Do not set tunnel config MSS/MTU on non-Android systems

IP_USER_MTU does not have the intended effect of limiting MSS size.

talpid-tunnel-config-client/src/lib.rs

@@ -102,15 +102,11 @@ pub type RelayConfigService = proto::ephemeral_peer_client::EphemeralPeerClient<
 pub const CONFIG_SERVICE_PORT: u16 = 1337;
 
 /// MTU to set on the tunnel config client socket. We want a low value to prevent fragmentation.
-/// This is needed for two reasons:
-/// 1. Especially on Android, we've found that the real MTU is often lower than the default MTU, and
-///    we cannot lower it further. This causes the outer packets to be dropped. Also, MTU detection
-///    will likely occur after the PQ handshake, so we cannot assume that the MTU is already
-///    correctly configured.
-/// 2. MH + PQ on macOS has connection issues during the handshake due to PF blocking packet
-///    fragments for not having a port. In the longer term this might be fixed by allowing the
-///    handshake to work even if there is fragmentation.
-#[cfg(not(target_os = "ios"))]
+/// Especially on Android, we've found that the real MTU is often lower than the default MTU, and
+/// we cannot lower it further. This causes the outer packets to be dropped. Also, MTU detection
+/// will likely occur after the PQ handshake, so we cannot assume that the MTU is already
+/// correctly configured.
+#[cfg(target_os = "android")]
 const CONFIG_CLIENT_MTU: u16 = 576;
 
 pub struct EphemeralPeer {
@@ -253,10 +249,7 @@ async fn new_client(addr: Ipv4Addr) -> Result<RelayConfigService, Error> {
         .connect_with_connector(service_fn(move |_| async move {
             let sock = TcpSocket::new_v4()?;
 
-            #[cfg(target_os = "windows")]
-            try_set_tcp_sock_mtu(sock.as_raw_socket(), CONFIG_CLIENT_MTU);
-
-            #[cfg(not(target_os = "windows"))]
+            #[cfg(target_os = "android")]
             try_set_tcp_sock_mtu(&addr, sock.as_raw_fd(), CONFIG_CLIENT_MTU);
 
             sock.connect(SocketAddr::new(addr, CONFIG_SERVICE_PORT))
@@ -268,31 +261,7 @@ async fn new_client(addr: Ipv4Addr) -> Result<RelayConfigService, Error> {
     Ok(RelayConfigService::new(conn))
 }
 
-#[cfg(windows)]
-fn try_set_tcp_sock_mtu(sock: RawSocket, mtu: u16) {
-    let mtu = u32::from(mtu);
-    log::debug!("Config client socket MTU: {mtu}");
-
-    let raw_sock = usize::try_from(sock).unwrap();
-
-    let result = unsafe {
-        setsockopt(
-            raw_sock,
-            IPPROTO_IP,
-            IP_USER_MTU,
-            &mtu as *const _ as _,
-            std::ffi::c_int::try_from(std::mem::size_of_val(&mtu)).unwrap(),
-        )
-    };
-    if result != 0 {
-        log::error!(
-            "Failed to set user MTU on config client socket: {}",
-            std::io::Error::last_os_error()
-        );
-    }
-}
-
-#[cfg(not(any(target_os = "windows", target_os = "ios")))]
+#[cfg(target_os = "android")]
 fn try_set_tcp_sock_mtu(dest: &IpAddr, sock: RawFd, mut mtu: u16) {
     const IPV4_HEADER_SIZE: u16 = 20;
     const IPV6_HEADER_SIZE: u16 = 40;