fix(quinn-udp): use TOS for IPv4-mapped IPv6 dst addrs

quinn-udp/src/unix.rs

@@ -586,7 +586,10 @@ fn prepare_msg(
     hdr.msg_controllen = CMSG_LEN as _;
     let mut encoder = unsafe { cmsg::Encoder::new(hdr) };
     let ecn = transmit.ecn.map_or(0, |x| x as libc::c_int);
-    if transmit.destination.is_ipv4() {
+    // True for IPv4 or IPv4-Mapped IPv6
+    let is_ipv4 = transmit.destination.is_ipv4()
+        || matches!(transmit.destination.ip(), IpAddr::V6(addr) if addr.to_ipv4_mapped().is_some());
+    if is_ipv4 {
         if !sendmsg_einval {
             encoder.push(libc::IPPROTO_IP, libc::IP_TOS, ecn as IpTosTy);
         }

quinn-udp/src/windows.rs

@@ -182,7 +182,10 @@ impl UdpSocketState {
 
             // ECN is a C integer https://learn.microsoft.com/en-us/windows/win32/winsock/winsock-ecn
             let ecn = transmit.ecn.map_or(0, |x| x as c_int);
-            if transmit.destination.is_ipv4() {
+            // True for IPv4 or IPv4-Mapped IPv6
+            let is_ipv4 = transmit.destination.is_ipv4()
+                || matches!(transmit.destination.ip(), IpAddr::V6(addr) if addr.to_ipv4_mapped().is_some());
+            if is_ipv4 {
                 encoder.push(WinSock::IPPROTO_IP, WinSock::IP_ECN, ecn);
             } else {
                 encoder.push(WinSock::IPPROTO_IPV6, WinSock::IPV6_ECN, ecn);

quinn-udp/tests/tests.rs

@@ -91,6 +91,44 @@ fn ecn_v4() {
     }
 }
 
+#[test]
+fn ecn_v4_mapped_v6() {
+    let send = socket2::Socket::new(
+        socket2::Domain::IPV6,
+        socket2::Type::DGRAM,
+        Some(socket2::Protocol::UDP),
+    )
+    .unwrap();
+    send.set_only_v6(false).unwrap();
+    send.bind(&socket2::SockAddr::from(
+        "[::]:0".parse::<SocketAddr>().unwrap(),
+    ))
+    .unwrap();
+
+    let recv = UdpSocket::bind("127.0.0.1:0").unwrap();
+    let recv = Socket::from(recv);
+    let recv_v4_mapped_v6 = SocketAddr::V6(SocketAddrV6::new(
+        Ipv4Addr::LOCALHOST.to_ipv6_mapped(),
+        recv.local_addr().unwrap().as_socket().unwrap().port(),
+        0,
+        0,
+    ));
+
+    for codepoint in [EcnCodepoint::Ect0, EcnCodepoint::Ect1] {
+        test_send_recv(
+            &send,
+            &recv,
+            Transmit {
+                destination: recv_v4_mapped_v6,
+                ecn: Some(codepoint),
+                contents: Bytes::from_static(b"hello"),
+                segment_size: None,
+                src_ip: None,
+            },
+        );
+    }
+}
+
 #[test]
 #[cfg_attr(not(any(target_os = "linux", target_os = "windows")), ignore)]
 fn gso() {
@@ -154,35 +192,32 @@ fn test_send_recv(send: &Socket, recv: &Socket, transmit: Transmit) {
         }
         datagrams += segments;
 
+        assert_eq!(
+            meta.addr.port(),
+            send.local_addr().unwrap().as_socket().unwrap().port()
+        );
         let send_v6 = send.local_addr().unwrap().as_socket().unwrap().is_ipv6();
         let recv_v6 = recv.local_addr().unwrap().as_socket().unwrap().is_ipv6();
-        match send_v6 == recv_v6 {
-            true => assert_eq!(meta.addr, send.local_addr().unwrap().as_socket().unwrap()),
-            false => assert_eq!(
-                meta.addr,
-                to_v6_mapped(send.local_addr().unwrap().as_socket().unwrap())
-            ),
-        }
-        assert_eq!(meta.ecn, transmit.ecn);
+        let src = meta.addr.ip();
         let dst = meta.dst_ip.unwrap();
-        match (send_v6, recv_v6) {
-            (_, false) => assert_eq!(dst, Ipv4Addr::LOCALHOST),
-            // Windows gives us real IPv4 addrs, whereas *nix use IPv6-mapped IPv4
-            // addrs. Canonicalize to IPv6-mapped for robustness.
-            (false, true) => assert_eq!(ip_to_v6_mapped(dst), Ipv4Addr::LOCALHOST.to_ipv6_mapped()),
-            (true, true) => assert_eq!(dst, Ipv6Addr::LOCALHOST),
+        for addr in [src, dst] {
+            match (send_v6, recv_v6) {
+                (_, false) => assert_eq!(addr, Ipv4Addr::LOCALHOST),
+                // Windows gives us real IPv4 addrs, whereas *nix use IPv6-mapped IPv4
+                // addrs. Canonicalize to IPv6-mapped for robustness.
+                (false, true) => {
+                    assert_eq!(ip_to_v6_mapped(addr), Ipv4Addr::LOCALHOST.to_ipv6_mapped())
+                }
+                (true, true) => assert!(
+                    addr == Ipv6Addr::LOCALHOST || addr == Ipv4Addr::LOCALHOST.to_ipv6_mapped()
+                ),
+            }
         }
+        assert_eq!(meta.ecn, transmit.ecn);
     }
     assert_eq!(datagrams, expected_datagrams);
 }
 
-fn to_v6_mapped(x: SocketAddr) -> SocketAddr {
-    match x {
-        SocketAddr::V4(x) => SocketAddrV6::new(x.ip().to_ipv6_mapped(), x.port(), 0, 0).into(),
-        SocketAddr::V6(_) => x,
-    }
-}
-
 fn ip_to_v6_mapped(x: IpAddr) -> IpAddr {
     match x {
         IpAddr::V4(x) => IpAddr::V6(x.to_ipv6_mapped()),