fix(server): mark a mapped listener with the IPv4 option

crates/irtt-server/src/socket_options.rs

@@ -10,10 +10,40 @@
 //! there is no raw `setsockopt` or per-packet control message anywhere in the
 //! server.
 
-use std::io;
+use std::{
+    io,
+    net::{IpAddr, SocketAddr},
+};
 
 use socket2::SockRef;
 
+/// Whether this listener's replies are marked with the IPv4 TOS option rather
+/// than the IPv6 Traffic Class one.
+///
+/// The listener's own bound family answers this, not the peer's — one socket,
+/// one answer. The exception is an IPv4-mapped bound address, where the socket
+/// is `AF_INET6` but every packet it emits is IPv4, and the platforms disagree
+/// about it:
+///
+/// - **Linux** binds such a socket to the IPv4 side. `IPV6_TCLASS` does not
+///   reach the packets it sends — their TOS stays zero and a negotiated marking
+///   is silently lost — while `IP_TOS` on that same socket marks them. So a
+///   mapped bind is read as the IPv4 listener it is.
+/// - **macOS** rejects `IP_TOS` on an `AF_INET6` socket with `EINVAL` whatever
+///   it is bound to, and marks nothing through either option. Reading a mapped
+///   bind as IPv4 there would turn an unmarked reply into a *dropped* one, since
+///   an unappliable marking stops the send — worse than the marking simply not
+///   arriving. It keeps the IPv6 option and its existing behavior. (The mapped
+///   *wildcard* never reaches this at all: macOS normalizes that bind to `[::]`.)
+/// - **FreeBSD** refuses a mapped bind under its default `IPV6_V6ONLY`, so the
+///   case does not arise.
+pub(crate) fn marks_with_ipv4_option(addr: SocketAddr) -> bool {
+    match addr.ip() {
+        IpAddr::V4(_) => true,
+        IpAddr::V6(address) => cfg!(target_os = "linux") && address.to_ipv4_mapped().is_some(),
+    }
+}
+
 /// Applies `traffic_class` to a socket bound in the given family.
 ///
 /// `is_ipv4` is the *listener's* family, not the peer's.
@@ -119,3 +149,36 @@ fn unsupported_traffic_class(_traffic_class: u32, option: &'static str) -> io::R
         format!("{option} are unsupported on this target"),
     ))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Which listeners mark with the IPv4 option.
+    ///
+    /// The mapped forms are the point: such a socket is `AF_INET6` but sends
+    /// IPv4, and `IPV6_TCLASS` does not reach those packets. Linux marks them
+    /// through `IP_TOS`; the platforms that would reject that keep the IPv6
+    /// option, because an unappliable marking stops the send and a dropped
+    /// reply is worse than an unmarked one.
+    #[test]
+    fn a_mapped_listener_marks_with_the_ipv4_option_where_that_option_works() {
+        let mapped = cfg!(target_os = "linux");
+
+        for (addr, is_ipv4) in [
+            ("0.0.0.0:2112", true),
+            ("127.0.0.1:2112", true),
+            ("[::ffff:0.0.0.0]:2112", mapped),
+            ("[::ffff:127.0.0.1]:2112", mapped),
+            ("[::]:2112", false),
+            ("[::1]:2112", false),
+            ("[2001:db8::1]:2112", false),
+        ] {
+            assert_eq!(
+                marks_with_ipv4_option(addr.parse().unwrap()),
+                is_ipv4,
+                "{addr} would mark with the wrong option"
+            );
+        }
+    }
+}