rightkit-http 0.3.0

//! Outbound address policy: address classification and the SSRF guard behind every
//! [`crate::Client`] and [`crate::AsyncClient`] request.
//!
//! The classification mirrors `rightkit-browser`'s `policy.rs` (`classify_ip`,
//! `AddressClass`) with the same semantics, so a browser page and an HTTP call reach the
//! same verdict for the same address. A test cross-checks the two tables.
//!
//! How the guard works:
//!
//! 1. The host of the request URL (and of every redirect hop) is resolved once, by this
//!    crate's resolver. Each resolved address is classified. One denied address denies the
//!    whole host.
//! 2. Only the vetted addresses are handed to the connector, so the socket opens to an
//!    address that was checked. There is no second lookup between check and connect.
//! 3. Names that resolve to no address are denied.
//!
//! Default (public-web) policy denies loopback, RFC 1918, link-local (including
//! `169.254.169.254`), CGNAT `100.64/10`, multicast, reserved, unspecified and broadcast
//! addresses, IPv6 ULA `fc00::/7` and link-local `fe80::/10`, and IPv6 forms that embed an
//! IPv4 address (mapped, compatible, NAT64 `64:ff9b::/96`, 6to4 `2002::/16`) by the class of
//! the embedded IPv4 address. Link-local and cloud-metadata destinations cannot be allowed
//! by any hook.

use std::error::Error;
use std::fmt;
use std::io;
use std::net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr, ToSocketAddrs};
use std::str::FromStr;
use std::sync::mpsc;
use std::sync::Arc;
use std::time::Duration;

use crate::error::{HttpError, Phase};

/// Address class of a destination. Everything except `Public` is denied by default.
#[derive(Clone, Copy, Debug, Eq, PartialEq, Hash)]
pub enum AddressClass {
    Public,
    Loopback,
    /// `169.254/16`, `fe80::/10`; includes the cloud metadata endpoint.
    LinkLocal,
    /// RFC 1918, CGNAT `100.64/10`, ULA `fc00::/7`, site-local `fec0::/10`.
    Private,
    /// IETF protocol assignments, documentation, benchmarking, `240/4`, broadcast,
    /// `2001:db8::/32`, Teredo.
    Reserved,
    Unspecified,
    Multicast,
}

/// Classify one address. IPv6 forms that embed an IPv4 address take the embedded class.
pub fn classify_ip(ip: IpAddr) -> AddressClass {
    match ip {
        IpAddr::V4(v4) => classify_v4(v4),
        IpAddr::V6(v6) => classify_v6(v6),
    }
}

fn classify_v4(ip: Ipv4Addr) -> AddressClass {
    let [a, b, c, _] = ip.octets();
    if a == 127 {
        AddressClass::Loopback
    } else if a == 0 {
        AddressClass::Unspecified
    } else if a == 169 && b == 254 {
        AddressClass::LinkLocal
    } else if a == 10
        || (a == 172 && (16..=31).contains(&b))
        || (a == 192 && b == 168)
        || (a == 100 && (64..=127).contains(&b))
    {
        AddressClass::Private
    } else if (224..=239).contains(&a) {
        AddressClass::Multicast
    } else if a >= 240
        || (a == 192 && b == 0 && c == 0)
        || (a == 192 && b == 0 && c == 2)
        || (a == 198 && (b == 18 || b == 19))
        || (a == 198 && b == 51 && c == 100)
        || (a == 203 && b == 0 && c == 113)
    {
        AddressClass::Reserved
    } else {
        AddressClass::Public
    }
}

fn v4_from_segments(hi: u16, lo: u16) -> Ipv4Addr {
    Ipv4Addr::new((hi >> 8) as u8, hi as u8, (lo >> 8) as u8, lo as u8)
}

fn v4_from_octets(o: &[u8; 16], idx: [usize; 4]) -> Ipv4Addr {
    Ipv4Addr::new(o[idx[0]], o[idx[1]], o[idx[2]], o[idx[3]])
}

/// Every IPv4 address an IPv6 address can embed: IPv4-mapped `::ffff:a.b.c.d`, the
/// deprecated IPv4-compatible `::a.b.c.d`, NAT64 `64:ff9b::/96`, 6to4 `2002::/16`, and the
/// local-use NAT64 space `64:ff9b:1::/48` (RFC 8215). The local-use prefix may be deployed
/// at several lengths (RFC 6052 section 2.2), so each of the /48, /56, /64 and /96 layouts
/// is returned. `::`, `::1` and non-embedding addresses return nothing.
pub fn embedded_v4s(ip: IpAddr) -> Vec<Ipv4Addr> {
    let IpAddr::V6(v6) = ip else {
        return Vec::new();
    };
    if v6.is_loopback() || v6.is_unspecified() {
        return Vec::new();
    }
    let s = v6.segments();
    if let Some(v4) = v6.to_ipv4_mapped() {
        return vec![v4];
    }
    if s[..6] == [0, 0, 0, 0, 0, 0] || s[..6] == [0x64, 0xff9b, 0, 0, 0, 0] {
        return vec![v4_from_segments(s[6], s[7])];
    }
    if s[0] == 0x2002 {
        return vec![v4_from_segments(s[1], s[2])];
    }
    if s[0] == 0x64 && s[1] == 0xff9b && s[2] == 1 {
        let o = v6.octets();
        return vec![
            v4_from_octets(&o, [12, 13, 14, 15]),
            v4_from_octets(&o, [9, 10, 11, 12]),
            v4_from_octets(&o, [7, 9, 10, 11]),
            v4_from_octets(&o, [6, 7, 9, 10]),
        ];
    }
    Vec::new()
}

/// The IPv4 address embedded in an IPv6 address (see [`embedded_v4s`]): the first
/// non-public one when several layouts are possible, otherwise the first.
pub fn embedded_v4(ip: IpAddr) -> Option<Ipv4Addr> {
    let all = embedded_v4s(ip);
    all.iter()
        .copied()
        .find(|v4| classify_v4(*v4) != AddressClass::Public)
        .or_else(|| all.first().copied())
}

fn classify_v6(v6: Ipv6Addr) -> AddressClass {
    let s = v6.segments();
    if v6.is_loopback() {
        return AddressClass::Loopback;
    }
    if v6.is_unspecified() {
        return AddressClass::Unspecified;
    }
    let embedded = embedded_v4s(IpAddr::V6(v6));
    if !embedded.is_empty() {
        return embedded
            .into_iter()
            .map(classify_v4)
            .find(|c| *c != AddressClass::Public)
            .unwrap_or(AddressClass::Public);
    }
    if v6.is_multicast() {
        AddressClass::Multicast
    } else if s[0] & 0xffc0 == 0xfe80 {
        AddressClass::LinkLocal
    } else if s[0] & 0xfe00 == 0xfc00 || s[0] & 0xffc0 == 0xfec0 {
        AddressClass::Private
    } else if (s[0] == 0x2001 && s[1] == 0x0db8) || (s[0] == 0x2001 && s[1] == 0) {
        AddressClass::Reserved
    } else {
        AddressClass::Public
    }
}

/// Addresses no hook may allow: link-local (which holds `169.254.169.254`), AWS's IPv6
/// infrastructure block `fd00:ec2::/32` (metadata `fd00:ec2::254`, DNS `fd00:ec2::253`),
/// and the metadata addresses outside link-local: Alibaba `100.100.100.200`, Azure's
/// wire server `168.63.129.16`, and Oracle/other `192.0.0.192`. IPv6 forms embedding one
/// of these are covered too. Returns why the address is blocked.
fn hard_block(ip: IpAddr) -> Option<&'static str> {
    fn one(ip: IpAddr) -> Option<&'static str> {
        if classify_ip(ip) == AddressClass::LinkLocal {
            return Some("is link-local (cloud metadata range)");
        }
        match ip {
            IpAddr::V4(v4)
                if [[100, 100, 100, 200], [168, 63, 129, 16], [192, 0, 0, 192]]
                    .contains(&v4.octets()) =>
            {
                Some("is a cloud metadata address")
            }
            IpAddr::V6(v6) if v6.segments()[0] == 0xfd00 && v6.segments()[1] == 0x0ec2 => {
                Some("is in the AWS IPv6 metadata block fd00:ec2::/32")
            }
            _ => None,
        }
    }
    one(ip).or_else(|| {
        embedded_v4s(ip)
            .into_iter()
            .find_map(|v4| one(IpAddr::V4(v4)))
    })
}

/// How a host string spelled an IP address.
#[derive(Clone, Copy, Debug, Eq, PartialEq, Hash)]
pub enum HostIpKind {
    /// Four decimal parts without leading zeros: `1.1.1.1`.
    Ipv4DottedQuad,
    /// Any other IPv4 spelling `inet_aton` accepts: `127.1`, `0177.0.0.1`, `0x7f.0.0.1`,
    /// `2130706433`, `10.1.2`, and the dotted quad with a trailing dot.
    Ipv4Legacy,
    /// An IPv6 literal, with or without brackets, including IPv4-mapped forms.
    Ipv6,
}

/// A host string that is an IP address, as parsed by [`parse_host_ip`].
#[derive(Clone, Copy, Debug, Eq, PartialEq, Hash)]
pub struct ParsedHostIp {
    pub addr: IpAddr,
    pub kind: HostIpKind,
    /// True for a dotted quad or an IPv6 literal, which every parser reads the same way.
    /// False for [`HostIpKind::Ipv4Legacy`] spellings, which downstream parsers disagree on.
    pub is_canonical: bool,
}

/// Parse a host string as an IP address the way `inet_aton` and system resolvers do, and say
/// how it was spelled. The input is a host as it appears in a URL: ASCII case does not
/// matter, IPv6 may be bracketed, and one trailing dot is allowed (and makes an IPv4
/// spelling non-canonical).
///
/// Accepted, with the result:
///
/// | Input | `addr` | `kind` | `is_canonical` |
/// |---|---|---|---|
/// | `1.1.1.1` | 1.1.1.1 | `Ipv4DottedQuad` | true |
/// | `127.1` | 127.0.0.1 | `Ipv4Legacy` | false |
/// | `10.1.2` | 10.1.0.2 | `Ipv4Legacy` | false |
/// | `0177.0.0.1` (octal parts) | 127.0.0.1 | `Ipv4Legacy` | false |
/// | `0x7f.0.0.1`, `0x7f000001` (hex) | 127.0.0.1 | `Ipv4Legacy` | false |
/// | `2130706433` (one 32-bit part) | 127.0.0.1 | `Ipv4Legacy` | false |
/// | `1.1.1.1.` (trailing dot) | 1.1.1.1 | `Ipv4Legacy` | false |
/// | `::1`, `[::1]`, `::ffff:127.0.0.1` | as written | `Ipv6` | true |
///
/// `inet_aton` rules: one to four parts; each part is decimal, octal (a leading `0`) or hex
/// (`0x`); every part but the last is at most 255; the last part fills the remaining bytes
/// (so `a.b.c` has a 16-bit last part and `a.b` a 24-bit one). Returns `None` for hostnames,
/// for more than four parts, for out-of-range parts, for a digit outside the part's radix
/// (`08.1`), and for empty parts.
pub fn parse_host_ip(host: &str) -> Option<ParsedHostIp> {
    let trimmed = host.trim_matches(['[', ']']);
    if let Ok(ip) = trimmed.parse::<IpAddr>() {
        let (kind, is_canonical) = match ip {
            IpAddr::V4(_) => (HostIpKind::Ipv4DottedQuad, true),
            IpAddr::V6(_) => (HostIpKind::Ipv6, true),
        };
        return Some(ParsedHostIp {
            addr: ip,
            kind,
            is_canonical,
        });
    }
    let (body, trailing_dot) = match trimmed.strip_suffix('.') {
        Some(b) => (b, true),
        None => (trimmed, false),
    };
    let v4 = parse_legacy_ipv4(body)?;
    let canonical = !trailing_dot && body.eq_ignore_ascii_case(&v4.to_string());
    Some(ParsedHostIp {
        addr: IpAddr::V4(v4),
        kind: if canonical {
            HostIpKind::Ipv4DottedQuad
        } else {
            HostIpKind::Ipv4Legacy
        },
        is_canonical: canonical,
    })
}

/// Parse an IPv4 address with the `inet_aton` rules described at [`parse_host_ip`]. This
/// accepts every spelling, canonical or not, and does not accept a trailing dot, brackets
/// or IPv6; use [`parse_host_ip`] when the spelling matters.
pub fn parse_legacy_ipv4(host: &str) -> Option<Ipv4Addr> {
    let parts: Vec<&str> = host.split('.').collect();
    if parts.is_empty() || parts.len() > 4 {
        return None;
    }
    let mut values = Vec::with_capacity(parts.len());
    for part in &parts {
        let (digits, radix) =
            if let Some(hex) = part.strip_prefix("0x").or_else(|| part.strip_prefix("0X")) {
                (hex, 16)
            } else if part.len() > 1 && part.starts_with('0') {
                (&part[1..], 8)
            } else {
                (*part, 10)
            };
        if digits.is_empty() || !digits.chars().all(|c| c.is_digit(radix)) {
            return None;
        }
        values.push(u64::from_str_radix(digits, radix).ok()?);
    }
    let (last, init) = values.split_last()?;
    if init.iter().any(|v| *v > 255) {
        return None;
    }
    let tail_bytes = 4 - init.len();
    if *last >= 1u64 << (8 * tail_bytes) {
        return None;
    }
    let mut addr: u32 = 0;
    for v in init {
        addr = (addr << 8) | *v as u32;
    }
    addr = addr.checked_shl(8 * tail_bytes as u32).unwrap_or(0) | *last as u32;
    Some(Ipv4Addr::from(addr))
}

/// Hostnames that always mean "cloud instance metadata".
fn is_metadata_host(host: &str) -> bool {
    matches!(
        host,
        "metadata" | "metadata.google.internal" | "metadata.goog" | "instance-data"
    ) || host.ends_with(".metadata.google.internal")
}

/// Lowercase, strip IPv6 brackets and one trailing dot.
pub(crate) fn normalize_host(host: &str) -> String {
    let h = host.trim_matches(['[', ']']).to_ascii_lowercase();
    h.strip_suffix('.').map(str::to_string).unwrap_or(h)
}

/// A CIDR range such as `10.0.0.0/8` or `fd00::/8`.
#[derive(Clone, Copy, Debug, Eq, PartialEq, Hash)]
pub struct IpNet {
    addr: IpAddr,
    prefix: u8,
}

/// `IpNet` text or prefix length was invalid.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct ParseIpNetError(pub String);

impl fmt::Display for ParseIpNetError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "invalid CIDR range: {}", self.0)
    }
}
impl Error for ParseIpNetError {}

impl IpNet {
    /// `prefix` is bits of the network part: at most 32 for IPv4, 128 for IPv6.
    pub fn new(addr: IpAddr, prefix: u8) -> Result<Self, ParseIpNetError> {
        let max = if addr.is_ipv4() { 32 } else { 128 };
        if prefix > max {
            return Err(ParseIpNetError(format!("prefix {prefix} exceeds {max}")));
        }
        Ok(Self { addr, prefix })
    }

    pub fn addr(&self) -> IpAddr {
        self.addr
    }

    pub fn prefix(&self) -> u8 {
        self.prefix
    }

    /// True when `ip` is in the range. Families must match; use [`embedded_v4`] first to
    /// test an IPv6 form against an IPv4 range.
    pub fn contains(&self, ip: &IpAddr) -> bool {
        match (self.addr, ip) {
            (IpAddr::V4(net), IpAddr::V4(ip)) => {
                let mask = if self.prefix == 0 {
                    0
                } else {
                    u32::MAX << (32 - u32::from(self.prefix))
                };
                u32::from(net) & mask == u32::from(*ip) & mask
            }
            (IpAddr::V6(net), IpAddr::V6(ip)) => {
                let mask = if self.prefix == 0 {
                    0
                } else {
                    u128::MAX << (128 - u32::from(self.prefix))
                };
                u128::from(net) & mask == u128::from(*ip) & mask
            }
            _ => false,
        }
    }
}

impl FromStr for IpNet {
    type Err = ParseIpNetError;

    /// `a.b.c.d/len`, `x::/len`, or a bare address (a single-host range).
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let (addr, prefix) = match s.split_once('/') {
            Some((a, p)) => (a, Some(p)),
            None => (s, None),
        };
        let addr: IpAddr = addr
            .trim()
            .parse()
            .map_err(|_| ParseIpNetError(s.to_string()))?;
        let prefix = match prefix {
            Some(p) => p
                .trim()
                .parse::<u8>()
                .map_err(|_| ParseIpNetError(s.to_string()))?,
            None => {
                if addr.is_ipv4() {
                    32
                } else {
                    128
                }
            }
        };
        Self::new(addr, prefix)
    }
}

impl fmt::Display for IpNet {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}/{}", self.addr, self.prefix)
    }
}

/// Why an address or host was refused. Converts to [`crate::HttpError::AddressDenied`].
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct AddressDenial {
    pub host: String,
    /// The offending address; `None` when the denial is about the name or scheme.
    pub addr: Option<IpAddr>,
    pub class: Option<AddressClass>,
    pub reason: String,
}

impl fmt::Display for AddressDenial {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(
            f,
            "address denied for host '{}': {}",
            self.host, self.reason
        )
    }
}
impl Error for AddressDenial {}

type DenyHook = Arc<dyn Fn(&IpAddr) -> Option<String> + Send + Sync>;
type AllowHook = Arc<dyn Fn(&IpAddr) -> bool + Send + Sync>;
type ResolverHook = Arc<dyn Fn(&str) -> Vec<IpAddr> + Send + Sync>;

#[derive(Clone)]
enum Mode {
    PublicWeb,
    FirstParty(Vec<String>),
}

#[derive(Clone)]
struct AllowRule {
    hosts: Vec<String>,
    pred: AllowHook,
}

/// Outbound address policy carried by [`crate::ClientConfig::address_policy`].
///
/// The default is public-web policy: only public addresses are reachable. Callers add
/// extra denials with [`AddressPolicy::deny_ranges`] and [`AddressPolicy::deny_if`], and
/// open specific non-public destinations with [`AddressPolicy::allow_if`]. Custom denials
/// always win over allowances.
#[derive(Clone)]
pub struct AddressPolicy {
    mode: Mode,
    deny_ranges: Vec<IpNet>,
    deny_if: Vec<DenyHook>,
    allow: Vec<AllowRule>,
    /// First-party hosts allowed to resolve to non-public addresses.
    private_ok: Vec<String>,
    /// Follow an HTTPS to HTTP redirect (refused by default).
    allow_downgrade: bool,
    /// Accept non-canonical IPv4 host spellings (refused by default).
    allow_legacy_ipv4: bool,
    resolver: Option<ResolverHook>,
}

impl Default for AddressPolicy {
    fn default() -> Self {
        Self::public_web()
    }
}

impl fmt::Debug for AddressPolicy {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut d = f.debug_struct("AddressPolicy");
        match &self.mode {
            Mode::PublicWeb => d.field("mode", &"public-web"),
            Mode::FirstParty(hosts) => d.field("mode", &format_args!("first-party {hosts:?}")),
        };
        d.field("deny_ranges", &self.deny_ranges)
            .field("deny_if", &self.deny_if.len())
            .field(
                "allow_if_hosts",
                &self
                    .allow
                    .iter()
                    .flat_map(|r| r.hosts.iter())
                    .collect::<Vec<_>>(),
            )
            .field("private_resolution_hosts", &self.private_ok)
            .field("allow_insecure_redirects", &self.allow_downgrade)
            .field("allow_legacy_ipv4", &self.allow_legacy_ipv4)
            .field("custom_resolver", &self.resolver.is_some())
            .finish()
    }
}

impl AddressPolicy {
    /// Public-web policy: only public addresses are reachable. This is the default.
    pub fn public_web() -> Self {
        Self {
            mode: Mode::PublicWeb,
            deny_ranges: Vec::new(),
            deny_if: Vec::new(),
            allow: Vec::new(),
            private_ok: Vec::new(),
            allow_downgrade: false,
            allow_legacy_ipv4: false,
            resolver: None,
        }
    }

    /// First-party policy: only the listed hosts (exact match, ASCII case-insensitive, one
    /// trailing dot ignored), only over HTTPS, on any redirect hop as well. The resolved
    /// addresses of those hosts are classified with the public-web classes too, so a listed
    /// name that resolves to a private address is refused; opt a host out with
    /// [`AddressPolicy::allow_private_resolution`]. Link-local and metadata destinations stay
    /// denied and the custom denials below still apply.
    pub fn first_party(hosts: Vec<String>) -> Self {
        Self {
            mode: Mode::FirstParty(hosts.iter().map(|h| normalize_host(h)).collect()),
            ..Self::public_web()
        }
    }

    /// True for a policy built with [`AddressPolicy::first_party`].
    pub fn is_first_party(&self) -> bool {
        matches!(self.mode, Mode::FirstParty(_))
    }

    /// Accept non-canonical IPv4 host spellings (`127.1`, `0177.0.0.1`, `0x7f.0.0.1`,
    /// `2130706433`, `1.1.1.1.`) and read them as `inet_aton` does. They are refused by
    /// default with [`crate::HttpError::AddressDenied`], because parsers disagree about
    /// them (some read `0177.0.0.1` as decimal 177), and that mismatch is an SSRF bypass.
    /// No legitimate URL spells a host this way. Cloud metadata and the address classes
    /// still apply to an accepted spelling.
    pub fn allow_legacy_ipv4(mut self) -> Self {
        self.allow_legacy_ipv4 = true;
        self
    }

    /// Follow redirects from HTTPS to plain HTTP. Refused by default: the redirect target
    /// (often a signed URL) would travel unencrypted and the answer could not be trusted.
    pub fn allow_insecure_redirects(mut self) -> Self {
        self.allow_downgrade = true;
        self
    }

    /// False for a redirect from HTTPS to plain HTTP, unless
    /// [`AddressPolicy::allow_insecure_redirects`] was set. Other transports use it to apply
    /// the same rule.
    pub fn allows_redirect(&self, from_scheme: &str, to_scheme: &str) -> bool {
        self.allow_downgrade
            || !from_scheme.eq_ignore_ascii_case("https")
            || !to_scheme.eq_ignore_ascii_case("http")
    }

    /// Refuse an HTTPS to HTTP redirect unless [`AddressPolicy::allow_insecure_redirects`].
    pub(crate) fn check_redirect_scheme(
        &self,
        from_scheme: &str,
        to_scheme: &str,
        host: &str,
    ) -> Result<(), AddressDenial> {
        if self.allows_redirect(from_scheme, to_scheme) {
            return Ok(());
        }
        Err(denial(
            &normalize_host(host),
            None,
            None,
            "redirect from HTTPS to HTTP refused",
        ))
    }

    /// Let a first-party `host` resolve to non-public addresses (loopback, RFC 1918, ...),
    /// for example an on-premises server. Exact host match like [`AddressPolicy::first_party`].
    /// Link-local and metadata addresses, `deny_ranges` and `deny_if` still refuse. It has no
    /// effect on hosts that are not first-party.
    pub fn allow_private_resolution(mut self, host: impl Into<String>) -> Self {
        self.private_ok.push(normalize_host(&host.into()));
        self
    }

    /// Deny these ranges in addition to the class rules. An IPv6 address that embeds an
    /// IPv4 address is also tested by its embedded IPv4 address.
    pub fn deny_ranges(mut self, ranges: Vec<IpNet>) -> Self {
        self.deny_ranges.extend(ranges);
        self
    }

    /// Deny addresses for which `f` returns `Some(text)`. The text becomes the `reason` of
    /// [`crate::HttpError::AddressDenied`]. `f` also sees the embedded IPv4 address of an
    /// IPv6 form. Several hooks run in the order added.
    pub fn deny_if<F>(mut self, f: F) -> Self
    where
        F: Fn(&IpAddr) -> Option<String> + Send + Sync + 'static,
    {
        self.deny_if.push(Arc::new(f));
        self
    }

    /// Let the explicitly listed `hosts` (exact match, like [`AddressPolicy::first_party`])
    /// reach addresses for which `pred` is true, even when their class is denied by default.
    /// It does not apply to other hosts, never overrides link-local or metadata denials, and
    /// never overrides `deny_ranges` or `deny_if`.
    pub fn allow_if<F>(mut self, hosts: Vec<String>, pred: F) -> Self
    where
        F: Fn(&IpAddr) -> bool + Send + Sync + 'static,
    {
        self.allow.push(AllowRule {
            hosts: hosts.iter().map(|h| normalize_host(h)).collect(),
            pred: Arc::new(pred),
        });
        self
    }

    /// Replace the system resolver (offline tests, pinned DNS, split-horizon setups). The
    /// hook gets the normalized host name and returns every address for it.
    pub fn with_resolver<F>(mut self, f: F) -> Self
    where
        F: Fn(&str) -> Vec<IpAddr> + Send + Sync + 'static,
    {
        self.resolver = Some(Arc::new(f));
        self
    }

    /// Resolve `host` once and vet every address with exactly the rules the HTTP clients
    /// use: the address classes, `deny_ranges`, `deny_if`, `allow_if`, the first-party host
    /// pin and its private-resolution opt-outs, metadata names, and the rule that a name with
    /// no address is denied. Returns the vetted socket addresses on `port`. The error is the
    /// same [`HttpError::AddressDenied`] the clients return; a failed lookup is a
    /// [`HttpError::Transport`] or [`HttpError::Timeout`] in [`Phase::Connect`].
    ///
    /// Other transports (a WebSocket, a raw TCP or gRPC client) use it to apply the identical
    /// policy, and **must connect to the returned addresses and not look the name up again**.
    /// A transport that cannot pin the connection to these addresses leaves a DNS-rebinding
    /// window between this check and its own lookup.
    ///
    /// The host is treated as reached over HTTPS: a first-party policy checks the host pin
    /// but cannot see the caller's transport, so requiring TLS stays the caller's job. A
    /// system lookup is bounded at ten seconds.
    pub fn resolve_vetted(&self, host: &str, port: u16) -> Result<Vec<SocketAddr>, HttpError> {
        self.admit("https", host)?;
        let host = normalize_host(host);
        let addrs = match self.fixed_addrs(&host) {
            Some(addrs) => addrs,
            None => match self.lookup_blocking(&host, port, Some(Duration::from_secs(10))) {
                Ok(addrs) => addrs,
                Err(e) if e.kind() == io::ErrorKind::TimedOut => {
                    return Err(HttpError::timeout("name lookup", Phase::Connect))
                }
                Err(_) => return Err(HttpError::transport("host not found", Phase::Connect)),
            },
        };
        let vetted = self.vet(&host, addrs)?;
        Ok(vetted
            .into_iter()
            .map(|ip| SocketAddr::new(ip, port))
            .collect())
    }

    /// True when some `allow_if` rule lists `host`.
    fn host_listed(&self, host: &str) -> bool {
        self.allow
            .iter()
            .any(|rule| rule.hosts.iter().any(|h| h == host))
    }

    /// Vet a destination that a proxy will resolve, so its address cannot be checked or
    /// pinned here. IP literals (and `localhost`) are classified like any address. Names
    /// are refused when they are `localhost`, `*.localhost`, `*.local`, `*.internal`, or
    /// have a single label, unless an `allow_if` rule lists the host. First-party hosts are
    /// pinned by [`AddressPolicy::admit`] instead and skip the name rules.
    pub(crate) fn vet_via_proxy(&self, host: &str) -> Result<(), AddressDenial> {
        let host = normalize_host(host);
        if let Some(addrs) = self.fixed_addrs(&host) {
            return self.vet(&host, addrs).map(|_| ());
        }
        if matches!(self.mode, Mode::FirstParty(_)) || self.host_listed(&host) {
            return Ok(());
        }
        let reason = if host.ends_with(".local") {
            Some("mDNS name is refused through a proxy")
        } else if host.ends_with(".internal") {
            Some("internal name is refused through a proxy")
        } else if !host.contains('.') {
            Some("single-label name is refused through a proxy")
        } else {
            None
        };
        match reason {
            Some(r) => Err(denial(
                &host,
                None,
                Some(AddressClass::Private),
                format!("{r} (through a proxy only names and literals can be checked)"),
            )),
            None => Ok(()),
        }
    }

    /// Name- and scheme-level admission, before any lookup: metadata host names, and the
    /// first-party host and HTTPS pin.
    ///
    /// `host` should be the host exactly as the URL spelled it (trailing dot included), so a
    /// non-canonical IPv4 spelling can be refused.
    pub(crate) fn admit(&self, scheme: &str, host: &str) -> Result<(), AddressDenial> {
        if !self.allow_legacy_ipv4 {
            if let Some(parsed) = parse_host_ip(host) {
                if !parsed.is_canonical {
                    return Err(denial(
                        &normalize_host(host),
                        Some(parsed.addr),
                        Some(classify_ip(parsed.addr)),
                        format!(
                            "non-canonical IPv4 spelling is refused (it reads as {})",
                            parsed.addr
                        ),
                    ));
                }
            }
        }
        let host = normalize_host(host);
        if is_metadata_host(&host) {
            return Err(denial(
                &host,
                None,
                Some(AddressClass::LinkLocal),
                "host is a cloud metadata endpoint",
            ));
        }
        if let Mode::FirstParty(hosts) = &self.mode {
            if !scheme.eq_ignore_ascii_case("https") {
                return Err(denial(
                    &host,
                    None,
                    None,
                    "first-party client allows HTTPS only",
                ));
            }
            if !hosts.contains(&host) {
                return Err(denial(
                    &host,
                    None,
                    None,
                    "host is not in the first-party host list",
                ));
            }
        }
        Ok(())
    }

    /// Addresses that need no lookup: IP literals and `localhost` names.
    pub(crate) fn fixed_addrs(&self, host: &str) -> Option<Vec<IpAddr>> {
        let host = normalize_host(host);
        if let Some(parsed) = parse_host_ip(&host) {
            return Some(vec![parsed.addr]);
        }
        if host == "localhost" || host.ends_with(".localhost") {
            return Some(vec![IpAddr::V4(Ipv4Addr::LOCALHOST)]);
        }
        None
    }

    /// Resolve `host` with the configured hook or the system resolver. `limit` bounds the
    /// lookup, hook included. `Ok(empty)` means the name resolved to nothing.
    pub(crate) fn lookup_blocking(
        &self,
        host: &str,
        port: u16,
        limit: Option<Duration>,
    ) -> io::Result<Vec<IpAddr>> {
        let host = normalize_host(host);
        let hook = self.resolver.clone();
        let run = move || -> io::Result<Vec<IpAddr>> {
            match hook {
                Some(hook) => Ok(hook(&host)),
                None => Ok((host, port).to_socket_addrs()?.map(|a| a.ip()).collect()),
            }
        };
        match limit {
            None => run(),
            Some(limit) => {
                let (tx, rx) = mpsc::sync_channel(1);
                std::thread::spawn(move || tx.send(run()).ok());
                match rx.recv_timeout(limit) {
                    Ok(r) => r,
                    Err(_) => Err(io::Error::new(
                        io::ErrorKind::TimedOut,
                        "name lookup timed out",
                    )),
                }
            }
        }
    }

    /// The configured resolver hook, for the async resolver.
    #[cfg(feature = "async")]
    pub(crate) fn resolver_hook(&self) -> Option<ResolverHook> {
        self.resolver.clone()
    }

    /// Classify every resolved address and return them when all pass. One denied address
    /// denies the host. Names with no address are denied.
    pub(crate) fn vet(&self, host: &str, addrs: Vec<IpAddr>) -> Result<Vec<IpAddr>, AddressDenial> {
        self.vet_with(host, addrs)
    }

    fn vet_with(&self, host: &str, addrs: Vec<IpAddr>) -> Result<Vec<IpAddr>, AddressDenial> {
        let host = normalize_host(host);
        if addrs.is_empty() {
            return Err(denial(
                &host,
                None,
                None,
                "host did not resolve to any address",
            ));
        }
        let literal = parse_host_ip(&host).is_some();
        for &ip in &addrs {
            let class = classify_ip(ip);
            if let Some(why) = hard_block(ip) {
                return Err(denial(
                    &host,
                    Some(ip),
                    Some(class),
                    format!("{} {why}", subject(literal, ip)),
                ));
            }
            let forms: Vec<IpAddr> = std::iter::once(ip)
                .chain(embedded_v4s(ip).into_iter().map(IpAddr::V4))
                .collect();
            for form in forms.iter() {
                if let Some(net) = self.deny_ranges.iter().find(|n| n.contains(form)) {
                    return Err(denial(
                        &host,
                        Some(ip),
                        Some(class),
                        format!("{} is in denied range {net}", subject(literal, ip)),
                    ));
                }
                for hook in &self.deny_if {
                    if let Some(text) = hook(form) {
                        return Err(denial(&host, Some(ip), Some(class), text));
                    }
                }
            }
            let private_ok =
                matches!(self.mode, Mode::FirstParty(_)) && self.private_ok.contains(&host);
            if class != AddressClass::Public && !private_ok && !self.allowed(&host, &ip) {
                let reason = if literal {
                    format!("address {ip} is {class:?}")
                } else {
                    format!("host resolves to {class:?} address {ip}")
                };
                return Err(denial(&host, Some(ip), Some(class), reason));
            }
        }
        Ok(addrs)
    }

    fn allowed(&self, host: &str, ip: &IpAddr) -> bool {
        self.allow
            .iter()
            .any(|rule| rule.hosts.iter().any(|h| h == host) && (rule.pred)(ip))
    }
}

fn subject(literal: bool, ip: IpAddr) -> String {
    if literal {
        format!("address {ip}")
    } else {
        format!("resolved address {ip}")
    }
}

fn denial(
    host: &str,
    addr: Option<IpAddr>,
    class: Option<AddressClass>,
    reason: impl Into<String>,
) -> AddressDenial {
    AddressDenial {
        host: host.to_string(),
        addr,
        class,
        reason: reason.into(),
    }
}

/// Find an [`AddressDenial`] anywhere in an error's source chain.
pub(crate) fn denial_in_chain(err: &(dyn Error + 'static)) -> Option<AddressDenial> {
    let mut current: Option<&(dyn Error + 'static)> = Some(err);
    while let Some(e) = current {
        if let Some(d) = e.downcast_ref::<AddressDenial>() {
            return Some(d.clone());
        }
        current = e.source();
    }
    None
}

thread_local! {
    /// True while the current thread is running a request that goes through the proxy.
    static VIA_PROXY: std::cell::Cell<bool> = const { std::cell::Cell::new(false) };
}

/// Marks the current thread's ureq call as routed through the proxy, so the resolver
/// treats the proxy endpoint (host and port) as trusted configuration. Without the mark
/// the same host and port is an ordinary destination and gets the full check, which is the
/// case for `NO_PROXY` hosts.
pub(crate) struct ViaProxy(bool);

impl ViaProxy {
    pub(crate) fn enter(on: bool) -> Self {
        Self(VIA_PROXY.with(|c| c.replace(on)))
    }
}

impl Drop for ViaProxy {
    fn drop(&mut self) {
        VIA_PROXY.with(|c| c.set(self.0));
    }
}

/// `ureq` keeps at most this many addresses per lookup (its `ResolvedSocketAddrs` capacity).
const MAX_RESOLVED_ADDRS: usize = 16;

/// Resolver for `ureq`: admits the URI, resolves it once, vets every address, and returns
/// only the vetted socket addresses. `ureq`'s `TcpConnector` connects to exactly the
/// addresses this returns (`ConnectionDetails::addrs`), so the check and the connection
/// use the same lookup.
#[derive(Debug)]
pub(crate) struct VettingResolver {
    pub(crate) policy: Arc<AddressPolicy>,
    /// Proxy endpoint `(normalized host, port)`: resolved but trusted (configuration, not
    /// input), so no address-class denial and no first-party host pin, but only while the
    /// call is marked [`ViaProxy`].
    pub(crate) proxy: Option<(String, u16)>,
    /// Bound for a name lookup when the request has no resolve or global timeout.
    pub(crate) lookup_limit: Duration,
}

impl ureq::unversioned::resolver::Resolver for VettingResolver {
    fn resolve(
        &self,
        uri: &ureq::http::Uri,
        _config: &ureq::config::Config,
        timeout: ureq::unversioned::transport::NextTimeout,
    ) -> Result<ureq::unversioned::resolver::ResolvedSocketAddrs, ureq::Error> {
        let scheme = uri.scheme_str().unwrap_or("").to_ascii_lowercase();
        let default_port = match scheme.as_str() {
            "http" => 80,
            "https" => 443,
            _ => return Err(ureq::Error::BadUri("unsupported scheme".into())),
        };
        let raw_host = uri
            .host()
            .ok_or_else(|| ureq::Error::BadUri("missing host".into()))?;
        let host = normalize_host(raw_host);
        let port = uri.port_u16().unwrap_or(default_port);
        let is_proxy = VIA_PROXY.with(|c| c.get())
            && self
                .proxy
                .as_ref()
                .is_some_and(|(h, p)| *h == host && *p == port);
        if !is_proxy && !matches!(scheme.as_str(), "http" | "https") {
            return Err(ureq::Error::BadUri("unsupported scheme".into()));
        }
        if !is_proxy {
            self.policy.admit(&scheme, raw_host).map_err(to_ureq)?;
        }
        let addrs = match self.policy.fixed_addrs(&host) {
            Some(addrs) => addrs,
            None => {
                let limit = if timeout.after.is_not_happening() {
                    self.lookup_limit
                } else {
                    (*timeout.after).min(self.lookup_limit)
                };
                match self.policy.lookup_blocking(&host, port, Some(limit)) {
                    Ok(addrs) => addrs,
                    Err(e) if e.kind() == io::ErrorKind::TimedOut => {
                        return Err(ureq::Error::Timeout(timeout.reason))
                    }
                    Err(_) => return Err(ureq::Error::HostNotFound),
                }
            }
        };
        let vetted = if is_proxy {
            if addrs.is_empty() {
                return Err(ureq::Error::HostNotFound);
            }
            addrs
        } else {
            self.policy.vet(&host, addrs).map_err(to_ureq)?
        };
        let mut out = ureq::unversioned::resolver::Resolver::empty(self);
        for ip in vetted.into_iter().take(MAX_RESOLVED_ADDRS) {
            out.push(std::net::SocketAddr::new(ip, port));
        }
        Ok(out)
    }
}

fn to_ureq(d: AddressDenial) -> ureq::Error {
    ureq::Error::Io(io::Error::new(io::ErrorKind::PermissionDenied, d))
}

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

    fn ip(s: &str) -> IpAddr {
        s.parse().unwrap()
    }

    #[test]
    fn ipnet_parses_and_matches() {
        let net: IpNet = "10.0.0.0/8".parse().unwrap();
        assert!(net.contains(&ip("10.255.0.1")));
        assert!(!net.contains(&ip("11.0.0.1")));
        assert!(!net.contains(&ip("::1")));
        let v6: IpNet = "fd00::/8".parse().unwrap();
        assert!(v6.contains(&ip("fd12::1")));
        assert!(!v6.contains(&ip("fe80::1")));
        let host: IpNet = "203.0.113.9".parse().unwrap();
        assert_eq!(host.prefix(), 32);
        assert!(host.contains(&ip("203.0.113.9")) && !host.contains(&ip("203.0.113.8")));
        assert!("10.0.0.0/33".parse::<IpNet>().is_err());
        assert!("nonsense".parse::<IpNet>().is_err());
        let all: IpNet = "0.0.0.0/0".parse().unwrap();
        assert!(all.contains(&ip("8.8.8.8")));
    }

    #[test]
    fn embedded_forms_extract_the_v4() {
        assert_eq!(
            embedded_v4(ip("::ffff:10.1.2.3")),
            Some("10.1.2.3".parse().unwrap())
        );
        assert_eq!(
            embedded_v4(ip("::10.1.2.3")),
            Some("10.1.2.3".parse().unwrap())
        );
        assert_eq!(
            embedded_v4(ip("64:ff9b::a00:1")),
            Some("10.0.0.1".parse().unwrap())
        );
        assert_eq!(
            embedded_v4(ip("2002:0a00:0001::1")),
            Some("10.0.0.1".parse().unwrap())
        );
        assert_eq!(embedded_v4(ip("::1")), None);
        assert_eq!(embedded_v4(ip("::")), None);
        assert_eq!(embedded_v4(ip("2606:4700::1")), None);
        assert_eq!(embedded_v4(ip("1.2.3.4")), None);
    }

    #[test]
    fn host_ip_parsing_reports_the_spelling() {
        let k = |h: &str| parse_host_ip(h).map(|p| (p.addr, p.kind, p.is_canonical));
        let v4 = |s: &str| IpAddr::V4(s.parse().unwrap());
        assert_eq!(
            k("1.1.1.1"),
            Some((v4("1.1.1.1"), HostIpKind::Ipv4DottedQuad, true))
        );
        for (spelling, want) in [
            ("127.1", "127.0.0.1"),
            ("10.1.2", "10.1.0.2"),
            ("0177.0.0.1", "127.0.0.1"),
            ("0x7f.0.0.1", "127.0.0.1"),
            ("0X7F000001", "127.0.0.1"),
            ("2130706433", "127.0.0.1"),
            ("017700000001", "127.0.0.1"),
            ("0251.0376.0251.0376", "169.254.169.254"),
            ("1.1.1.1.", "1.1.1.1"),
            ("0", "0.0.0.0"),
        ] {
            assert_eq!(
                k(spelling),
                Some((v4(want), HostIpKind::Ipv4Legacy, false)),
                "{spelling}"
            );
        }
        for v6 in [
            "::1",
            "[::1]",
            "::ffff:127.0.0.1",
            "[::ffff:7f00:1]",
            "64:ff9b::a00:1",
        ] {
            let p = parse_host_ip(v6).unwrap();
            assert!(p.is_canonical && p.kind == HostIpKind::Ipv6, "{v6}");
        }
        for not_ip in [
            "example.com",
            "1.2.3.4.5",
            "256.1.1.1",
            "1.2.3.256",
            "4294967296",
            "08.1",
            "0x",
            "1..2",
            "",
            ".",
            "1.2.3.",
            "a.b.c.d",
            "127.0.0.1x",
        ] {
            let got = parse_host_ip(not_ip);
            // "1.2.3." has a trailing dot on a valid 3-part spelling: that is a legacy IPv4.
            if not_ip == "1.2.3." {
                assert!(got.is_some());
            } else {
                assert!(got.is_none(), "{not_ip}: {got:?}");
            }
        }
        assert_eq!(
            parse_legacy_ipv4("0x7f.1"),
            Some("127.0.0.1".parse().unwrap())
        );
        assert_eq!(parse_legacy_ipv4("1.1.1.1."), None);
        assert_eq!(parse_legacy_ipv4("::1"), None);
    }

    #[test]
    fn non_canonical_spellings_are_refused_by_default_and_allowed_on_request() {
        let strict = AddressPolicy::default();
        for h in ["0177.0.0.1", "0x7f.1", "2130706433", "127.1", "1.1.1.1."] {
            let d = strict.admit("http", h).unwrap_err();
            assert!(d.reason.contains("non-canonical"), "{h}: {d}");
            assert!(d.addr.is_some(), "{h}");
        }
        for h in [
            "1.1.1.1",
            "::ffff:1.1.1.1",
            "[::1]",
            "example.com",
            "example.com.",
        ] {
            assert!(strict.admit("http", h).is_ok(), "{h}");
        }
        let lax = AddressPolicy::default().allow_legacy_ipv4();
        assert!(lax.admit("http", "127.1").is_ok());
        // The address classes still apply to an accepted spelling.
        assert!(lax.vet("127.1", lax.fixed_addrs("127.1").unwrap()).is_err());
    }

    #[test]
    fn host_normalization() {
        assert_eq!(normalize_host("[::1]"), "::1");
        assert_eq!(normalize_host("Example.COM."), "example.com");
    }

    #[test]
    fn vet_denies_empty_resolution() {
        let d = AddressPolicy::default()
            .vet("nowhere.test", vec![])
            .unwrap_err();
        assert_eq!(d.addr, None);
        assert!(d.reason.contains("did not resolve"), "{d}");
    }
}