.timers.push(Reverse(timer));
                }
                info.set_status(*if_index, ServiceStatus::Probing);
            }
        }

        if !invalid_intf_addrs.is_empty() {
            let _ = self.send_cmd_to_self(Command::InvalidIntfAddrs(invalid_intf_addrs));
        }

        // RFC 6762 section 8.3.
        // ..The Multicast DNS responder MUST send at least two unsolicited
        //    responses, one second apart.
        let next_time = current_time_millis() + 1000;
        for if_index in outgoing_intfs {
            self.add_retransmission(
                next_time,
                Command::RegisterResend(info.get_fullname().to_string(), if_index),
            );
        }

        outgoing_addrs
    }

    /// Send probings or finish them if expired. Notify waiting services.
    fn probing_handler(&mut self) {
        let now = current_time_millis();
        let mut invalid_intf_addrs = HashSet::new();

        for (if_index, intf) in self.my_intfs.iter() {
            let Some(dns_registry) = self.dns_registry_map.get_mut(if_index) else {
                continue;
            };

            let (out, expired_probes) = check_probing(dns_registry, &mut self.timers, now);

            // send probing.
            if !out.questions().is_empty() {
                trace!("sending out probing of questions: {:?}", out.questions());
                if let Some(sock) = self.ipv4_sock.as_mut() {
                    if let Err(InternalError::IntfAddrInvalid(intf_addr)) =
                        send_dns_outgoing(&out, intf, &sock.pktinfo, self.port, None, None)
                    {
                        invalid_intf_addrs.insert(intf_addr);
                    }
                }
                if let Some(sock) = self.ipv6_sock.as_mut() {
                    if let Err(InternalError::IntfAddrInvalid(intf_addr)) =
                        send_dns_outgoing(&out, intf, &sock.pktinfo, self.port, None, None)
                    {
                        invalid_intf_addrs.insert(intf_addr);
                    }
                }
            }

            // For finished probes, wake up services that are waiting for the probes.
            let waiting_services =
                handle_expired_probes(expired_probes, &intf.name, dns_registry, &mut self.monitors);

            for service_name in waiting_services {
                // service names are lowercase
                if let Some(info) = self.my_services.get_mut(&service_name.to_lowercase()) {
                    if info.get_status(*if_index) == ServiceStatus::Announced {
                        debug!("service {} already announced", info.get_fullname());
                        continue;
                    }

                    let announced_v4 = if let Some(sock) = self.ipv4_sock.as_mut() {
                        match announce_service_on_intf(
                            dns_registry,
                            info,
                            intf,
                            &sock.pktinfo,
                            self.port,
                        ) {
                            Ok(announced) => announced,
                            Err(InternalError::IntfAddrInvalid(intf_addr)) => {
                                invalid_intf_addrs.insert(intf_addr);
                                false
                            }
                        }
                    } else {
                        false
                    };
                    let announced_v6 = if let Some(sock) = self.ipv6_sock.as_mut() {
                        match announce_service_on_intf(
                            dns_registry,
                            info,
                            intf,
                            &sock.pktinfo,
                            self.port,
                        ) {
                            Ok(announced) => announced,
                            Err(InternalError::IntfAddrInvalid(intf_addr)) => {
                                invalid_intf_addrs.insert(intf_addr);
                                false
                            }
                        }
                    } else {
                        false
                    };

                    if announced_v4 || announced_v6 {
                        let next_time = now + 1000;
                        let command =
                            Command::RegisterResend(info.get_fullname().to_string(), *if_index);
                        self.retransmissions.push(ReRun { next_time, command });
                        self.timers.push(Reverse(next_time));

                        let fullname = dns_registry.resolve_name(&service_name).to_string();

                        let hostname = dns_registry.resolve_name(info.get_hostname());

                        debug!("wake up: announce service {} on {}", fullname, intf.name);
                        notify_monitors(
                            &mut self.monitors,
                            DaemonEvent::Announce(fullname, format!("{}:{}", hostname, &intf.name)),
                        );

                        info.set_status(*if_index, ServiceStatus::Announced);
                    }
                }
            }
        }

        if !invalid_intf_addrs.is_empty() {
            let _ = self.send_cmd_to_self(Command::InvalidIntfAddrs(invalid_intf_addrs));
        }
    }

    fn unregister_service(
        &self,
        info: &ServiceInfo,
        intf: &MyIntf,
        sock: &PktInfoUdpSocket,
    ) -> Vec<u8> {
        let is_ipv4 = sock.domain() == Domain::IPV4;

        let mut out = DnsOutgoing::new(FLAGS_QR_RESPONSE | FLAGS_AA);
        out.add_answer_at_time(
            DnsPointer::new(
                info.get_type(),
                RRType::PTR,
                CLASS_IN,
                0,
                info.get_fullname().to_string(),
            ),
            0,
        );

        if let Some(sub) = info.get_subtype() {
            trace!("Adding subdomain {}", sub);
            out.add_answer_at_time(
                DnsPointer::new(
                    sub,
                    RRType::PTR,
                    CLASS_IN,
                    0,
                    info.get_fullname().to_string(),
                ),
                0,
            );
        }

        out.add_answer_at_time(
            DnsSrv::new(
                info.get_fullname(),
                CLASS_IN | CLASS_CACHE_FLUSH,
                0,
                info.get_priority(),
                info.get_weight(),
                info.get_port(),
                info.get_hostname().to_string(),
            ),
            0,
        );
        out.add_answer_at_time(
            DnsTxt::new(
                info.get_fullname(),
                CLASS_IN | CLASS_CACHE_FLUSH,
                0,
                info.generate_txt(),
            ),
            0,
        );

        let if_addrs = if is_ipv4 {
            info.get_addrs_on_my_intf_v4(intf)
        } else {
            info.get_addrs_on_my_intf_v6(intf)
        };

        if if_addrs.is_empty() {
            return vec![];
        }

        for address in if_addrs {
            out.add_answer_at_time(
                DnsAddress::new(
                    info.get_hostname(),
                    ip_address_rr_type(&address),
                    CLASS_IN | CLASS_CACHE_FLUSH,
                    0,
                    address,
                    intf.into(),
                ),
                0,
            );
        }

        // Only (at most) one packet is expected to be sent out.
        let sent_vec = match send_dns_outgoing(&out, intf, sock, self.port, None, None) {
            Ok(sent_vec) => sent_vec,
            Err(InternalError::IntfAddrInvalid(intf_addr)) => {
                let invalid_intf_addrs = HashSet::from([intf_addr]);
                let _ = self.send_cmd_to_self(Command::InvalidIntfAddrs(invalid_intf_addrs));
                vec![]
            }
        };
        sent_vec.into_iter().next().unwrap_or_default()
    }

    /// Binds a channel `listener` to querying mDNS hostnames.
    ///
    /// If there is already a `listener`, it will be updated, i.e. overwritten.
    fn add_hostname_resolver(
        &mut self,
        hostname: String,
        listener: Sender<HostnameResolutionEvent>,
        timeout: Option<u64>,
    ) {
        let real_timeout = timeout.map(|t| current_time_millis() + t);
        self.hostname_resolvers
            .insert(hostname.to_lowercase(), (listener, real_timeout));
        if let Some(t) = real_timeout {
            self.add_timer(t);
        }
    }

    /// Sends a multicast query for `name` with `qtype`.
    fn send_query(&self, name: &str, qtype: RRType) {
        self.send_query_vec(&[(name, qtype)]);
    }

    /// Sends a query on a specific interface without known answers.
    ///
    /// Used when a new interface is added so the responder won't suppress
    /// its response due to known-answer suppression (RFC 6762 Section 7.1).
    fn send_query_on_intf(&self, name: &str, qtype: RRType, intf: &MyIntf) {
        let mut out = DnsOutgoing::new(FLAGS_QR_QUERY);
        out.add_question(name, qtype);

        let mut invalid_intf_addrs = HashSet::new();
        if let Some(sock) = self.ipv4_sock.as_ref() {
            if let Err(InternalError::IntfAddrInvalid(intf_addr)) =
                send_dns_outgoing(&out, intf, &sock.pktinfo, self.port, None, None)
            {
                invalid_intf_addrs.insert(intf_addr);
            }
        }
        if let Some(sock) = self.ipv6_sock.as_ref() {
            if let Err(InternalError::IntfAddrInvalid(intf_addr)) =
                send_dns_outgoing(&out, intf, &sock.pktinfo, self.port, None, None)
            {
                invalid_intf_addrs.insert(intf_addr);
            }
        }
        if !invalid_intf_addrs.is_empty() {
            let _ = self.send_cmd_to_self(Command::InvalidIntfAddrs(invalid_intf_addrs));
        }
    }

    /// Sends out a list of `questions` (i.e. DNS questions) via multicast.
    fn send_query_vec(&self, questions: &[(&str, RRType)]) {
        let mut out = DnsOutgoing::new(FLAGS_QR_QUERY);
        let now = current_time_millis();

        for (name, qtype) in questions {
            out.add_question(name, *qtype);

            for record in self.cache.get_known_answers(name, *qtype, now) {
                /*
                RFC 6762 section 7.1: https://datatracker.ietf.org/doc/html/rfc6762#section-7.1
                ...
                    When a Multicast DNS querier sends a query to which it already knows
                    some answers, it populates the Answer Section of the DNS query
                    message with those answers.
                 */
                trace!("add known answer: {:?}", record.record);
                let mut new_record = record.record.clone();
                new_record.get_record_mut().update_ttl(now);
                out.add_answer_box(new_record);
            }
        }

        let mut invalid_intf_addrs = HashSet::new();
        for (_, intf) in self.my_intfs.iter() {
            if let Some(sock) = self.ipv4_sock.as_ref() {
                if let Err(InternalError::IntfAddrInvalid(intf_addr)) =
                    send_dns_outgoing(&out, intf, &sock.pktinfo, self.port, None, None)
                {
                    invalid_intf_addrs.insert(intf_addr);
                }
            }
            if let Some(sock) = self.ipv6_sock.as_ref() {
                if let Err(InternalError::IntfAddrInvalid(intf_addr)) =
                    send_dns_outgoing(&out, intf, &sock.pktinfo, self.port, None, None)
                {
                    invalid_intf_addrs.insert(intf_addr);
                }
            }
        }

        if !invalid_intf_addrs.is_empty() {
            let _ = self.send_cmd_to_self(Command::InvalidIntfAddrs(invalid_intf_addrs));
        }
    }

    /// Reads one UDP datagram from the socket of `intf`.
    ///
    /// Returns false if failed to receive a packet,
    /// otherwise returns true.
    fn handle_read(&mut self, event_key: usize) -> bool {
        let sock_opt = match event_key {
            IPV4_SOCK_EVENT_KEY => &mut self.ipv4_sock,
            IPV6_SOCK_EVENT_KEY => &mut self.ipv6_sock,
            _ => {
                debug!("handle_read: unknown token {}", event_key);
                return false;
            }
        };
        let Some(sock) = sock_opt.as_mut() else {
            debug!("handle_read: socket not available for token {}", event_key);
            return false;
        };
        let mut buf = vec![0u8; MAX_MSG_ABSOLUTE];

        // Read the next mDNS UDP datagram.
        //
        // If the datagram is larger than `buf`, excess bytes may or may not
        // be truncated by the socket layer depending on the platform's libc.
        // In any case, such large datagram will not be decoded properly and
        // this function should return false but should not crash.
        let (sz, pktinfo) = match sock.pktinfo.recv(&mut buf) {
            Ok(sz) => sz,
            Err(e) => {
                if e.kind() != std::io::ErrorKind::WouldBlock {
                    debug!("listening socket read failed: {}", e);
                }
                return false;
            }
        };

        // Find the interface that received the packet.
        let pkt_if_index = pktinfo.if_index as u32;
        let Some(my_intf) = self.my_intfs.get(&pkt_if_index) else {
            debug!(
                "handle_read: no interface found for pktinfo if_index: {}",
                pktinfo.if_index
            );
            return true; // We still return true to indicate that we read something.
        };

        // Drop packets for an IP version that has been disabled on this interface.
        // This is needed because some times the socket layer may still receive packets
        // for an IP version even after we left the multicast group for that IP version.
        // We want to drop such packets to avoid unnecessary processing.
        let is_ipv4 = event_key == IPV4_SOCK_EVENT_KEY;
        if (is_ipv4 && my_intf.next_ifaddr_v4().is_none())
            || (!is_ipv4 && my_intf.next_ifaddr_v6().is_none())
        {
            debug!(
                "handle_read: dropping {} packet on intf {} (disabled)",
                if is_ipv4 { "IPv4" } else { "IPv6" },
                my_intf.name
            );
            return true;
        }

        buf.truncate(sz); // reduce potential processing errors

        match DnsIncoming::new(buf, my_intf.into()) {
            Ok(msg) => {
                if msg.is_query() {
                    let querier_addr = pktinfo.addr_src;
                    self.handle_query(msg, pkt_if_index, querier_addr);
                } else if msg.is_response() {
                    self.handle_response(msg, pkt_if_index);
                } else {
                    debug!("Invalid message: not query and not response");
                }
            }
            Err(e) => debug!("Invalid incoming DNS message: {}", e),
        }

        true
    }

    /// Returns true, if sent query. Returns false if SRV already exists.
    fn query_unresolved(&mut self, instance: &str) -> bool {
        if !valid_instance_name(instance) {
            trace!("instance name {} not valid", instance);
            return false;
        }

        if let Some(records) = self.cache.get_srv(instance) {
            for record in records {
                if let Some(srv) = record.record.any().downcast_ref::<DnsSrv>() {
                    if self.cache.get_addr(srv.host()).is_none() {
                        self.send_query_vec(&[(srv.host(), RRType::A), (srv.host(), RRType::AAAA)]);
                        return true;
                    }
                }
            }
        } else {
            self.send_query(instance, RRType::ANY);
            return true;
        }

        false
    }

    /// Checks if `ty_domain` has records in the cache. If yes, sends the
    /// cached records via `sender`.
    fn query_cache_for_service(
        &mut self,
        ty_domain: &str,
        sender: &Sender<ServiceEvent>,
        now: u64,
    ) {
        let mut resolved: HashSet<String> = HashSet::new();
        let mut unresolved: HashSet<String> = HashSet::new();

        if let Some(records) = self.cache.get_ptr(ty_domain) {
            for record in records.iter().filter(|r| !r.record.expires_soon(now)) {
                if let Some(ptr) = record.record.any().downcast_ref::<DnsPointer>() {
                    let mut new_event = None;
                    match self.resolve_service_from_cache(ty_domain, ptr.alias()) {
                        Ok(resolved_service) => {
                            if resolved_service.is_valid() {
                                debug!("Resolved service from cache: {}", ptr.alias());
                                new_event =
                                    Some(ServiceEvent::ServiceResolved(Box::new(resolved_service)));
                            } else {
                                debug!("Resolved service is not valid: {}", ptr.alias());
                            }
                        }
                        Err(err) => {
                            debug!("Error while resolving service from cache: {}", err);
                            continue;
                        }
                    }

                    match sender.send(ServiceEvent::ServiceFound(
                        ty_domain.to_string(),
                        ptr.alias().to_string(),
                    )) {
                        Ok(()) => debug!("sent service found {}", ptr.alias()),
                        Err(e) => {
                            debug!("failed to send service found: {}", e);
                            continue;
                        }
                    }

                    if let Some(event) = new_event {
                        resolved.insert(ptr.alias().to_string());
                        match sender.send(event) {
                            Ok(()) => debug!("sent service resolved: {}", ptr.alias()),
                            Err(e) => debug!("failed to send service resolved: {}", e),
                        }
                    } else {
                        unresolved.insert(ptr.alias().to_string());
                    }
                }
            }
        }

        for instance in resolved.drain() {
            self.pending_resolves.remove(&instance);
            self.resolved.insert(instance);
        }

        for instance in unresolved.drain() {
            self.add_pending_resolve(instance);
        }
    }

    /// Checks if `hostname` has records in the cache. If yes, sends the
    /// cached records via `sender`.
    fn query_cache_for_hostname(
        &mut self,
        hostname: &str,
        sender: Sender<HostnameResolutionEvent>,
    ) {
        let addresses_map = self.cache.get_addresses_for_host(hostname);
        for (name, addresses) in addresses_map {
            match sender.send(HostnameResolutionEvent::AddressesFound(name, addresses)) {
                Ok(()) => trace!("sent hostname addresses found"),
                Err(e) => debug!("failed to send hostname addresses found: {}", e),
            }
        }
    }

    fn add_pending_resolve(&mut self, instance: String) {
        if !self.pending_resolves.contains(&instance) {
            let next_time = current_time_millis() + RESOLVE_WAIT_IN_MILLIS;
            self.add_retransmission(next_time, Command::Resolve(instance.clone(), 1));
            self.pending_resolves.insert(instance);
        }
    }

    /// Creates a `ResolvedService` from the cache.
    fn resolve_service_from_cache(
        &self,
        ty_domain: &str,
        fullname: &str,
    ) -> Result<ResolvedService> {
        let now = current_time_millis();
        let mut resolved_service = ResolvedService {
            ty_domain: ty_domain.to_string(),
            sub_ty_domain: None,
            fullname: fullname.to_string(),
            host: String::new(),
            port: 0,
            addresses: HashSet::new(),
            txt_properties: TxtProperties::new(),
        };

        // Be sure setting `subtype` if available even when querying for the parent domain.
        if let Some(subtype) = self.cache.get_subtype(fullname) {
            trace!(
                "ty_domain: {} found subtype {} for instance: {}",
                ty_domain,
                subtype,
                fullname
            );
            if resolved_service.sub_ty_domain.is_none() {
                resolved_service.sub_ty_domain = Some(subtype.to_string());
            }
        }

        // resolve SRV record
        if let Some(records) = self.cache.get_srv(fullname) {
            if let Some(answer) = records.iter().find(|r| !r.record.expires_soon(now)) {
                if let Some(dns_srv) = answer.record.any().downcast_ref::<DnsSrv>() {
                    resolved_service.host = dns_srv.host().to_string();
                    resolved_service.port = dns_srv.port();
                }
            }
        }

        // resolve TXT record
        if let Some(records) = self.cache.get_txt(fullname) {
            if let Some(record) = records.iter().find(|r| !r.record.expires_soon(now)) {
                if let Some(dns_txt) = record.record.any().downcast_ref::<DnsTxt>() {
                    resolved_service.txt_properties = dns_txt.text().into();
                }
            }
        }

        // resolve A and AAAA records
        if let Some(records) = self.cache.get_addr(&resolved_service.host) {
            for answer in records.iter() {
                if let Some(dns_a) = answer.record.any().downcast_ref::<DnsAddress>() {
                    if dns_a.expires_soon(now) {
                        trace!(
                            "Addr expired or expires soon: {}",
                            dns_a.address().to_ip_addr()
                        );
                    } else {
                        let scoped = dns_a.address();
                        if let ScopedIp::V4(v4) = &scoped {
                            // Merge interface_ids if this V4 addr already exists.
                            // Linear scan by IP since Eq/Hash include interface_ids.
                            let existing = resolved_service
                                .addresses
                                .iter()
                                .find(|a| a.to_ip_addr() == IpAddr::V4(*v4.addr()))
                                .cloned();
                            if let Some(mut existing) = existing {
                                resolved_service.addresses.remove(&existing);
                                if let ScopedIp::V4(existing_v4) = &mut existing {
                                    for id in v4.interface_ids() {
                                        existing_v4.add_interface_id(id.clone());
                                    }
                                }
                                resolved_service.addresses.insert(existing);
                            } else {
                                resolved_service.addresses.insert(scoped);
                            }
                        } else {
                            resolved_service.addresses.insert(scoped);
                        }
                    }
                }
            }
        }

        Ok(resolved_service)
    }

    fn handle_poller_events(&mut self, events: &mio::Events) {
        for ev in events.iter() {
            trace!("event received with key {:?}", ev.token());
            if ev.token().0 == SIGNAL_SOCK_EVENT_KEY {
                // Drain signals as we will drain commands as well.
                self.signal_sock_drain();

                if let Err(e) = self.poller.registry().reregister(
                    &mut self.signal_sock,
                    ev.token(),
                    mio::Interest::READABLE,
                ) {
                    debug!("failed to modify poller for signal socket: {}", e);
                }
                continue; // Next event.
            }

            // Read until no more packets available.
            while self.handle_read(ev.token().0) {}

            // we continue to monitor this socket.
            if ev.token().0 == IPV4_SOCK_EVENT_KEY {
                // Re-register the IPv4 socket for reading.
                if let Some(sock) = self.ipv4_sock.as_mut() {
                    if let Err(e) =
                        self.poller
                            .registry()
                            .reregister(sock, ev.token(), mio::Interest::READABLE)
                    {
                        debug!("modify poller for IPv4 socket: {}", e);
                    }
                }
            } else if ev.token().0 == IPV6_SOCK_EVENT_KEY {
                // Re-register the IPv6 socket for reading.
                if let Some(sock) = self.ipv6_sock.as_mut() {
                    if let Err(e) =
                        self.poller
                            .registry()
                            .reregister(sock, ev.token(), mio::Interest::READABLE)
                    {
                        debug!("modify poller for IPv6 socket: {}", e);
                    }
                }
            }
        }
    }

    /// Deal with incoming response packets.  All answers
    /// are held in the cache, and listeners are notified.
    fn handle_response(&mut self, mut msg: DnsIncoming, if_index: u32) {
        let now = current_time_millis();

        // remove records that are expired.
        let mut record_predicate = |record: &DnsRecordBox| {
            if !record.get_record().is_expired(now) {
                return true;
            }

            debug!("record is expired, removing it from cache.");
            if self.cache.remove(record) {
                // for PTR records, send event to listeners
                if let Some(dns_ptr) = record.any().downcast_ref::<DnsPointer>() {
                    call_service_listener(
                        &self.service_queriers,
                        dns_ptr.get_name(),
                        ServiceEvent::ServiceRemoved(
                            dns_ptr.get_name().to_string(),
                            dns_ptr.alias().to_string(),
                        ),
                    );
                }
            }
            false
        };
        msg.answers_mut().retain(&mut record_predicate);
        msg.authorities_mut().retain(&mut record_predicate);
        msg.additionals_mut().retain(&mut record_predicate);

        // check possible conflicts and handle them.
        self.conflict_handler(&msg, if_index);

        // check if the message is for us.
        let mut is_for_us = true; // assume it is for us.

        // If there are any PTR records in the answers, there should be
        // at least one PTR for us. Otherwise, the message is not for us.
        // If there are no PTR records at all, assume this message is for us.
        for answer in msg.answers() {
            if answer.get_type() == RRType::PTR {
                if self.service_queriers.contains_key(answer.get_name()) {
                    is_for_us = true;
                    break; // OK to break: at least one PTR for us.
                } else {
                    is_for_us = false;
                }
            } else if answer.get_type() == RRType::A || answer.get_type() == RRType::AAAA {
                // If there is a hostname querier for this address, then it is for us.
                let answer_lowercase = answer.get_name().to_lowercase();
                if self.hostname_resolvers.contains_key(&answer_lowercase) {
                    is_for_us = true;
                    break; // OK to break: at least one hostname for us.
                }
            }
        }

        // if we explicitily want to accept unsolicited responses, we should consider all messages as for us.
        if self.accept_unsolicited {
            is_for_us = true;
        }

        /// Represents a DNS record change that involves one service instance.
        struct InstanceChange {
            ty: RRType,   // The type of DNS record for the instance.
            name: String, // The name of the record.
        }

        // Go through all answers to get the new and updated records.
        // For new PTR records, send out ServiceFound immediately. For others,
        // collect them into `changes`.
        //
        // Note: we don't try to identify the update instances based on
        // each record immediately as the answers are likely related to each
        // other.
        let mut changes = Vec::new();
        let mut timers = Vec::new();
        let Some(my_intf) = self.my_intfs.get(&if_index) else {
            return;
        };
        for record in msg.all_records() {
            match self
                .cache
                .add_or_update(my_intf, record, &mut timers, is_for_us)
            {
                Some((dns_record, true)) => {
                    timers.push(dns_record.record.get_record().get_expire_time());
                    timers.push(dns_record.record.get_record().get_refresh_time());

                    let ty = dns_record.record.get_type();
                    let name = dns_record.record.get_name();

                    // Only process PTR that does not expire soon (i.e. TTL > 1).
                    if ty == RRType::PTR && dns_record.record.get_record().get_ttl() > 1 {
                        if self.service_queriers.contains_key(name) {
                            timers.push(dns_record.record.get_record().get_refresh_time());
                        }

                        // send ServiceFound
                        if let Some(dns_ptr) = dns_record.record.any().downcast_ref::<DnsPointer>()
                        {
                            debug!("calling listener with service found: {name}");
                            call_service_listener(
                                &self.service_queriers,
                                name,
                                ServiceEvent::ServiceFound(
                                    name.to_string(),
                                    dns_ptr.alias().to_string(),
                                ),
                            );
                            changes.push(InstanceChange {
                                ty,
                                name: dns_ptr.alias().to_string(),
                            });
                        }
                    } else {
                        changes.push(InstanceChange {
                            ty,
                            name: name.to_string(),
                        });
                    }
                }
                Some((dns_record, false)) => {
                    timers.push(dns_record.record.get_record().get_expire_time());
                    timers.push(dns_record.record.get_record().get_refresh_time());
                }
                _ => {}
            }
        }

        // Add timers for the new records.
        for t in timers {
            self.add_timer(t);
        }

        // Go through remaining changes to see if any hostname resolutions were found or updated.
        for change in changes
            .iter()
            .filter(|change| change.ty == RRType::A || change.ty == RRType::AAAA)
        {
            let addr_map = self.cache.get_addresses_for_host(&change.name);
            for (name, addresses) in addr_map {
                call_hostname_resolution_listener(
                    &self.hostname_resolvers,
                    &change.name,
                    HostnameResolutionEvent::AddressesFound(name, addresses),
                )
            }
        }

        // Identify the instances that need to be "resolved".
        let mut updated_instances = HashSet::new();
        for update in changes {
            match update.ty {
                RRType::PTR | RRType::SRV | RRType::TXT => {
                    updated_instances.insert(update.name);
                }
                RRType::A | RRType::AAAA => {
                    let instances = self.cache.get_instances_on_host(&update.name);
                    updated_instances.extend(instances);
                }
                _ => {}
            }
        }

        self.resolve_updated_instances(&updated_instances);
    }

    fn conflict_handler(&mut self, msg: &DnsIncoming, if_index: u32) {
        let Some(my_intf) = self.my_intfs.get(&if_index) else {
            debug!("handle_response: no intf found for index {if_index}");
            return;
        };

        let Some(dns_registry) = self.dns_registry_map.get_mut(&if_index) else {
            return;
        };

        for answer in msg.answers().iter() {
            let mut new_records = Vec::new();

            let name = answer.get_name();
            let Some(probe) = dns_registry.probing.get_mut(name) else {
                continue;
            };

            // check against possible multicast forwarding
            if answer.get_type() == RRType::A || answer.get_type() == RRType::AAAA {
                if let Some(answer_addr) = answer.any().downcast_ref::<DnsAddress>() {
                    if answer_addr.interface_id.index != if_index {
                        debug!(
                            "conflict handler: answer addr {:?} not in the subnet of intf {}",
                            answer_addr, my_intf.name
                        );
                        continue;
                    }
                }

                // double check if any other address record matches rrdata,
                // as there could be multiple addresses for the same name.
                let any_match = probe.records.iter().any(|r| {
                    r.get_type() == answer.get_type()
                        && r.get_class() == answer.get_class()
                        && r.rrdata_match(answer.as_ref())
                });
                if any_match {
                    continue; // no conflict for this answer.
                }
            }

            probe.records.retain(|record| {
                if record.get_type() == answer.get_type()
                    && record.get_class() == answer.get_class()
                    && !record.rrdata_match(answer.as_ref())
                {
                    debug!(
                        "found conflict name: '{name}' record: {}: {} PEER: {}",
                        record.get_type(),
                        record.rdata_print(),
                        answer.rdata_print()
                    );

                    // create a new name for this record
                    // then remove the old record in probing.
                    let mut new_record = record.clone();
                    let new_name = match record.get_type() {
                        RRType::A => hostname_change(name),
                        RRType::AAAA => hostname_change(name),
                        _ => name_change(name),
                    };
                    new_record.get_record_mut().set_new_name(new_name);
                    new_records.push(new_record);
                    return false; // old record is dropped from the probe.
                }

                true
            });

            // ?????
            // if probe.records.is_empty() {
            //     dns_registry.probing.remove(name);
            // }

            // Probing again with the new names.
            let create_time = current_time_millis() + fastrand::u64(0..250);

            let waiting_services = probe.waiting_services.clone();

            for record in new_records {
                if dns_registry.update_hostname(name, record.get_name(), create_time) {
                    self.timers.push(Reverse(create_time));
                }

                // remember the name changes (note: `name` might not be the original, it could be already changed once.)
                dns_registry.name_changes.insert(
                    record.get_record().get_original_name().to_string(),
                    record.get_name().to_string(),
                );

                let new_probe = match dns_registry.probing.get_mut(record.get_name()) {
                    Some(p) => p,
                    None => {
                        let new_probe = dns_registry
                            .probing
                            .entry(record.get_name().to_string())
                            .or_insert_with(|| {
                                debug!("conflict handler: new probe of {}", record.get_name());
                                Probe::new(create_time)
                            });
                        self.timers.push(Reverse(new_probe.next_send));
                        new_probe
                    }
                };

                debug!(
                    "insert record with new name '{}' {} into probe",
                    record.get_name(),
                    record.get_type()
                );
                new_probe.insert_record(record);

                new_probe.waiting_services.extend(waiting_services.clone());
            }
        }
    }

    /// Resolve the updated (including new) instances.
    ///
    /// Note: it is possible that more than 1 PTR pointing to the same
    /// instance. For example, a regular service type PTR and a sub-type
    /// service type PTR can both point to the same service instance.
    /// This loop automatically handles the sub-type PTRs.
    fn resolve_updated_instances(&mut self, updated_instances: &HashSet<String>) {
        if updated_instances.is_empty() {
            return;
        }

        let mut resolved: HashSet<String> = HashSet::new();
        let mut unresolved: HashSet<String> = HashSet::new();
        let mut removed_instances = HashMap::new();

        let now = current_time_millis();

        for (ty_domain, records) in self.cache.all_ptr().iter() {
            if !self.service_queriers.contains_key(ty_domain) {
                // No need to resolve if not in our queries.
                continue;
            }

            for ptr in records.iter().filter(|r| !r.record.expires_soon(now)) {
                let Some(dns_ptr) = ptr.record.any().downcast_ref::<DnsPointer>() else {
                    continue;
                };

                let instance = dns_ptr.alias();
                if !updated_instances.contains(instance) {
                    continue;
                }

                let Ok(resolved_service) = self.resolve_service_from_cache(ty_domain, instance)
                else {
                    continue;
                };

                debug!("resolve_updated_instances: from cache: {instance}");
                if resolved_service.is_valid() {
                    debug!("call queriers to resolve {instance}");
                    resolved.insert(instance.to_string());
                    let event = ServiceEvent::ServiceResolved(Box::new(resolved_service));
                    call_service_listener(&self.service_queriers, ty_domain, event);
                } else {
                    debug!("Resolved service is not valid: {instance}");
                    if self.resolved.remove(dns_ptr.alias()) {
                        removed_instances
                            .entry(ty_domain.to_string())
                            .or_insert_with(HashSet::new)
                            .insert(instance.to_string());
                    }
                    unresolved.insert(instance.to_string());
                }
            }
        }

        for instance in resolved.drain() {
            self.pending_resolves.remove(&instance);
            self.resolved.insert(instance);
        }

        for instance in unresolved.drain() {
            self.add_pending_resolve(instance);
        }

        if !removed_instances.is_empty() {
            debug!(
                "resolve_updated_instances: removed {}",
                &removed_instances.len()
            );
            self.notify_service_removal(removed_instances);
        }
    }

    /// Handle incoming query packets, figure out whether and what to respond.
    fn handle_query(&mut self, msg: DnsIncoming, if_index: u32, querier_addr: SocketAddr) {
        let querier_ip = querier_addr.ip();
        let is_ipv4 = querier_ip.is_ipv4();
        let sock_opt = if is_ipv4 {
            &self.ipv4_sock
        } else {
            &self.ipv6_sock
        };
        let Some(sock) = sock_opt.as_ref() else {
            debug!("handle_query: socket not available for intf {}", if_index);
            return;
        };
        let mut out = DnsOutgoing::new(FLAGS_QR_RESPONSE | FLAGS_AA);

        // Special meta-query "_services._dns-sd._udp.<Domain>".
        // See https://datatracker.ietf.org/doc/html/rfc6763#section-9
        const META_QUERY: &str = "_services._dns-sd._udp.local.";

        let Some(dns_registry) = self.dns_registry_map.get_mut(&if_index) else {
            debug!("missing dns registry for intf {}", if_index);
            return;
        };

        let Some(intf) = self.my_intfs.get(&if_index) else {
            debug!("handle_query: no intf found for index {if_index}");
            return;
        };

        for question in msg.questions().iter() {
            let qtype = question.entry_type();
            let q_name = question.entry_name();

            if qtype == RRType::PTR {
                for service in self.my_services.values() {
                    if service.get_status(if_index) != ServiceStatus::Announced {
                        continue;
                    }

                    if service.matches_type_or_subtype(q_name) {
                        out.add_answer_with_additionals(&msg, service, intf, dns_registry, is_ipv4);
                    } else if q_name == META_QUERY {
                        let ttl = service.get_other_ttl();
                        let alias = service.get_type().to_string();
                        let ptr = DnsPointer::new(q_name, RRType::PTR, CLASS_IN, ttl, alias);
                        if !out.add_answer(&msg, ptr) {
                            trace!("answer was not added for meta-query {:?}", &question);
                        }
                    }
                }
            } else {
                // Simultaneous Probe Tiebreaking (RFC 6762 section 8.2)
                if qtype == RRType::ANY && msg.num_authorities() > 0 {
                    if let Some(probe) = dns_registry.probing.get_mut(q_name) {
                        probe.tiebreaking(&msg, q_name);
                    }
                }

                if qtype == RRType::A || qtype == RRType::AAAA || qtype == RRType::ANY {
                    for service in self.my_services.values() {
                        if service.get_status(if_index) != ServiceStatus::Announced {
                            continue;
                        }

                        let service_hostname = dns_registry.resolve_name(service.get_hostname());

                        if service_hostname.to_lowercase() == question.entry_name().to_lowercase() {
                            // Pick addresses based on the question type, not the
                            // socket family. RFC 6762 doesn't require A queries
                            // to come over IPv4 transport — Android's getaddrinfo
                            // routinely sends both A and AAAA queries over its
                            // preferred IPv6 mDNS socket and expects A records
                            // to be answered with v4 addresses.
                            let mut intf_addrs: Vec<IpAddr> = Vec::new();
                            if qtype == RRType::A || qtype == RRType::ANY {
                                intf_addrs.extend(service.get_addrs_on_my_intf_v4(intf));
                            }
                            if qtype == RRType::AAAA || qtype == RRType::ANY {
                                intf_addrs.extend(service.get_addrs_on_my_intf_v6(intf));
                            }
                            if intf_addrs.is_empty()
                                && (qtype == RRType::A || qtype == RRType::AAAA)
                            {
                                let t = match qtype {
                                    RRType::A => "TYPE_A",
                                    RRType::AAAA => "TYPE_AAAA",
                                    _ => "invalid_type",
                                };
                                trace!(
                                    "Cannot find valid addrs for {} response on intf {:?}",
                                    t,
                                    &intf
                                );
                                continue;
                            }
                            for address in intf_addrs {
                                out.add_answer(
                                    &msg,
                                    DnsAddress::new(
                                        service_hostname,
                                        ip_address_rr_type(&address),
                                        CLASS_IN | CLASS_CACHE_FLUSH,
                                        service.get_host_ttl(),
                                        address,
                                        intf.into(),
                                    ),
                                );
                            }
                        }
                    }
                }

                let query_name = q_name.to_lowercase();
                let service_opt = self
                    .my_services
                    .iter()
                    .find(|(k, _v)| dns_registry.resolve_name(k.as_str()) == query_name)
                    .map(|(_, v)| v);

                let Some(service) = service_opt else {
                    continue;
                };

                if service.get_status(if_index) != ServiceStatus::Announced {
                    continue;
                }

                let intf_addrs = if is_ipv4 {
                    service.get_addrs_on_my_intf_v4(intf)
                } else {
                    service.get_addrs_on_my_intf_v6(intf)
                };
                if intf_addrs.is_empty() {
                    debug!(
                        "Cannot find valid addrs for TYPE_SRV response on intf {:?}",
                        &intf
                    );
                    continue;
                }

                add_answer_of_service(
                    &mut out,
                    &msg,
                    question.entry_name(),
                    service,
                    qtype,
                    intf_addrs,
                );
            }
        }

        if out.answers_count() > 0 {
            debug!("sending response on intf {}", &intf.name);
            out.set_id(msg.id());

            // Pick a source IfAddr on `intf` whose subnet contains the querier's IP.
            // It's OK if it's None, `send_dns_outgoing` will then pick one address.
            let matched_source = intf
                .addrs
                .iter()
                .find(|if_addr| valid_ip_on_intf(&querier_ip, if_addr));

            // RFC 6762 §6.7 (Legacy Unicast Responses): if the querier's source
            // port is not 5353, it's a one-shot legacy querier (e.g. Android's
            // getaddrinfo, iOS resolver fallback). The response MUST be unicast
            // back to the querier's source IP and port; multicast replies will
            // never reach the querier's ephemeral socket. Legacy unicast
            // responses must also echo the question section and clear the
            // cache-flush bit, since legacy resolvers don't understand it.
            let unicast_dest = if querier_addr.port() != MDNS_PORT {
                Some(querier_addr)
            } else {
                None
            };

            if unicast_dest.is_some() {
                for q in msg.questions() {
                    out.add_question(q.entry_name(), q.entry_type());
                }
                out.clear_cache_flush_bits();
            }

            if let Err(InternalError::IntfAddrInvalid(intf_addr)) = send_dns_outgoing(
                &out,
                intf,
                &sock.pktinfo,
                self.port,
                matched_source,
                unicast_dest,
            ) {
                let invalid_intf_addr = HashSet::from([intf_addr]);
                let _ = self.send_cmd_to_self(Command::InvalidIntfAddrs(invalid_intf_addr));
            }

            let if_name = intf.name.clone();

            self.increase_counter(Counter::Respond, 1);
            self.notify_monitors(DaemonEvent::Respond(if_name));
        }

        self.increase_counter(Counter::KnownAnswerSuppression, out.known_answer_count());
    }

    /// Increases the value of `counter` by `count`.
    fn increase_counter(&mut self, counter: Counter, count: i64) {
        let key = counter.to_string();
        match self.counters.get_mut(&key) {
            Some(v) => *v += count,
            None => {
                self.counters.insert(key, count);
            }
        }
    }

    /// Sets the value of `counter` to `count`.
    fn set_counter(&mut self, counter: Counter, count: i64) {
        let key = counter.to_string();
        self.counters.insert(key, count);
    }

    fn signal_sock_drain(&self) {
        let mut signal_buf = [0; 1024];

        // This recv is non-blocking as the socket is non-blocking.
        while let Ok(sz) = self.signal_sock.recv(&mut signal_buf) {
            trace!(
                "signal socket recvd: {}",
                String::from_utf8_lossy(&signal_buf[0..sz])
            );
        }
    }

    fn add_retransmission(&mut self, next_time: u64, command: Command) {
        self.retransmissions.push(ReRun { next_time, command });
        self.add_timer(next_time);
    }

    /// Sends service removal event to listeners for expired service records.
    /// `expired`: map of service type domain to set of instance names.
    fn notify_service_removal(&self, expired: HashMap<String, HashSet<String>>) {
        for (ty_domain, sender) in self.service_queriers.iter() {
            if let Some(instances) = expired.get(ty_domain) {
                for instance_name in instances {
                    let event = ServiceEvent::ServiceRemoved(
                        ty_domain.to_string(),
                        instance_name.to_string(),
                    );
                    match sender.send(event) {
                        Ok(()) => debug!("notify_service_removal: sent ServiceRemoved to listener of {ty_domain}: {instance_name}"),
                        Err(e) => debug!("Failed to send event: {}", e),
                    }
                }
            }
        }
    }

    /// The entry point that executes all commands received by the daemon.
    ///
    /// `repeating`: whether this is a retransmission.
    fn exec_command(&mut self, command: Command, repeating: bool) {
        trace!("exec_command: {:?} repeating: {}", &command, repeating);
        match command {
            Command::Browse(ty, next_delay, cache_only, listener) => {
                self.exec_command_browse(repeating, ty, next_delay, cache_only, listener);
            }

            Command::ResolveHostname(hostname, next_delay, listener, timeout) => {
                self.exec_command_resolve_hostname(
                    repeating, hostname, next_delay, listener, timeout,
                );
            }

            Command::Register(service_info) => {
                self.register_service(*service_info);
                self.increase_counter(Counter::Register, 1);
            }

            Command::RegisterResend(fullname, intf) => {
                trace!("register-resend service: {fullname} on {}", &intf);
                if let Err(InternalError::IntfAddrInvalid(intf_addr)) =
                    self.exec_command_register_resend(fullname, intf)
                {
                    let invalid_intf_addr = HashSet::from([intf_addr]);
                    let _ = self.send_cmd_to_self(Command::InvalidIntfAddrs(invalid_intf_addr));
                }
            }

            Command::Unregister(fullname, resp_s) => {
                trace!("unregister service {} repeat {}", &fullname, &repeating);
                self.exec_command_unregister(repeating, fullname, resp_s);
            }

            Command::UnregisterResend(packet, if_index, is_ipv4) => {
                self.exec_command_unregister_resend(packet, if_index, is_ipv4);
            }

            Command::StopBrowse(ty_domain) => self.exec_command_stop_browse(ty_domain),

            Command::StopResolveHostname(hostname) => {
                self.exec_command_stop_resolve_hostname(hostname.to_lowercase())
            }

            Command::Resolve(instance, try_count) => self.exec_command_resolve(instance, try_count),

            Command::GetMetrics(resp_s) => self.exec_command_get_metrics(resp_s),

            Command::GetStatus(resp_s) => match resp_s.send(self.status.clone()) {
                Ok(()) => trace!("Sent status to the client"),
                Err(e) => debug!("Failed to send status: {}", e),
            },

            Command::Monitor(resp_s) => {
                self.monitors.push(resp_s);
            }

            Command::SetOption(daemon_opt) => {
                self.process_set_option(daemon_opt);
            }

            Command::GetOption(resp_s) => {
                let val = DaemonOptionVal {
                    _service_name_len_max: self.service_name_len_max,
                    ip_check_interval: self.ip_check_interval,
                };
                if let Err(e) = resp_s.send(val) {
                    debug!("Failed to send options: {}", e);
                }
            }

            Command::Verify(instance_fullname, timeout) => {
                self.exec_command_verify(instance_fullname, timeout, repeating);
            }

            Command::InvalidIntfAddrs(invalid_intf_addrs) => {
                for intf_addr in invalid_intf_addrs {
                    self.del_interface_addr(&intf_addr);
                }

                self.check_ip_changes();
            }

            _ => {
                debug!("unexpected command: {:?}", &command);
            }
        }
    }

    fn exec_command_get_metrics(&mut self, resp_s: Sender<HashMap<String, i64>>) {
        self.set_counter(Counter::CachedPTR, self.cache.ptr_count() as i64);
        self.set_counter(Counter::CachedSRV, self.cache.srv_count() as i64);
        self.set_counter(Counter::CachedAddr, self.cache.addr_count() as i64);
        self.set_counter(Counter::CachedTxt, self.cache.txt_count() as i64);
        self.set_counter(Counter::CachedNSec, self.cache.nsec_count() as i64);
        self.set_counter(Counter::CachedSubtype, self.cache.subtype_count() as i64);
        self.set_counter(Counter::Timer, self.timers.len() as i64);

        let dns_registry_probe_count: usize = self
            .dns_registry_map
            .values()
            .map(|r| r.probing.len())
            .sum();
        self.set_counter(Counter::DnsRegistryProbe, dns_registry_probe_count as i64);

        let dns_registry_active_count: usize = self
            .dns_registry_map
            .values()
            .map(|r| r.active.values().map(|a| a.len()).sum::<usize>())
            .sum();
        self.set_counter(Counter::DnsRegistryActive, dns_registry_active_count as i64);

        let dns_registry_timer_count: usize = self
            .dns_registry_map
            .values()
            .map(|r| r.new_timers.len())
            .sum();
        self.set_counter(Counter::DnsRegistryTimer, dns_registry_timer_count as i64);

        let dns_registry_name_change_count: usize = self
            .dns_registry_map
            .values()
            .map(|r| r.name_changes.len())
            .sum();
        self.set_counter(
            Counter::DnsRegistryNameChange,
            dns_registry_name_change_count as i64,
        );

        // Send the metrics to the client.
        if let Err(e) = resp_s.send(self.counters.clone()) {
            debug!("Failed to send metrics: {}", e);
        }
    }

    fn exec_command_browse(
        &mut self,
        repeating: bool,
        ty: String,
        next_delay: u32,
        cache_only: bool,
        listener: Sender<ServiceEvent>,
    ) {
        let pretty_addrs: Vec<String> = self
            .my_intfs
            .iter()
            .map(|(if_index, itf)| format!("{} ({if_index})", itf.name))
            .collect();

        if let Err(e) = listener.send(ServiceEvent::SearchStarted(format!(
            "{ty} on {} interfaces [{}]",
            pretty_addrs.len(),
            pretty_addrs.join(", ")
        ))) {
            debug!(
                "Failed to send SearchStarted({})(repeating:{}): {}",
                &ty, repeating, e
            );
            return;
        }

        let now = current_time_millis();
        if !repeating {
            // Binds a `listener` to querying mDNS domain type `ty`.
            //
            // If there is already a `listener`, it will be updated, i.e. overwritten.
            self.service_queriers.insert(ty.clone(), listener.clone());

            // if we already have the records in our cache, just send them
            self.query_cache_for_service(&ty, &listener, now);
        }

        if cache_only {
            // If cache_only is true, we do not send a query.
            match listener.send(ServiceEvent::SearchStopped(ty.clone())) {
                Ok(()) => debug!("SearchStopped sent for {}", &ty),
                Err(e) => debug!("Failed to send SearchStopped: {}", e),
            }
            return;
        }

        self.send_query(&ty, RRType::PTR);
        self.increase_counter(Counter::Browse, 1);

        let next_time = now + (next_delay * 1000) as u64;
        let max_delay = 60 * 60;
        let delay = cmp::min(next_delay * 2, max_delay);
        self.add_retransmission(next_time, Command::Browse(ty, delay, cache_only, listener));
    }

    fn exec_command_resolve_hostname(
        &mut self,
        repeating: bool,
        hostname: String,
        next_delay: u32,
        listener: Sender<HostnameResolutionEvent>,
        timeout: Option<u64>,
    ) {
        let addr_list: Vec<_> = self.my_intfs.iter().collect();
        if let Err(e) = listener.send(HostnameResolutionEvent::SearchStarted(format!(
            "{} on addrs {:?}",
            &hostname, &addr_list
        ))) {
            debug!(
                "Failed to send ResolveStarted({})(repeating:{}): {}",
                &hostname, repeating, e
            );
            return;
        }
        if !repeating {
            self.add_hostname_resolver(hostname.to_owned(), listener.clone(), timeout);
            // if we already have the records in our cache, just send them
            self.query_cache_for_hostname(&hostname, listener.clone());
        }

        self.send_query_vec(&[(&hostname, RRType::A), (&hostname, RRType::AAAA)]);
        self.increase_counter(Counter::ResolveHostname, 1);

        let now = current_time_millis();
        let next_time = now + u64::from(next_delay) * 1000;
        let max_delay = 60 * 60;
        let delay = cmp::min(next_delay * 2, max_delay);

        // Only add retransmission if it does not exceed the hostname resolver timeout, if any.
        if self
            .hostname_resolvers
            .get(&hostname)
            .and_then(|(_sender, timeout)| *timeout)
            .map(|timeout| next_time < timeout)
            .unwrap_or(true)
        {
            self.add_retransmission(
                next_time,
                Command::ResolveHostname(hostname, delay, listener, None),
            );
        }
    }

    fn exec_command_resolve(&mut self, instance: String, try_count: u16) {
        let pending_query = self.query_unresolved(&instance);
        let max_try = 3;
        if pending_query && try_count < max_try {
            // Note that if the current try already succeeds, the next retransmission
            // will be no-op as the cache has been updated.
            let next_time = current_time_millis() + RESOLVE_WAIT_IN_MILLIS;
            self.add_retransmission(next_time, Command::Resolve(instance, try_count + 1));
        }
    }

    fn exec_command_unregister(
        &mut self,
        repeating: bool,
        fullname: String,
        resp_s: Sender<UnregisterStatus>,
    ) {
        let response = match self.my_services.remove_entry(&fullname) {
            None => {
                debug!("unregister: cannot find such service {}", &fullname);
                UnregisterStatus::NotFound
            }
            Some((_k, info)) => {
                let mut timers = Vec::new();

                for (if_index, intf) in self.my_intfs.iter() {
                    if let Some(sock) = self.ipv4_sock.as_ref() {
                        let packet = self.unregister_service(&info, intf, &sock.pktinfo);
                        // repeat for one time just in case some peers miss the message
                        if !repeating && !packet.is_empty() {
                            let next_time = current_time_millis() + 120;
                            self.retransmissions.push(ReRun {
                                next_time,
                                command: Command::UnregisterResend(packet, *if_index, true),
                            });
                            timers.push(next_time);
                        }
                    }

                    // ipv6
                    if let Some(sock) = self.ipv6_sock.as_ref() {
                        let packet = self.unregister_service(&info, intf, &sock.pktinfo);
                        if !repeating && !packet.is_empty() {
                            let next_time = current_time_millis() + 120;
                            self.retransmissions.push(ReRun {
                                next_time,
                                command: Command::UnregisterResend(packet, *if_index, false),
                            });
                            timers.push(next_time);
                        }
                    }
                }

                for t in timers {
                    self.add_timer(t);
                }

                self.increase_counter(Counter::Unregister, 1);
                UnregisterStatus::OK
            }
        };
        if let Err(e) = resp_s.send(response) {
            debug!("unregister: failed to send response: {}", e);
        }
    }

    fn exec_command_unregister_resend(&mut self, packet: Vec<u8>, if_index: u32, is_ipv4: bool) {
        let Some(intf) = self.my_intfs.get(&if_index) else {
            return;
        };
        let sock_opt = if is_ipv4 {
            &self.ipv4_sock
        } else {
            &self.ipv6_sock
        };
        let Some(sock) = sock_opt else {
            return;
        };

        let if_addr = if is_ipv4 {
            match intf.next_ifaddr_v4() {
                Some(addr) => addr,
                None => return,
            }
        } else {
            match intf.next_ifaddr_v6() {
                Some(addr) => addr,
                None => return,
            }
        };

        debug!("UnregisterResend from {:?}", if_addr);
        multicast_on_intf(
            &packet[..],
            &intf.name,
            intf.index,
            if_addr,
            &sock.pktinfo,
            self.port,
        );

        self.increase_counter(Counter::UnregisterResend, 1);
    }

    fn exec_command_stop_browse(&mut self, ty_domain: String) {
        match self.service_queriers.remove_entry(&ty_domain) {
            None => debug!("StopBrowse: cannot find querier for {}", &ty_domain),
            Some((ty, sender)) => {
                // Remove pending browse commands in the reruns.
                trace!("StopBrowse: removed queryer for {}", &ty);
                let mut i = 0;
                while i < self.retransmissions.len() {
                    if let Command::Browse(t, _, _, _) = &self.retransmissions[i].command {
                        if t == &ty {
                            self.retransmissions.remove(i);
                            trace!("StopBrowse: removed retransmission for {}", &ty);
                            continue;
                        }
                    }
                    i += 1;
                }

                // Remove cache entries.
                self.cache.remove_service_type(&ty_domain);

                // Notify the client.
                match sender.send(ServiceEvent::SearchStopped(ty_domain)) {
                    Ok(()) => trace!("Sent SearchStopped to the listener"),
                    Err(e) => debug!("Failed to send SearchStopped: {}", e),
                }
            }
        }
    }

    fn exec_command_stop_resolve_hostname(&mut self, hostname: String) {
        if let Some((host, (sender, _timeout))) = self.hostname_resolvers.remove_entry(&hostname) {
            // Remove pending resolve commands in the reruns.
            trace!("StopResolve: removed queryer for {}", &host);
            let mut i = 0;
            while i < self.retransmissions.len() {
                if let Command::Resolve(t, _) = &self.retransmissions[i].command {
                    if t == &host {
                        self.retransmissions.remove(i);
                        trace!("StopResolve: removed retransmission for {}", &host);
                        continue;
                    }
                }
                i += 1;
            }

            // Notify the client.
            match sender.send(HostnameResolutionEvent::SearchStopped(hostname)) {
                Ok(()) => trace!("Sent SearchStopped to the listener"),
                Err(e) => debug!("Failed to send SearchStopped: {}", e),
            }
        }
    }

    fn exec_command_register_resend(&mut self, fullname: String, if_index: u32) -> MyResult<()> {
        let Some(info) = self.my_services.get_mut(&fullname) else {
            trace!("announce: cannot find such service {}", &fullname);
            return Ok(());
        };

        let Some(dns_registry) = self.dns_registry_map.get_mut(&if_index) else {
            return Ok(());
        };

        let Some(intf) = self.my_intfs.get(&if_index) else {
            return Ok(());
        };

        let announced_v4 = if let Some(sock) = self.ipv4_sock.as_ref() {
            announce_service_on_intf(dns_registry, info, intf, &sock.pktinfo, self.port)?
        } else {
            false
        };
        let announced_v6 = if let Some(sock) = self.ipv6_sock.as_ref() {
            announce_service_on_intf(dns_registry, info, intf, &sock.pktinfo, self.port)?
        } else {
            false
        };

        if announced_v4 || announced_v6 {
            let hostname = dns_registry.resolve_name(info.get_hostname());
            let service_name = dns_registry.resolve_name(&fullname).to_string();

            debug!("resend: announce service {service_name} on {}", intf.name);

            notify_monitors(
                &mut self.monitors,
                DaemonEvent::Announce(service_name, format!("{}:{}", hostname, &intf.name)),
            );
            info.set_status(if_index, ServiceStatus::Announced);
        } else {
            debug!("register-resend should not fail");
        }

        self.increase_counter(Counter::RegisterResend, 1);
        Ok(())
    }

    fn exec_command_verify(&mut self, instance: String, timeout: Duration, repeating: bool) {
        /*
        RFC 6762 section 10.4:
        ...
        When the cache receives this hint that it should reconfirm some
        record, it MUST issue two or more queries for the resource record in
        dispute.  If no response is received within ten seconds, then, even
        though its TTL may indicate that it is not yet due to expire, that
        record SHOULD be promptly flushed from the cache.
        */
        let now = current_time_millis();
        let expire_at = if repeating {
            None
        } else {
            Some(now + timeout.as_millis() as u64)
        };

        // send query for the resource records.
        let record_vec = self.cache.service_verify_queries(&instance, expire_at);

        if !record_vec.is_empty() {
            let query_vec: Vec<(&str, RRType)> = record_vec
                .iter()
                .map(|(record, rr_type)| (record.as_str(), *rr_type))
                .collect();
            self.send_query_vec(&query_vec);

            if let Some(new_expire) = expire_at {
                self.add_timer(new_expire); // ensure a check for the new expire time.

                // schedule a resend 1 second later
                self.add_retransmission(now + 1000, Command::Verify(instance, timeout));
            }
        }
    }

    /// Refresh cached service records with active queriers
    fn refresh_active_services(&mut self) {
        let mut query_ptr_count = 0;
        let mut query_srv_count = 0;
        let mut new_timers = HashSet::new();
        let mut query_addr_count = 0;

        for (ty_domain, _sender) in self.service_queriers.iter() {
            let refreshed_timers = self.cache.refresh_due_ptr(ty_domain);
            if !refreshed_timers.is_empty() {
                trace!("sending refresh query for PTR: {}", ty_domain);
                self.send_query(ty_domain, RRType::PTR);
                query_ptr_count += 1;
                new_timers.extend(refreshed_timers);
            }

            let (instances, timers) = self.cache.refresh_due_srv_txt(ty_domain);
            for (instance, types) in instances {
                trace!("sending refresh query for: {}", &instance);
                let query_vec = types
                    .into_iter()
                    .map(|ty| (instance.as_str(), ty))
                    .collect::<Vec<_>>();
                self.send_query_vec(&query_vec);
                query_srv_count += 1;
            }
            new_timers.extend(timers);
            let (hostnames, timers) = self.cache.refresh_due_hosts(ty_domain);
            for hostname in hostnames.iter() {
                trace!("sending refresh queries for A and AAAA:  {}", hostname);
                self.send_query_vec(&[(hostname, RRType::A), (hostname, RRType::AAAA)]);
                query_addr_count += 2;
            }
            new_timers.extend(timers);
        }

        for timer in new_timers {
            self.add_timer(timer);
        }

        self.increase_counter(Counter::CacheRefreshPTR, query_ptr_count);
        self.increase_counter(Counter::CacheRefreshSrvTxt, query_srv_count);
        self.increase_counter(Counter::CacheRefreshAddr, query_addr_count);
    }
}

/// Adds one or more answers of a service for incoming msg and RR entry name.
fn add_answer_of_service(
    out: &mut DnsOutgoing,
    msg: &DnsIncoming,
    entry_name: &str,
    service: &ServiceInfo,
    qtype: RRType,
    intf_addrs: Vec<IpAddr>,
) {
    if qtype == RRType::SRV || qtype == RRType::ANY {
        out.add_answer(
            msg,
            DnsSrv::new(
                entry_name,
                CLASS_IN | CLASS_CACHE_FLUSH,
                service.get_host_ttl(),
                service.get_priority(),
                service.get_weight(),
                service.get_port(),
                service.get_hostname().to_string(),
            ),
        );
    }

    if qtype == RRType::TXT || qtype == RRType::ANY {
        out.add_answer(
            msg,
            DnsTxt::new(
                entry_name,
                CLASS_IN | CLASS_CACHE_FLUSH,
                service.get_other_ttl(),
                service.generate_txt(),
            ),
        );
    }

    if qtype == RRType::SRV {
        for address in intf_addrs {
            out.add_additional_answer(DnsAddress::new(
                service.get_hostname(),
                ip_address_rr_type(&address),
                CLASS_IN | CLASS_CACHE_FLUSH,
                service.get_host_ttl(),
                address,
                InterfaceId::default(),
            ));
        }
    }
}

/// All possible events sent to the client from the daemon
/// regarding service discovery.
#[derive(Clone, Debug)]
#[non_exhaustive]
pub enum ServiceEvent {
    /// Started searching for a service type.
    SearchStarted(String),

    /// Found a specific (service_type, fullname).
    ServiceFound(String, String),

    /// Resolved a service instance in a ResolvedService struct.
    ServiceResolved(Box<ResolvedService>),

    /// A service instance (service_type, fullname) was removed.
    ServiceRemoved(String, String),

    /// Stopped searching for a service type.
    SearchStopped(String),
}

/// All possible events sent to the client from the daemon
/// regarding host resolution.
#[derive(Clone, Debug)]
#[non_exhaustive]
pub enum HostnameResolutionEvent {
    /// Started searching for the ip address of a hostname.
    SearchStarted(String),
    /// One or more addresses for a hostname has been found.
    AddressesFound(String, HashSet<ScopedIp>),
    /// One or more addresses for a hostname has been removed.
    AddressesRemoved(String, HashSet<ScopedIp>),
    /// The search for the ip address of a hostname has timed out.
    SearchTimeout(String),
    /// Stopped searching for the ip address of a hostname.
    SearchStopped(String),
}

/// Some notable events from the daemon besides [`ServiceEvent`].
/// These events are expected to happen infrequently.
#[derive(Clone, Debug)]
#[non_exhaustive]
pub enum DaemonEvent {
    /// Daemon unsolicitly announced a service from an interface.
    Announce(String, String),

    /// Daemon encountered an error.
    Error(Error),

    /// Daemon detected a new IP address from the host.
    IpAdd(IpAddr),

    /// Daemon detected a IP address removed from the host.
    IpDel(IpAddr),

    /// Daemon resolved a name conflict by changing one of its names.
    /// see [DnsNameChange] for more details.
    NameChange(DnsNameChange),

    /// Send out a multicast response via an interface.
    Respond(String),
}

/// Represents a name change due to a name conflict resolution.
/// See [RFC 6762 section 9](https://datatracker.ietf.org/doc/html/rfc6762#section-9)
#[derive(Clone, Debug)]
pub struct DnsNameChange {
    /// The original name set in `ServiceInfo` by the user.
    pub original: String,

    /// A new name is created by appending a suffix after the original name.
    ///
    /// - for a service instance name, the suffix is `(N)`, where N starts at 2.
    /// - for a host name, the suffix is `-N`, where N starts at 2.
    ///
    /// For example:
    ///
    /// - Service name `foo._service-type._udp` becomes `foo (2)._service-type._udp`
    /// - Host name `foo.local.` becomes `foo-2.local.`
    pub new_name: String,

    /// The resource record type
    pub rr_type: RRType,

    /// The interface where the name conflict and its change happened.
    pub intf_name: String,
}

/// Commands supported by the daemon
#[derive(Debug)]
enum Command {
    /// Browsing for a service type (ty_domain, next_time_delay_in_seconds, channel::sender)
    Browse(String, u32, bool, Sender<ServiceEvent>),

    /// Resolve a hostname to IP addresses.
    ResolveHostname(String, u32, Sender<HostnameResolutionEvent>, Option<u64>), // (hostname, next_time_delay_in_seconds, sender, timeout_in_milliseconds)

    /// Register a service
    Register(Box<ServiceInfo>),

    /// Unregister a service
    Unregister(String, Sender<UnregisterStatus>), // (fullname)

    /// Announce again a service to local network
    RegisterResend(String, u32), // (fullname)

    /// Resend unregister packet.
    UnregisterResend(Vec<u8>, u32, bool), // (packet content, if_index, is_ipv4)

    /// Stop browsing a service type
    StopBrowse(String), // (ty_domain)

    /// Stop resolving a hostname
    StopResolveHostname(String), // (hostname)

    /// Send query to resolve a service instance.
    /// This is used when a PTR record exists but SRV & TXT records are missing.
    Resolve(String, u16), // (service_instance_fullname, try_count)

    /// Read the current values of the counters
    GetMetrics(Sender<Metrics>),

    /// Get the current status of the daemon.
    GetStatus(Sender<DaemonStatus>),

    /// Monitor noticeable events in the daemon.
    Monitor(Sender<DaemonEvent>),

    SetOption(DaemonOption),

    GetOption(Sender<DaemonOptionVal>),

    /// Proactively confirm a DNS resource record.
    ///
    /// The intention is to check if a service name or IP address still valid
    /// before its TTL expires.
    Verify(String, Duration),

    /// Invalidate some interface addresses.
    InvalidIntfAddrs(HashSet<Interface>),

    Exit(Sender<DaemonStatus>),
}

impl fmt::Display for Command {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Browse(_, _, _, _) => write!(f, "Command Browse"),
            Self::ResolveHostname(_, _, _, _) => write!(f, "Command ResolveHostname"),
            Self::Exit(_) => write!(f, "Command Exit"),
            Self::GetStatus(_) => write!(f, "Command GetStatus"),
            Self::GetMetrics(_) => write!(f, "Command GetMetrics"),
            Self::Monitor(_) => write!(f, "Command Monitor"),
            Self::Register(_) => write!(f, "Command Register"),
            Self::RegisterResend(_, _) => write!(f, "Command RegisterResend"),
            Self::SetOption(_) => write!(f, "Command SetOption"),
            Self::GetOption(_) => write!(f, "Command GetOption"),
            Self::StopBrowse(_) => write!(f, "Command StopBrowse"),
            Self::StopResolveHostname(_) => write!(f, "Command StopResolveHostname"),
            Self::Unregister(_, _) => write!(f, "Command Unregister"),
            Self::UnregisterResend(_, _, _) => write!(f, "Command UnregisterResend"),
            Self::Resolve(_, _) => write!(f, "Command Resolve"),
            Self::Verify(_, _) => write!(f, "Command VerifyResource"),
            Self::InvalidIntfAddrs(_) => write!(f, "Command InvalidIntfAddrs"),
        }
    }
}

struct DaemonOptionVal {
    _service_name_len_max: u8,
    ip_check_interval: u64,
}

#[derive(Debug)]
enum DaemonOption {
    ServiceNameLenMax(u8),
    IpCheckInterval(u64),
    EnableInterface(Vec<IfKind>),
    DisableInterface(Vec<IfKind>),
    MulticastLoopV4(bool),
    MulticastLoopV6(bool),
    AcceptUnsolicited(bool),
    IncludeAppleP2P(bool),
    #[cfg(test)]
    TestDownInterface(String),
    #[cfg(test)]
    TestUpInterface(String),
}

/// The length of Service Domain name supported in this lib.
const DOMAIN_LEN: usize = "._tcp.local.".len();

/// Validate the length of "service_name" in a "_<service_name>.<domain_name>." string.
fn check_service_name_length(ty_domain: &str, limit: u8) -> Result<()> {
    if ty_domain.len() <= DOMAIN_LEN + 1 {
        // service name cannot be empty or only '_'.
        return Err(e_fmt!("Service type name cannot be empty: {}", ty_domain));
    }

    let service_name_len = ty_domain.len() - DOMAIN_LEN - 1; // exclude the leading `_`
    if service_name_len > limit as usize {
        return Err(e_fmt!("Service name length must be <= {} bytes", limit));
    }
    Ok(())
}

/// Checks if `name` ends with a valid domain: '._tcp.local.' or '._udp.local.'
fn check_domain_suffix(name: &str) -> Result<()> {
    if !(name.ends_with("._tcp.local.") || name.ends_with("._udp.local.")) {
        return Err(e_fmt!(
            "mDNS service {} must end with '._tcp.local.' or '._udp.local.'",
            name
        ));
    }

    Ok(())
}

/// Validate the service name in a fully qualified name.
///
/// A Full Name = <Instance>.<Service>.<Domain>
/// The only `<Domain>` supported are "._tcp.local." and "._udp.local.".
///
/// Note: this function does not check for the length of the service name.
/// Instead, `register_service` method will check the length.
fn check_service_name(fullname: &str) -> Result<()> {
    check_domain_suffix(fullname)?;

    let remaining: Vec<&str> = fullname[..fullname.len() - DOMAIN_LEN].split('.').collect();
    let name = remaining.last().ok_or_else(|| e_fmt!("No service name"))?;

    if &name[0..1] != "_" {
        return Err(e_fmt!("Service name must start with '_'"));
    }

    let name = &name[1..];

    if name.contains("--") {
        return Err(e_fmt!("Service name must not contain '--'"));
    }

    if name.starts_with('-') || name.ends_with('-') {
        return Err(e_fmt!("Service name (%s) may not start or end with '-'"));
    }

    let ascii_count = name.chars().filter(|c| c.is_ascii_alphabetic()).count();
    if ascii_count < 1 {
        return Err(e_fmt!(
            "Service name must contain at least one letter (eg: 'A-Za-z')"
        ));
    }

    Ok(())
}

/// Validate a hostname.
fn check_hostname(hostname: &str) -> Result<()> {
    if !hostname.ends_with(".local.") {
        return Err(e_fmt!("Hostname must end with '.local.': {hostname}"));
    }

    if hostname == ".local." {
        return Err(e_fmt!(
            "The part of the hostname before '.local.' cannot be empty"
        ));
    }

    if hostname.len() > 255 {
        return Err(e_fmt!("Hostname length must be <= 255 bytes"));
    }

    Ok(())
}

fn call_service_listener(
    listeners_map: &HashMap<String, Sender<ServiceEvent>>,
    ty_domain: &str,
    event: ServiceEvent,
) {
    if let Some(listener) = listeners_map.get(ty_domain) {
        match listener.send(event) {
            Ok(()) => trace!("Sent event to listener successfully"),
            Err(e) => debug!("Failed to send event: {}", e),
        }
    }
}

fn call_hostname_resolution_listener(
    listeners_map: &HashMap<String, (Sender<HostnameResolutionEvent>, Option<u64>)>,
    hostname: &str,
    event: HostnameResolutionEvent,
) {
    let hostname_lower = hostname.to_lowercase();
    if let Some(listener) = listeners_map.get(&hostname_lower).map(|(l, _)| l) {
        match listener.send(event) {
            Ok(()) => trace!("Sent event to listener successfully"),
            Err(e) => debug!("Failed to send event: {}", e),
        }
    }
}

/// Returns valid network interfaces in the host system.
/// Operational down interfaces are excluded.
/// Loopback interfaces are excluded if `with_loopback` is false.
fn my_ip_interfaces(with_loopback: bool) -> Vec<Interface> {
    my_ip_interfaces_inner(with_loopback, false)
}

fn my_ip_interfaces_inner(with_loopback: bool, with_apple_p2p: bool) -> Vec<Interface> {
    if_addrs::get_if_addrs()
        .unwrap_or_default()
        .into_iter()
        .filter(|i| {
            i.is_oper_up()
                && !i.is_p2p()
                && (!i.is_loopback() || with_loopback)
                && (with_apple_p2p || !is_apple_p2p_by_name(&i.name))
        })
        .collect()
}

/// Checks if the interface name indicates it's an Apple peer-to-peer interface,
/// which should be ignored by default.
fn is_apple_p2p_by_name(name: &str) -> bool {
    let p2p_prefixes = ["awdl", "llw"];
    p2p_prefixes.iter().any(|prefix| name.starts_with(prefix))
}

/// Send an outgoing mDNS query or response, and returns the packet bytes.
/// Returns empty vec if no valid interface address is found.
fn send_dns_outgoing(
    out: &DnsOutgoing,
    my_intf: &MyIntf,
    sock: &PktInfoUdpSocket,
    port: u16,
    source: Option<&IfAddr>,
    unicast_dest: Option<SocketAddr>,
) -> MyResult<Vec<Vec<u8>>> {
    let if_name = &my_intf.name;

    let if_addr = match source {
        Some(addr) => addr,
        None => {
            if sock.domain() == Domain::IPV4 {
                match my_intf.next_ifaddr_v4() {
                    Some(addr) => addr,
                    None => return Ok(vec![]),
                }
            } else {
                match my_intf.next_ifaddr_v6() {
                    Some(addr) => addr,
                    None => return Ok(vec![]),
                }
            }
        }
    };

    send_dns_outgoing_impl(
        out,
        if_name,
        my_intf.index,
        if_addr,
        sock,
        port,
        unicast_dest,
    )
}

/// Send an outgoing mDNS query or response, and returns the packet bytes.
fn send_dns_outgoing_impl(
    out: &DnsOutgoing,
    if_name: &str,
    if_index: u32,
    if_addr: &IfAddr,
    sock: &PktInfoUdpSocket,
    port: u16,
    unicast_dest: Option<SocketAddr>,
) -> MyResult<Vec<Vec<u8>>> {
    let qtype = if out.is_query() {
        "query"
    } else {
        if out.answers_count() == 0 && out.additionals().is_empty() {
            return Ok(vec![]); // no need to send empty response
        }
        "response"
    };
    trace!(
        "send {}: {} questions {} answers {} authorities {} additional",
        qtype,
        out.questions().len(),
        out.answers_count(),
        out.authorities().len(),
        out.additionals().len()
    );

    match if_addr.ip() {
        IpAddr::V4(ipv4) => {
            if let Err(e) = sock.set_multicast_if_v4(&ipv4) {
                debug!(
                    "send_dns_outgoing: failed to set multicast interface for IPv4 {}: {}",
                    ipv4, e
                );
                // cannot send without a valid interface
                if e.kind() == std::io::ErrorKind::AddrNotAvailable {
                    let intf_addr = Interface {
                        name: if_name.to_string(),
                        addr: if_addr.clone(),
                        index: Some(if_index),
                        oper_status: if_addrs::IfOperStatus::Down,
                        is_p2p: false,
                        #[cfg(windows)]
                        adapter_name: String::new(),
                    };
                    return Err(InternalError::IntfAddrInvalid(intf_addr));
                }
                return Ok(vec![]); // non-fatal other failure
            }
        }
        IpAddr::V6(ipv6) => {
            if let Err(e) = sock.set_multicast_if_v6(if_index) {
                debug!(
                    "send_dns_outgoing: failed to set multicast interface for IPv6 {}: {}",
                    ipv6, e
                );
                // cannot send without a valid interface
                if e.kind() == std::io::ErrorKind::AddrNotAvailable {
                    let intf_addr = Interface {
                        name: if_name.to_string(),
                        addr: if_addr.clone(),
                        index: Some(if_index),
                        oper_status: if_addrs::IfOperStatus::Down,
                        is_p2p: false,
                        #[cfg(windows)]
                        adapter_name: String::new(),
                    };
                    return Err(InternalError::IntfAddrInvalid(intf_addr));
                }
                return Ok(vec![]); // non-fatal other failure
            }
        }
    }

    let packet_list = out.to_data_on_wire();
    for packet in packet_list.iter() {
        match unicast_dest {
            Some(dest) => unicast_on_intf(packet, if_name, dest, sock),
            None => multicast_on_intf(packet, if_name, if_index, if_addr, sock, port),
        }
    }
    Ok(packet_list)
}

/// Sends a unicast packet directly to `dest` (used for RFC 6762 §6.7
/// legacy unicast responses).
fn unicast_on_intf(packet: &[u8], if_name: &str, dest: SocketAddr, socket: &PktInfoUdpSocket) {
    if packet.len() > MAX_MSG_ABSOLUTE {
        debug!("Drop over-sized packet ({})", packet.len());
        return;
    }

    let sock_addr = dest.into();
    match socket.send_to(packet, &sock_addr) {
        Ok(sz) => trace!(
            "sent unicast {} bytes on interface {} to {}",
            sz,
            if_name,
            dest
        ),
        Err(e) => trace!(
            "Failed to send unicast to {} via {:?}: {}",
            dest,
            &if_name,
            e
        ),
    }
}

/// Sends a multicast packet, and returns the packet bytes.
fn multicast_on_intf(
    packet: &[u8],
    if_name: &str,
    if_index: u32,
    if_addr: &IfAddr,
    socket: &PktInfoUdpSocket,
    port: u16,
) {
    if packet.len() > MAX_MSG_ABSOLUTE {
        debug!("Drop over-sized packet ({})", packet.len());
        return;
    }

    let addr: SocketAddr = match if_addr {
        if_addrs::IfAddr::V4(_) => SocketAddrV4::new(GROUP_ADDR_V4, port).into(),
        if_addrs::IfAddr::V6(_) => {
            let mut sock = SocketAddrV6::new(GROUP_ADDR_V6, port, 0, 0);
            sock.set_scope_id(if_index); // Choose iface for multicast
            sock.into()
        }
    };

    // Sends out `packet` to `addr` on the socket.
    let sock_addr = addr.into();
    match socket.send_to(packet, &sock_addr) {
        Ok(sz) => trace!(
            "sent out {} bytes on interface {} (idx {}) addr {}",
            sz,
            if_name,
            if_index,
            if_addr.ip()
        ),
        Err(e) => trace!("Failed to send to {} via {:?}: {}", addr, &if_name, e),
    }
}

/// Returns true if `name` is a valid instance name of format:
/// <instance>.<service_type>.<_udp|_tcp>.local.
/// Note: <instance> could contain '.' as well.
fn valid_instance_name(name: &str) -> bool {
    name.split('.').count() >= 5
}

fn notify_monitors(monitors: &mut Vec<Sender<DaemonEvent>>, event: DaemonEvent) {
    monitors.retain(|sender| {
        if let Err(e) = sender.try_send(event.clone()) {
            debug!("notify_monitors: try_send: {}", &e);
            if matches!(e, TrySendError::Disconnected(_)) {
                return false; // This monitor is dropped.
            }
        }
        true
    });
}

/// Check if all unique records passed "probing", and if yes, create a packet
/// to announce the service.
fn prepare_announce(
    info: &ServiceInfo,
    intf: &MyIntf,
    dns_registry: &mut DnsRegistry,
    is_ipv4: bool,
) -> Option<DnsOutgoing> {
    let intf_addrs = if is_ipv4 {
        info.get_addrs_on_my_intf_v4(intf)
    } else {
        info.get_addrs_on_my_intf_v6(intf)
    };

    if intf_addrs.is_empty() {
        debug!(
            "prepare_announce (ipv4: {is_ipv4}): no valid addrs on interface {}",
            &intf.name
        );
        return None;
    }

    // check if we changed our name due to conflicts.
    let service_fullname = dns_registry.resolve_name(info.get_fullname());

    debug!(
        "prepare to announce service {service_fullname} on {:?}",
        &intf_addrs
    );

    let mut probing_count = 0;
    let mut out = DnsOutgoing::new(FLAGS_QR_RESPONSE | FLAGS_AA);
    let create_time = current_time_millis() + fastrand::u64(0..250);

    out.add_answer_at_time(
        DnsPointer::new(
            info.get_type(),
            RRType::PTR,
            CLASS_IN,
            info.get_other_ttl(),
            service_fullname.to_string(),
        ),
        0,
    );

    if let Some(sub) = info.get_subtype() {
        trace!("Adding subdomain {}", sub);
        out.add_answer_at_time(
            DnsPointer::new(
                sub,
                RRType::PTR,
                CLASS_IN,
                info.get_other_ttl(),
                service_fullname.to_string(),
            ),
            0,
        );
    }

    // SRV records.
    let hostname = dns_registry.resolve_name(info.get_hostname()).to_string();

    let mut srv = DnsSrv::new(
        info.get_fullname(),
        CLASS_IN | CLASS_CACHE_FLUSH,
        info.get_host_ttl(),
        info.get_priority(),
        info.get_weight(),
        info.get_port(),
        hostname,
    );

    if let Some(new_name) = dns_registry.name_changes.get(info.get_fullname()) {
        srv.get_record_mut().set_new_name(new_name.to_string());
    }

    if !info.requires_probe()
        || dns_registry.is_probing_done(&srv, info.get_fullname(), create_time)
    {
        out.add_answer_at_time(srv, 0);
    } else {
        probing_count += 1;
    }

    // TXT records.

    let mut txt = DnsTxt::new(
        info.get_fullname(),
        CLASS_IN | CLASS_CACHE_FLUSH,
        info.get_other_ttl(),
        info.generate_txt(),
    );

    if let Some(new_name) = dns_registry.name_changes.get(info.get_fullname()) {
        txt.get_record_mut().set_new_name(new_name.to_string());
    }

    if !info.requires_probe()
        || dns_registry.is_probing_done(&txt, info.get_fullname(), create_time)
    {
        out.add_answer_at_time(txt, 0);
    } else {
        probing_count += 1;
    }

    // Address records. (A and AAAA)

    let hostname = info.get_hostname();
    for address in intf_addrs {
        let mut dns_addr = DnsAddress::new(
            hostname,
            ip_address_rr_type(&address),
            CLASS_IN | CLASS_CACHE_FLUSH,
            info.get_host_ttl(),
            address,
            intf.into(),
        );

        if let Some(new_name) = dns_registry.name_changes.get(hostname) {
            dns_addr.get_record_mut().set_new_name(new_name.to_string());
        }

        if !info.requires_probe()
            || dns_registry.is_probing_done(&dns_addr, info.get_fullname(), create_time)
        {
            out.add_answer_at_time(dns_addr, 0);
        } else {
            probing_count += 1;
        }
    }

    if probing_count > 0 {
        return None;
    }

    Some(out)
}

/// Send an unsolicited response for owned service via `intf` and `sock`.
/// Returns true if sent out successfully for IPv4 or IPv6.
fn announce_service_on_intf(
    dns_registry: &mut DnsRegistry,
    info: &ServiceInfo,
    intf: &MyIntf,
    sock: &PktInfoUdpSocket,
    port: u16,
) -> MyResult<bool> {
    let is_ipv4 = sock.domain() == Domain::IPV4;
    if let Some(out) = prepare_announce(info, intf, dns_registry, is_ipv4) {
        let _ = send_dns_outgoing(&out, intf, sock, port, None, None)?;
        return Ok(true);
    }

    Ok(false)
}

/// Returns a new name based on the `original` to avoid conflicts.
/// If the name already contains a number in parentheses, increments that number.
///
/// Examples:
/// - `foo.local.` becomes `foo (2).local.`
/// - `foo (2).local.` becomes `foo (3).local.`
/// - `foo (9)` becomes `foo (10)`
fn name_change(original: &str) -> String {
    let mut parts: Vec<_> = original.split('.').collect();
    let Some(first_part) = parts.get_mut(0) else {
        return format!("{original} (2)");
    };

    let mut new_name = format!("{first_part} (2)");

    // check if there is already has `(<num>)` suffix.
    if let Some(paren_pos) = first_part.rfind(" (") {
        // Check if there's a closing parenthesis
        if let Some(end_paren) = first_part[paren_pos..].find(')') {
            let absolute_end_pos = paren_pos + end_paren;
            // Only process if the closing parenthesis is the last character
            if absolute_end_pos == first_part.len() - 1 {
                let num_start = paren_pos + 2; // Skip " ("
                                               // Try to parse the number between parentheses
                if let Ok(number) = first_part[num_start..absolute_end_pos].parse::<u32>() {
                    let base_name = &first_part[..paren_pos];
                    new_name = format!("{} ({})", base_name, number + 1)
                }
            }
        }
    }

    *first_part = &new_name;
    parts.join(".")
}

/// Returns a new name based on the `original` to avoid conflicts.
/// If the name already contains a hyphenated number, increments that number.
///
/// Examples:
/// - `foo.local.` becomes `foo-2.local.`
/// - `foo-2.local.` becomes `foo-3.local.`
/// - `foo` becomes `foo-2`
fn hostname_change(original: &str) -> String {
    let mut parts: Vec<_> = original.split('.').collect();
    let Some(first_part) = parts.get_mut(0) else {
        return format!("{original}-2");
    };

    let mut new_name = format!("{first_part}-2");

    // check if there is already a `-<num>` suffix
    if let Some(hyphen_pos) = first_part.rfind('-') {
        // Try to parse everything after the hyphen as a number
        if let Ok(number) = first_part[hyphen_pos + 1..].parse::<u32>() {
            let base_name = &first_part[..hyphen_pos];
            new_name = format!("{}-{}", base_name, number + 1);
        }
    }

    *first_part = &new_name;
    parts.join(".")
}

/// Check probes in a registry and returns: a probing packet to send out, and a list of probe names
/// that are finished.
fn check_probing(
    dns_registry: &mut DnsRegistry,
    timers: &mut BinaryHeap<Reverse<u64>>,
    now: u64,
) -> (DnsOutgoing, Vec<String>) {
    let mut expired_probes = Vec::new();
    let mut out = DnsOutgoing::new(FLAGS_QR_QUERY);

    for (name, probe) in dns_registry.probing.iter_mut() {
        if now >= probe.next_send {
            if probe.expired(now) {
                // move the record to active
                expired_probes.push(name.clone());
            } else {
                out.add_question(name, RRType::ANY);

                /*
                RFC 6762 section 8.2: https://datatracker.ietf.org/doc/html/rfc6762#section-8.2
                ...
                for tiebreaking to work correctly in all
                cases, the Authority Section must contain *all* the records and
                proposed rdata being probed for uniqueness.
                    */
                for record in probe.records.iter() {
                    out.add_authority(record.clone());
                }

                probe.update_next_send(now);

                // add timer
                timers.push(Reverse(probe.next_send));
            }
        }
    }

    (out, expired_probes)
}

/// Process expired probes on an interface and return a list of services
/// that are waiting for the probe to finish.
///
/// `DnsNameChange` events are sent to the monitors.
fn handle_expired_probes(
    expired_probes: Vec<String>,
    intf_name: &str,
    dns_registry: &mut DnsRegistry,
    monitors: &mut Vec<Sender<DaemonEvent>>,
) -> HashSet<String> {
    let mut waiting_services = HashSet::new();

    for name in expired_probes {
        let Some(probe) = dns_registry.probing.remove(&name) else {
            continue;
        };

        // send notifications about name changes
        for record in probe.records.iter() {
            if let Some(new_name) = record.get_record().get_new_name() {
                dns_registry
                    .name_changes
                    .insert(name.clone(), new_name.to_string());

                let event = DnsNameChange {
                    original: record.get_record().get_original_name().to_string(),
                    new_name: new_name.to_string(),
                    rr_type: record.get_type(),
                    intf_name: intf_name.to_string(),
                };
                debug!("Name change event: {:?}", &event);
                notify_monitors(monitors, DaemonEvent::NameChange(event));
            }
        }

        // move RR from probe to active.
        debug!(
            "probe of '{name}' finished: move {} records to active. ({} waiting services)",
            probe.records.len(),
            probe.waiting_services.len(),
        );

        // Move records to active and plan to wake up services if records are not empty.
        if !probe.records.is_empty() {
            match dns_registry.active.get_mut(&name) {
                Some(records) => {
                    records.extend(probe.records);
                }
                None => {
                    dns_registry.active.insert(name, probe.records);
                }
            }

            waiting_services.extend(probe.waiting_services);
        }
    }

    waiting_services
}

/// Resolves `IfKind::Addr(ip)` to `IndexV4(if_index)` or `IndexV6(if_index)`.
fn resolve_addr_to_index(if_kind: IfKind, interfaces: &[Interface]) -> IfKind {
    if let IfKind::Addr(addr) = &if_kind {
        if let Some(intf) = interfaces.iter().find(|intf| &intf.ip() == addr) {
            let if_index = intf.index.unwrap_or(0);
            return if addr.is_ipv4() {
                IfKind::IndexV4(if_index)
            } else {
                IfKind::IndexV6(if_index)
            };
        }
    }
    if_kind
}

#[cfg(test)]
mod tests {
    use super::{
        _new_socket_bind, check_domain_suffix, check_service_name_length, hostname_change,
        my_ip_interfaces, name_change, send_dns_outgoing_impl, valid_instance_name,
        valid_ip_on_intf, HostnameResolutionEvent, MyIntf, ServiceDaemon, ServiceEvent,
        ServiceInfo, GROUP_ADDR_V4, MDNS_PORT,
    };
    use crate::{
        dns_parser::{
            DnsEntryExt, DnsIncoming, DnsOutgoing, DnsPointer, InterfaceId, RRType, ScopedIp,
            CLASS_IN, FLAGS_AA, FLAGS_QR_QUERY, FLAGS_QR_RESPONSE,
        },
        service_daemon::{add_answer_of_service, check_hostname},
    };
    use if_addrs::{IfAddr, Ifv4Addr};
    use std::{
        collections::HashSet,
        net::{IpAddr, Ipv4Addr, UdpSocket},
        time::{Duration, Instant, SystemTime},
    };
    use test_log::test;

    #[test]
    fn test_response_source_ifaddr_match() {
        // When an interface has multiple IPs on unrelated subnets,
        // handle_query should pick the IfAddr whose subnet contains the querier,
        // and fall back to None if none match.
        let ifaddr_a = IfAddr::V4(Ifv4Addr {
            ip: Ipv4Addr::new(192, 168, 1, 148),
            netmask: Ipv4Addr::new(255, 255, 255, 0),
            broadcast: None,
            prefixlen: 24,
        });
        let ifaddr_b = IfAddr::V4(Ifv4Addr {
            ip: Ipv4Addr::new(10, 238, 0, 51),
            netmask: Ipv4Addr::new(255, 255, 255, 0),
            broadcast: None,
            prefixlen: 24,
        });

        let intf = MyIntf {
            name: "dummy0".to_string(),
            index: 1,
            addrs: HashSet::from([ifaddr_a.clone(), ifaddr_b.clone()]),
        };

        let pick = |querier: IpAddr| -> Option<IfAddr> {
            intf.addrs
                .iter()
                .find(|a| valid_ip_on_intf(&querier, a))
                .cloned()
        };

        assert_eq!(
            pick(IpAddr::V4(Ipv4Addr::new(192, 168, 1, 2))),
            Some(ifaddr_a)
        );
        assert_eq!(
            pick(IpAddr::V4(Ipv4Addr::new(10, 238, 0, 99))),
            Some(ifaddr_b)
        );
        // Querier not on any local subnet: fall back to None.
        assert_eq!(pick(IpAddr::V4(Ipv4Addr::new(172, 16, 0, 1))), None);
    }

    #[test]
    fn test_instance_name() {
        assert!(valid_instance_name("my-laser._printer._tcp.local."));
        assert!(valid_instance_name("my-laser.._printer._tcp.local."));
        assert!(!valid_instance_name("_printer._tcp.local."));
    }

    #[test]
    fn test_legacy_unicast_response() {
        // RFC 6762 §6.7: a query whose UDP source port is not 5353 (a
        // "legacy" / "one-shot" querier, e.g. Android's getaddrinfo) must
        // get its response via unicast, sent back to the querier's source
        // address, with the question echoed and the cache-flush bit cleared.
        //
        // This test sends such a query from an ephemeral port and asserts
        // the response arrives on that same socket. The socket is not joined
        // to the mDNS multicast group, so a multicast-only reply would never
        // reach it — simply receiving the response proves it was unicast.

        let intf_ip = match my_ip_interfaces(false)
            .into_iter()
            .find_map(|intf| match intf.ip() {
                IpAddr::V4(ip) => Some(ip),
                IpAddr::V6(_) => None,
            }) {
            Some(ip) => ip,
            None => {
                println!("No IPv4 interface available; skipping test.");
                return;
            }
        };

        // Register a service with a unique hostname on this host.
        let daemon = ServiceDaemon::new().expect("Failed to create daemon");
        let unique = SystemTime::now()
            .duration_since(SystemTime::UNIX_EPOCH)
            .unwrap()
            .as_micros();
        let hostname = format!("legacy-unicast-test-{unique}.local.");
        let service_info = ServiceInfo::new(
            "_legacy-uni._udp.local.",
            "test_instance",
            &hostname,
            &[IpAddr::V4(intf_ip)] as &[IpAddr],
            5353, // arbitrary; the test only resolves the hostname
            None,
        )
        .expect("invalid service info");
        daemon.register(service_info).expect("register service");

        // A one-shot querier: ephemeral source port, not 5353. Binding to
        // `intf_ip` directs the multicast query out that interface, which is
        // one the daemon is listening on.
        let querier = UdpSocket::bind((intf_ip, 0)).expect("bind querier socket");
        querier
            .set_multicast_loop_v4(true)
            .expect("enable multicast loopback");
        querier
            .set_read_timeout(Some(Duration::from_millis(500)))
            .expect("set read timeout");
        assert_ne!(
            querier.local_addr().unwrap().port(),
            MDNS_PORT,
            "querier must use an ephemeral (non-5353) source port"
        );

        // Build a one-question A-record query for our hostname.
        let mut query = DnsOutgoing::new(FLAGS_QR_QUERY);
        query.add_question(&hostname, RRType::A);
        let query_packet = query
            .to_data_on_wire()
            .pop()
            .expect("query serialized to one packet");

        let if_id = InterfaceId {
            name: "test".to_string(),
            index: 0,
        };

        // The service is announced asynchronously after register(), so retry
        // the query until our answer comes back or the deadline passes.
        let deadline = Instant::now() + Duration::from_secs(8);
        let mut response = None;
        'outer: while Instant::now() < deadline {
            querier
                .send_to(&query_packet, (GROUP_ADDR_V4, MDNS_PORT))
                .expect("send query");

            // Drain whatever has arrived; on read timeout the loop ends and
            // we re-send the query.
            let mut buf = [0u8; 1500];
            while let Ok((len, from)) = querier.recv_from(&mut buf) {
                let Ok(msg) = DnsIncoming::new(buf[..len].to_vec(), if_id.clone()) else {
                    continue;
                };
                if msg.is_response()
                    && msg
                        .answers()
                        .iter()
                        .any(|a| a.get_name().eq_ignore_ascii_case(&hostname))
                {
                    response = Some((msg, from));
                    break 'outer;
                }
            }
        }

        let (msg, from) = response.expect(
            "expected a unicast response to the legacy query; \
             a multicast-only reply would never reach this un-joined socket",
        );

        // The reply came back to our ephemeral socket, from the mDNS port.
        assert_eq!(
            from.port(),
            MDNS_PORT,
            "response should originate from the mDNS port"
        );

        // RFC 6762 §6.7: the original question must be echoed.
        assert!(
            msg.questions()
                .iter()
                .any(|q| q.entry_name().eq_ignore_ascii_case(&hostname)),
            "legacy unicast response must echo the question section"
        );

        // RFC 6762 §6.7 / §10.2: the answer must be the A record we asked
        // for, with the cache-flush bit cleared.
        let answer = msg
            .answers()
            .iter()
            .find(|a| a.get_name().eq_ignore_ascii_case(&hostname))
            .expect("response contains an answer for our hostname");
        assert_eq!(
            answer.get_type(),
            RRType::A,
            "an A query should be answered with an A record"
        );
        assert!(
            !answer.get_cache_flush(),
            "legacy unicast responses must clear the cache-flush bit"
        );

        daemon.shutdown().unwrap();
    }

    #[test]
    fn test_check_service_name_length() {
        let result = check_service_name_length("_tcp", 100);
        assert!(result.is_err());
        if let Err(e) = result {
            println!("{}", e);
        }
    }

    #[test]
    fn test_check_hostname() {
        // valid hostnames
        for hostname in &[
            "my_host.local.",
            &("A".repeat(255 - ".local.".len()) + ".local."),
        ] {
            let result = check_hostname(hostname);
            assert!(result.is_ok());
        }

        // erroneous hostnames
        for hostname in &[
            "my_host.local",
            ".local.",
            &("A".repeat(256 - ".local.".len()) + ".local."),
        ] {
            let result = check_hostname(hostname);
            assert!(result.is_err());
            if let Err(e) = result {
                println!("{}", e);
            }
        }
    }

    #[test]
    fn test_check_domain_suffix() {
        assert!(check_domain_suffix("_missing_dot._tcp.local").is_err());
        assert!(check_domain_suffix("_missing_bar.tcp.local.").is_err());
        assert!(check_domain_suffix("_mis_spell._tpp.local.").is_err());
        assert!(check_domain_suffix("_mis_spell._upp.local.").is_err());
        assert!(check_domain_suffix("_has_dot._tcp.local.").is_ok());
        assert!(check_domain_suffix("_goodname._udp.local.").is_ok());
    }

    #[test]
    fn test_service_with_temporarily_invalidated_ptr() {
        // Create a daemon
        let d = ServiceDaemon::new().expect("Failed to create daemon");

        let service = "_test_inval_ptr._udp.local.";
        let host_name = "my_host_tmp_invalidated_ptr.local.";
        let intfs: Vec<_> = my_ip_interfaces(false);
        let intf_ips: Vec<_> = intfs.iter().map(|intf| intf.ip()).collect();
        let port = 5201;
        let my_service =
            ServiceInfo::new(service, "my_instance", host_name, &intf_ips[..], port, None)
                .expect("invalid service info")
                .enable_addr_auto();
        let result = d.register(my_service.clone());
        assert!(result.is_ok());

        // Browse for a service
        let browse_chan = d.browse(service).unwrap();
        let timeout = Duration::from_secs(2);
        let mut resolved = false;

        while let Ok(event) = browse_chan.recv_timeout(timeout) {
            match event {
                ServiceEvent::ServiceResolved(info) => {
                    resolved = true;
                    println!("Resolved a service of {}", &info.fullname);
                    break;
                }
                e => {
                    println!("Received event {:?}", e);
                }
            }
        }

        assert!(resolved);

        println!("Stopping browse of {}", service);
        // Pause browsing so restarting will cause a new immediate query.
        // Unregistering will not work here, it will invalidate all the records.
        d.stop_browse(service).unwrap();

        // Ensure the search is stopped.
        // Reduces the chance of receiving an answer adding the ptr back to the
        // cache causing the later browse to return directly from the cache.
        // (which invalidates what this test is trying to test for.)
        let mut stopped = false;
        while let Ok(event) = browse_chan.recv_timeout(timeout) {
            match event {
                ServiceEvent::SearchStopped(_) => {
                    stopped = true;
                    println!("Stopped browsing service");
                    break;
                }
                // Other `ServiceResolved` messages may be received
                // here as they come from different interfaces.
                // That's fine for this test.
                e => {
                    println!("Received event {:?}", e);
                }
            }
        }

        assert!(stopped);

        // Invalidate the ptr from the service to the host.
        let invalidate_ptr_packet = DnsPointer::new(
            my_service.get_type(),
            RRType::PTR,
            CLASS_IN,
            0,
            my_service.get_fullname().to_string(),
        );

        let mut packet_buffer = DnsOutgoing::new(FLAGS_QR_RESPONSE | FLAGS_AA);
        packet_buffer.add_additional_answer(invalidate_ptr_packet);

        for intf in intfs {
            let sock = _new_socket_bind(&intf, true).unwrap();
            send_dns_outgoing_impl(
                &packet_buffer,
                &intf.name,
                intf.index.unwrap_or(0),
                &intf.addr,
                &sock.pktinfo,
                MDNS_PORT,
                None,
            )
            .unwrap();
        }

        println!(
            "Sent PTR record invalidation. Starting second browse for {}",
            service
        );

        // Restart the browse to force the sender to re-send the announcements.
        let browse_chan = d.browse(service).unwrap();

        resolved = false;
        while let Ok(event) = browse_chan.recv_timeout(timeout) {
            match event {
                ServiceEvent::ServiceResolved(info) => {
                    resolved = true;
                    println!("Resolved a service of {}", &info.fullname);
                    break;
                }
                e => {
                    println!("Received event {:?}", e);
                }
            }
        }

        assert!(resolved);
        d.shutdown().unwrap();
    }

    #[test]
    fn test_expired_srv() {
        // construct service info
        let service_type = "_expired-srv._udp.local.";
        let instance = "test_instance";
        let host_name = "expired_srv_host.local.";
        let mut my_service = ServiceInfo::new(service_type, instance, host_name, "", 5023, None)
            .unwrap()
            .enable_addr_auto();
        // let fullname = my_service.get_fullname().to_string();

        // set SRV to expire soon.
        let new_ttl = 3; // for testing only.
        my_service._set_host_ttl(new_ttl);

        // register my service
        let mdns_server = ServiceDaemon::new().expect("Failed to create mdns server");
        let result = mdns_server.register(my_service);
        assert!(result.is_ok());

        let mdns_client = ServiceDaemon::new().expect("Failed to create mdns client");
        let browse_chan = mdns_client.browse(service_type).unwrap();
        let timeout = Duration::from_secs(2);
        let mut resolved = false;

        while let Ok(event) = browse_chan.recv_timeout(timeout) {
            if let ServiceEvent::ServiceResolved(info) = event {
                resolved = true;
                println!("Resolved a service of {}", &info.fullname);
                break;
            }
        }

        assert!(resolved);

        // Exit the server so that no more responses.
        mdns_server.shutdown().unwrap();

        // SRV record in the client cache will expire.
        let expire_timeout = Duration::from_secs(new_ttl as u64);
        while let Ok(event) = browse_chan.recv_timeout(expire_timeout) {
            if let ServiceEvent::ServiceRemoved(service_type, full_name) = event {
                println!("Service removed: {}: {}", &service_type, &full_name);
                break;
            }
        }
    }

    #[test]
    fn test_hostname_resolution_address_removed() {
        // Create a mDNS server
        let server = ServiceDaemon::new().expect("Failed to create server");
        let hostname = "addr_remove_host._tcp.local.";
        let service_ip_addr: ScopedIp = my_ip_interfaces(false)
            .iter()
            .find(|iface| iface.ip().is_ipv4())
            .map(|iface| iface.into())
            .unwrap();

        let mut my_service = ServiceInfo::new(
            "_host_res_test._tcp.local.",
            "my_instance",
            hostname,
            service_ip_addr.to_ip_addr(),
            1234,
            None,
        )
        .expect("invalid service info");

        // Set a short TTL for addresses for testing.
        let addr_ttl = 2;
        my_service._set_host_ttl(addr_ttl); // Expire soon

        server.register(my_service).unwrap();

        // Create a mDNS client for resolving the hostname.
        let client = ServiceDaemon::new().expect("Failed to create client");
        let event_receiver = client.resolve_hostname(hostname, None).unwrap();
        let resolved = loop {
            match event_receiver.recv() {
                Ok(HostnameResolutionEvent::AddressesFound(found_hostname, addresses)) => {
                    assert!(found_hostname == hostname);
                    assert!(addresses.contains(&service_ip_addr));
                    println!("address found: {:?}", &addresses);
                    break true;
                }
                Ok(HostnameResolutionEvent::SearchStopped(_)) => break false,
                Ok(_event) => {}
                Err(_) => break false,
            }
        };

        assert!(resolved);

        // Shutdown the server so no more responses / refreshes for addresses.
        server.shutdown().unwrap();

        // Wait till hostname address record expires, with 1 second grace period.
        let timeout = Duration::from_secs(addr_ttl as u64 + 1);
        let removed = loop {
            match event_receiver.recv_timeout(timeout) {
                Ok(HostnameResolutionEvent::AddressesRemoved(removed_host, addresses)) => {
                    assert!(removed_host == hostname);
                    assert!(addresses.contains(&service_ip_addr));

                    println!(
                        "address removed: hostname: {} addresses: {:?}",
                        &hostname, &addresses
                    );
                    break true;
                }
                Ok(_event) => {}
                Err(_) => {
                    break false;
                }
            }
        };

        assert!(removed);

        client.shutdown().unwrap();
    }

    #[test]
    fn test_refresh_ptr() {
        // construct service info
        let service_type = "_refresh-ptr._udp.local.";
        let instance = "test_instance";
        let host_name = "refresh_ptr_host.local.";
        let service_ip_addr = my_ip_interfaces(false)
            .iter()
            .find(|iface| iface.ip().is_ipv4())
            .map(|iface| iface.ip())
            .unwrap();

        let mut my_service = ServiceInfo::new(
            service_type,
            instance,
            host_name,
            service_ip_addr,
            5023,
            None,
        )
        .unwrap();

        let new_ttl = 3; // for testing only.
        my_service._set_other_ttl(new_ttl);

        // register my service
        let mdns_server = ServiceDaemon::new().expect("Failed to create mdns server");
        let result = mdns_server.register(my_service);
        assert!(result.is_ok());

        let mdns_client = ServiceDaemon::new().expect("Failed to create mdns client");
        let browse_chan = mdns_client.browse(service_type).unwrap();
        let timeout = Duration::from_millis(1500); // Give at least 1 second for the service probing.
        let mut resolved = false;

        // resolve the service first.
        while let Ok(event) = browse_chan.recv_timeout(timeout) {
            if let ServiceEvent::ServiceResolved(info) = event {
                resolved = true;
                println!("Resolved a service of {}", &info.fullname);
                break;
            }
        }

        assert!(resolved);

        // wait over 80% of TTL, and refresh PTR should be sent out.
        let timeout = Duration::from_millis(new_ttl as u64 * 1000 * 90 / 100);
        while let Ok(event) = browse_chan.recv_timeout(timeout) {
            println!("event: {:?}", &event);
        }

        // verify refresh counter.
        let metrics_chan = mdns_client.get_metrics().unwrap();
        let metrics = metrics_chan.recv_timeout(timeout).unwrap();
        let ptr_refresh_counter = metrics["cache-refresh-ptr"];
        assert_eq!(ptr_refresh_counter, 1);
        let srvtxt_refresh_counter = metrics["cache-refresh-srv-txt"];
        assert_eq!(srvtxt_refresh_counter, 1);

        // Exit the server so that no more responses.
        mdns_server.shutdown().unwrap();
        mdns_client.shutdown().unwrap();
    }

    #[test]
    fn test_name_change() {
        assert_eq!(name_change("foo.local."), "foo (2).local.");
        assert_eq!(name_change("foo (2).local."), "foo (3).local.");
        assert_eq!(name_change("foo (9).local."), "foo (10).local.");
        assert_eq!(name_change("foo"), "foo (2)");
        assert_eq!(name_change("foo (2)"), "foo (3)");
        assert_eq!(name_change(""), " (2)");

        // Additional edge cases
        assert_eq!(name_change("foo (abc)"), "foo (abc) (2)"); // Invalid number
        assert_eq!(name_change("foo (2"), "foo (2 (2)"); // Missing closing parenthesis
        assert_eq!(name_change("foo (2) extra"), "foo (2) extra (2)"); // Extra text after number
    }

    #[test]
    fn test_hostname_change() {
        assert_eq!(hostname_change("foo.local."), "foo-2.local.");
        assert_eq!(hostname_change("foo"), "foo-2");
        assert_eq!(hostname_change("foo-2.local."), "foo-3.local.");
        assert_eq!(hostname_change("foo-9"), "foo-10");
        assert_eq!(hostname_change("test-42.domain."), "test-43.domain.");
    }

    #[test]
    fn test_add_answer_txt_ttl() {
        // construct a simple service info
        let service_type = "_test_add_answer._udp.local.";
        let instance = "test_instance";
        let host_name = "add_answer_host.local.";
        let service_intf = my_ip_interfaces(false)
            .into_iter()
            .find(|iface| iface.ip().is_ipv4())
            .unwrap();
        let service_ip_addr = service_intf.ip();
        let my_service = ServiceInfo::new(
            service_type,
            instance,
            host_name,
            service_ip_addr,
            5023,
            None,
        )
        .unwrap();

        // construct a DnsOutgoing message
        let mut out = DnsOutgoing::new(FLAGS_QR_RESPONSE | FLAGS_AA);

        // Construct a dummy DnsIncoming message
        let mut dummy_data = out.to_data_on_wire();
        let interface_id = InterfaceId::from(&service_intf);
        let incoming = DnsIncoming::new(dummy_data.pop().unwrap(), interface_id).unwrap();

        // Add an answer of TXT type for the service.
        let if_addrs = vec![service_intf.ip()];
        add_answer_of_service(
            &mut out,
            &incoming,
            instance,
            &my_service,
            RRType::TXT,
            if_addrs,
        );

        // Check if the answer was added correctly
        assert!(
            out.answers_count() > 0,
            "No answers added to the outgoing message"
        );

        // Check if the first answer is of type TXT
        let answer = out._answers().first().unwrap();
        assert_eq!(answer.0.get_type(), RRType::TXT);

        // Check TTL is set properly for the TXT record
        assert_eq!(answer.0.get_record().get_ttl(), my_service.get_other_ttl());
    }

    #[test]
    fn test_interface_flip() {
        // start a server
        let ty_domain = "_intf-flip._udp.local.";
        let host_name = "intf_flip.local.";
        let now = SystemTime::now()
            .duration_since(SystemTime::UNIX_EPOCH)
            .unwrap();
        let instance_name = now.as_micros().to_string(); // Create a unique name.
        let port = 5200;

        // Get a single IPv4 address
        let (ip_addr1, intf_name) = my_ip_interfaces(false)
            .iter()
            .find(|iface| iface.ip().is_ipv4())
            .map(|iface| (iface.ip(), iface.name.clone()))
            .unwrap();

        println!("Using interface {} with IP {}", intf_name, ip_addr1);

        // Register the service.
        let service1 = ServiceInfo::new(ty_domain, &instance_name, host_name, ip_addr1, port, None)
            .expect("valid service info");
        let server1 = ServiceDaemon::new().expect("failed to start server");
        server1
            .register(service1)
            .expect("Failed to register service1");

        // wait for the service announced.
        std::thread::sleep(Duration::from_secs(2));

        // start a client
        let client = ServiceDaemon::new().expect("failed to start client");

        let receiver = client.browse(ty_domain).unwrap();

        let timeout = Duration::from_secs(3);
        let mut got_data = false;

        while let Ok(event) = receiver.recv_timeout(timeout) {
            if let ServiceEvent::ServiceResolved(_) = event {
                println!("Received ServiceResolved event");
                got_data = true;
                break;
            }
        }

        assert!(got_data, "Should receive ServiceResolved event");

        // Set a short IP check interval to detect interface changes quickly.
        client.set_ip_check_interval(1).unwrap();

        // Now shutdown the interface and expect the client to lose the service.
        println!("Shutting down interface {}", &intf_name);
        client.test_down_interface(&intf_name).unwrap();

        let mut got_removed = false;

        while let Ok(event) = receiver.recv_timeout(timeout) {
            if let ServiceEvent::ServiceRemoved(ty_domain, instance) = event {
                got_removed = true;
                println!("removed: {ty_domain} : {instance}");
                break;
            }
        }
        assert!(got_removed, "Should receive ServiceRemoved event");

        println!("Bringing up interface {}", &intf_name);
        client.test_up_interface(&intf_name).unwrap();
        let mut got_data = false;
        while let Ok(event) = receiver.recv_timeout(timeout) {
            if let ServiceEvent::ServiceResolved(resolved) = event {
                got_data = true;
                println!("Received ServiceResolved: {:?}", resolved);
                break;
            }
        }
        assert!(
            got_data,
            "Should receive ServiceResolved event after interface is back up"
        );

        server1.shutdown().unwrap();
        client.shutdown().unwrap();
    }

    #[test]
    fn test_cache_only() {
        // construct service info
        let service_type = "_cache_only._udp.local.";
        let instance = "test_instance";
        let host_name = "cache_only_host.local.";
        let service_ip_addr = my_ip_interfaces(false)
            .iter()
            .find(|iface| iface.ip().is_ipv4())
            .map(|iface| iface.ip())
            .unwrap();

        let mut my_service = ServiceInfo::new(
            service_type,
            instance,
            host_name,
            service_ip_addr,
            5023,
            None,
        )
        .unwrap();

        let new_ttl = 3; // for testing only.
        my_service._set_other_ttl(new_ttl);

        let mdns_client = ServiceDaemon::new().expect("Failed to create mdns client");

        // make a single browse request to record that we are interested in the service.  This ensures that
        // subsequent announcements are cached.
        let browse_chan = mdns_client.browse_cache(service_type).unwrap();
        std::thread::sleep(Duration::from_secs(2));

        // register my service
        let mdns_server = ServiceDaemon::new().expect("Failed to create mdns server");
        let result = mdns_server.register(my_service);
        assert!(result.is_ok());

        let timeout = Duration::from_millis(1500); // Give at least 1 second for the service probing.
        let mut resolved = false;

        // resolve the service.
        while let Ok(event) = browse_chan.recv_timeout(timeout) {
            if let ServiceEvent::ServiceResolved(info) = event {
                resolved = true;
                println!("Resolved a service of {}", &info.get_fullname());
                break;
            }
        }

        assert!(resolved);

        // Exit the server so that no more responses.
        mdns_server.shutdown().unwrap();
        mdns_client.shutdown().unwrap();
    }

    #[test]
    fn test_cache_only_unsolicited() {
        let service_type = "_c_unsolicit._udp.local.";
        let instance = "test_instance";
        let host_name = "c_unsolicit_host.local.";
        let service_ip_addr = my_ip_interfaces(false)
            .iter()
            .find(|iface| iface.ip().is_ipv4())
            .map(|iface| iface.ip())
            .unwrap();

        let my_service = ServiceInfo::new(
            service_type,
            instance,
            host_name,
            service_ip_addr,
            5023,
            None,
        )
        .unwrap();

        // register my service
        let mdns_server = ServiceDaemon::new().expect("Failed to create mdns server");
        let result = mdns_server.register(my_service);
        assert!(result.is_ok());

        let mdns_client = ServiceDaemon::new().expect("Failed to create mdns client");
        mdns_client.accept_unsolicited(true).unwrap();

        // Wait a bit for the service announcements to go out, before calling browse_cache.  This ensures
        // that the announcements are treated as unsolicited
        std::thread::sleep(Duration::from_secs(2));
        let browse_chan = mdns_client.browse_cache(service_type).unwrap();
        let timeout = Duration::from_millis(1500); // Give at least 1 second for the service probing.
        let mut resolved = false;

        // resolve the service.
        while let Ok(event) = browse_chan.recv_timeout(timeout) {
            if let ServiceEvent::ServiceResolved(info) = event {
                resolved = true;
                println!("Resolved a service of {}", &info.get_fullname());
                break;
            }
        }

        assert!(resolved);

        // Exit the server so that no more responses.
        mdns_server.shutdown().unwrap();
        mdns_client.shutdown().unwrap();
    }

    #[test]
    fn test_custom_port_isolation() {
        // This test verifies:
        // 1. Daemons on a custom port can communicate with each other
        // 2. Daemons on different ports are isolated (no cross-talk)

        let service_type = "_custom_port._udp.local.";
        let instance_custom = "custom_port_instance";
        let instance_default = "default_port_instance";
        let host_name = "custom_port_host.local.";

        let service_ip_addr = my_ip_interfaces(false)
            .iter()
            .find(|iface| iface.ip().is_ipv4())
            .map(|iface| iface.ip())
            .expect("Test requires an IPv4 interface");

        // Create service info for custom port (5454)
        let service_custom = ServiceInfo::new(
            service_type,
            instance_custom,
            host_name,
            service_ip_addr,
            8080,
            None,
        )
        .unwrap();

        // Create service info for default port (5353)
        let service_default = ServiceInfo::new(
            service_type,
            instance_default,
            host_name,
            service_ip_addr,
            8081,
            None,
        )
        .unwrap();

        // Create two daemons on custom port 5454
        let custom_port = 5454u16;
        let server_custom =
            ServiceDaemon::new_with_port(custom_port).expect("Failed to create custom port server");
        let client_custom =
            ServiceDaemon::new_with_port(custom_port).expect("Failed to create custom port client");

        // Create daemon on default port (5353)
        let server_default = ServiceDaemon::new().expect("Failed to create default port server");

        // Register service on custom port
        server_custom
            .register(service_custom.clone())
            .expect("Failed to register custom port service");

        // Register service on default port
        server_default
            .register(service_default.clone())
            .expect("Failed to register default port service");

        // Browse from custom port client
        let browse_custom = client_custom
            .browse(service_type)
            .expect("Failed to browse on custom port");

        let timeout = Duration::from_secs(3);
        let mut found_custom = false;
        let mut found_default_on_custom = false;

        // Custom port client should find the custom port service
        while let Ok(event) = browse_custom.recv_timeout(timeout) {
            if let ServiceEvent::ServiceResolved(info) = event {
                println!(
                    "Custom port client resolved: {} on port {}",
                    info.get_fullname(),
                    info.get_port()
                );
                if info.get_fullname().starts_with(instance_custom) {
                    found_custom = true;
                    assert_eq!(info.get_port(), 8080);
                }
                if info.get_fullname().starts_with(instance_default) {
                    found_default_on_custom = true;
                }
            }
        }

        assert!(
            found_custom,
            "Custom port client should find service on custom port"
        );
        assert!(
            !found_default_on_custom,
            "Custom port client should NOT find service on default port"
        );

        // Now verify the default port daemon can find its own services
        // but not the custom port services
        let client_default = ServiceDaemon::new().expect("Failed to create default port client");
        let browse_default = client_default
            .browse(service_type)
            .expect("Failed to browse on default port");

        let mut found_default = false;
        let mut found_custom_on_default = false;

        while let Ok(event) = browse_default.recv_timeout(timeout) {
            if let ServiceEvent::ServiceResolved(info) = event {
                println!(
                    "Default port client resolved: {} on port {}",
                    info.get_fullname(),
                    info.get_port()
                );
                if info.get_fullname().starts_with(instance_default) {
                    found_default = true;
                    assert_eq!(info.get_port(), 8081);
                }
                if info.get_fullname().starts_with(instance_custom) {
                    found_custom_on_default = true;
                }
            }
        }

        assert!(
            found_default,
            "Default port client should find service on default port"
        );
        assert!(
            !found_custom_on_default,
            "Default port client should NOT find service on custom port"
        );

        // Cleanup
        server_custom.shutdown().unwrap();
        client_custom.shutdown().unwrap();
        server_default.shutdown().unwrap();
        client_default.shutdown().unwrap();
    }
}