fix(udp): honor IPv4 source addresses on Apple platforms

quinn-udp/Cargo.toml

@@ -36,6 +36,9 @@ windows-sys = { workspace = true }
 criterion = { version = "0.8", default-features = false, features = ["async_tokio"] }
 tokio = { workspace = true, features = ["rt", "rt-multi-thread", "net"] }
 
+[target.'cfg(target_vendor = "apple")'.dev-dependencies]
+nix = { version = "0.31.3", default-features = false, features = ["net"] }
+
 [build-dependencies]
 cfg_aliases = { workspace = true }
 

quinn-udp/src/apple_fast.rs

@@ -133,12 +133,14 @@ fn prepare_msg_x(
     if let Some(ip) = &transmit.src_ip {
         match ip {
             IpAddr::V4(v4) => {
-                if encode_src_ip {
-                    let addr = libc::in_addr {
+                let pktinfo = libc::in_pktinfo {
+                    ipi_ifindex: 0,
+                    ipi_spec_dst: libc::in_addr {
                         s_addr: u32::from_ne_bytes(v4.octets()),
-                    };
-                    encoder.push(libc::IPPROTO_IP, libc::IP_RECVDSTADDR, addr);
-                }
+                    },
+                    ipi_addr: libc::in_addr { s_addr: 0 },
+                };
+                encoder.push(libc::IPPROTO_IP, libc::IP_PKTINFO, pktinfo);
             }
             IpAddr::V6(v6) => {
                 let pktinfo = libc::in6_pktinfo {

quinn-udp/src/unix.rs

@@ -685,7 +685,7 @@ fn prepare_msg(
     hdr: &mut libc::msghdr,
     iov: &mut libc::iovec,
     ctrl: &mut cmsg::Aligned<[u8; cmsg::LEN]>,
-    #[allow(unused_variables)] // only used on FreeBSD & macOS
+    #[allow(unused_variables)] // only used on BSD & Solaris
     encode_src_ip: bool,
     sendmsg_einval: bool,
 ) {
@@ -734,7 +734,7 @@ fn prepare_msg(
     if let Some(ip) = &transmit.src_ip {
         match ip {
             IpAddr::V4(v4) => {
-                #[cfg(any(target_os = "linux", target_os = "android"))]
+                #[cfg(any(target_os = "linux", target_os = "android", apple))]
                 {
                     let pktinfo = libc::in_pktinfo {
                         ipi_ifindex: 0,
@@ -745,7 +745,7 @@ fn prepare_msg(
                     };
                     encoder.push(libc::IPPROTO_IP, libc::IP_PKTINFO, pktinfo);
                 }
-                #[cfg(any(bsd, apple, solarish))]
+                #[cfg(any(bsd, solarish))]
                 {
                     if encode_src_ip {
                         let addr = libc::in_addr {

quinn-udp/tests/tests.rs

@@ -79,6 +79,59 @@ fn basic_src_ip() {
     );
 }
 
+#[test]
+#[cfg(apple)]
+fn ipv4_source_address_override() {
+    use nix::{ifaddrs::getifaddrs, net::if_::InterfaceFlags};
+
+    let Some(src_ip) = getifaddrs()
+        .unwrap()
+        .filter(|interface| interface.flags.contains(InterfaceFlags::IFF_UP))
+        .filter_map(|interface| Some(interface.address?.as_sockaddr_in()?.ip()))
+        .find(|ip| {
+            *ip != Ipv4Addr::LOCALHOST
+                && !ip.is_unspecified()
+                && !ip.is_multicast()
+                && !ip.is_broadcast()
+        })
+    else {
+        eprintln!("skipping source override test: no alternate assigned IPv4 address");
+        return;
+    };
+
+    let send = UdpSocket::bind((Ipv4Addr::UNSPECIFIED, 0)).unwrap();
+    let recv = UdpSocket::bind((Ipv4Addr::LOCALHOST, 0)).unwrap();
+    recv.set_read_timeout(Some(std::time::Duration::from_secs(3)))
+        .unwrap();
+    let send_state = UdpSocketState::new((&send).into()).unwrap();
+    #[cfg(apple_fast)]
+    // SAFETY: sendmsg_x is available on the macOS test host.
+    unsafe {
+        send_state.set_apple_fast_path();
+    }
+    UdpSockRef::from(&send).set_nonblocking(false).unwrap();
+
+    let contents = b"hello";
+    let sent = send_state
+        .try_send(
+            (&send).into(),
+            &Transmit {
+                destination: recv.local_addr().unwrap(),
+                ecn: None,
+                contents,
+                segment_size: None,
+                src_ip: Some(src_ip.into()),
+            },
+        )
+        .unwrap();
+    assert_eq!(sent.get(), 1);
+
+    let mut buf = [0; 32];
+    let (received, source) = recv.recv_from(&mut buf).unwrap();
+    assert_eq!(&buf[..received], contents);
+    assert_eq!(source.ip(), src_ip);
+}
+
 // The WASI backend sets no ECN codepoint and always reports `None`.
 #[test]
 #[cfg(not(target_os = "wasi"))]