arcbox-daemon 0.8.2

//! Registers the daemon's `*.<domain>` names with mDNSResponder.
//!
//! The domain ends in `.local`, which RFC 6762 gives to mDNS, so macOS
//! resolves it through mDNSResponder. The unicast server on 127.0.0.1 still
//! answers through `/etc/resolver/<domain>`, and stays the one that makes a
//! miss fail fast; this mirror adds the same names as host-local Bonjour
//! records so they resolve on a Mac that has no resolver file, as
//! OrbStack's `orb.local` names do. Both answer from the same table.
//!
//! Every Bonjour operation needs Local Network access (TN3179). A `launchd`
//! agent inherits its app's privilege and the user is asked once; a daemon
//! started from a third-party terminal inherits that terminal's. Without it
//! mDNSResponder answers `kDNSServiceErr_PolicyDenied`, the mirror logs the
//! denial once and retries every [`RETRY_DENIED`], and names keep resolving
//! through the resolver file for clients that are allowed to resolve
//! `.local` names at all.

mod dns_sd;
mod mirror;

use std::io;
use std::net::IpAddr;
use std::os::fd::{AsRawFd, RawFd};
use std::sync::Arc;
use std::time::Duration;

use arcbox_core::Runtime;
use arcbox_net::NetworkManager;
use tokio::io::Interest;
use tokio::io::unix::AsyncFd;
use tokio::sync::broadcast::error::RecvError;
use tokio_util::sync::CancellationToken;
use tracing::{debug, info, warn};

use dns_sd::{Connection, RecordHandle};
use mirror::{Event, Mirror, Registrar};

/// How long after a policy denial the mirror asks again. The user may grant
/// Local Network access in System Settings at any time, and mDNSResponder
/// does not revisit a refused registration on its own.
const RETRY_DENIED: Duration = Duration::from_secs(60);

/// Starts the mirror for `domain` when it is an mDNS domain.
///
/// The caller decides ownership: only the daemon that owns the host's
/// resolver for `domain` may announce its names, or the production and a
/// development daemon would both claim `host.arcbox.local`.
pub fn spawn(runtime: &Arc<Runtime>, domain: &str, shutdown: &CancellationToken) {
    if !is_mdns_domain(domain) {
        debug!(domain, "not an mDNS domain; no Bonjour records");
        return;
    }
    let network_manager = Arc::clone(runtime.network_manager());
    let domain = domain.to_owned();
    let shutdown = shutdown.clone();
    drop(tokio::spawn(async move {
        if let Err(error) = run(network_manager, &domain, shutdown).await {
            warn!(%error, domain, "mDNS mirror stopped; names resolve through the resolver file only");
        }
    }));
}

/// RFC 6762: `.local` is the mDNS domain. DNS names compare case-insensitively.
fn is_mdns_domain(domain: &str) -> bool {
    domain
        .rsplit('.')
        .next()
        .is_some_and(|tld| tld.eq_ignore_ascii_case("local"))
}

struct Fd(RawFd);

impl AsRawFd for Fd {
    fn as_raw_fd(&self) -> RawFd {
        self.0
    }
}

impl Registrar for Connection {
    type Handle = RecordHandle;

    fn register(&mut self, fqdn: &str, ip: IpAddr) -> io::Result<RecordHandle> {
        self.register_record(fqdn, ip)
    }

    fn remove(&mut self, handle: RecordHandle) {
        self.remove_record(handle);
    }
}

async fn run(
    network_manager: Arc<NetworkManager>,
    domain: &str,
    shutdown: CancellationToken,
) -> io::Result<()> {
    // Subscribe before the snapshot so no change falls between the two.
    let mut changes = network_manager.subscribe_dns_changes();
    let connection = Connection::open()?;
    let fd = connection.fd();
    let mut mirror = Mirror::new(connection);
    // Declared after the mirror so it is dropped, and deregistered, before
    // the connection closes the socket.
    let socket = AsyncFd::with_interest(Fd(fd), Interest::READABLE)?;
    mirror.resync(network_manager.local_dns_entries());

    let mut announced = false;
    loop {
        // Captured by value: a borrow of the mirror held across the select
        // would make the future require `Connection: Sync`.
        let denied = mirror.is_denied();
        let retry = async move {
            if denied {
                tokio::time::sleep(RETRY_DENIED).await;
            } else {
                std::future::pending::<()>().await;
            }
        };
        tokio::select! {
            () = shutdown.cancelled() => break,
            change = changes.recv() => match change {
                Ok(change) => mirror.apply(change),
                Err(RecvError::Lagged(_)) => mirror.resync(network_manager.local_dns_entries()),
                Err(RecvError::Closed) => break,
            },
            guard = socket.readable() => {
                let mut guard = guard?;
                // Edge-triggered: clear first, then drain, so a reply that
                // lands after the drain raises a new event.
                guard.clear_ready();
                let mut replies = Vec::new();
                while mirror.registrar_mut().has_pending_reply() {
                    replies.extend(mirror.registrar_mut().process()?);
                }
                for reply in replies {
                    match mirror.handle_reply(reply.handle, reply.error) {
                        Some(Event::Confirmed { fqdn }) => {
                            if !announced {
                                announced = true;
                                info!("mDNSResponder serves this host's *.{domain} names");
                            }
                            debug!(fqdn, "mDNS record live");
                        }
                        Some(Event::Denied) => warn!(
                            retry_secs = RETRY_DENIED.as_secs(),
                            "mDNSResponder refused the *.{domain} records: this process has no \
                             Local Network access (System Settings › Privacy & Security › Local \
                             Network); names resolve through /etc/resolver/{domain} meanwhile"
                        ),
                        Some(Event::Recovered) => {
                            info!("mDNSResponder accepts the *.{domain} records again");
                        }
                        Some(Event::Refused { fqdn, error }) => {
                            warn!(fqdn, error, "mDNSResponder refused a record");
                        }
                        None => {}
                    }
                }
            }
            () = retry => mirror.retry_denied(),
        }
    }
    Ok(())
}

#[cfg(test)]
mod tests {
    use super::is_mdns_domain;

    #[test]
    fn only_local_domains_are_announced() {
        assert!(is_mdns_domain("arcbox.local"));
        assert!(is_mdns_domain("worktree.dev.arcbox.local"));
        assert!(is_mdns_domain("ARCBOX.LOCAL"));
        assert!(is_mdns_domain("local"));
        assert!(!is_mdns_domain("arcbox.test"));
        assert!(!is_mdns_domain("notlocal"));
        assert!(!is_mdns_domain("local.example"));
    }
}