Support TCP_MAXSEG TCP Maximum Segment Size socket options

CHANGELOG.md

@@ -34,6 +34,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   (#[1503](https://github.com/nix-rust/nix/pull/1503))
 - Enabled `pwritev` and `preadv` for more operating systems.
   (#[1511](https://github.com/nix-rust/nix/pull/1511))
+ Added support for `TCP_MAXSEG` TCP Maximum Segment Size socket options
+  (#[1292](https://github.com/nix-rust/nix/pull/1292))
 - Added `Ipv4RecvErr` and `Ipv6RecvErr` sockopts and associated control messages.
   (#[1514](https://github.com/nix-rust/nix/pull/1514))
 - Added `AsRawFd` implementation on `PollFd`.

src/sys/socket/sockopt.rs

@@ -265,6 +265,13 @@ sockopt_impl!(Both, TcpKeepAlive, libc::IPPROTO_TCP, libc::TCP_KEEPALIVE, u32);
           target_os = "linux",
           target_os = "nacl"))]
 sockopt_impl!(Both, TcpKeepIdle, libc::IPPROTO_TCP, libc::TCP_KEEPIDLE, u32);
+cfg_if! {
+    if #[cfg(any(target_os = "android", target_os = "linux"))] {
+        sockopt_impl!(Both, TcpMaxSeg, libc::IPPROTO_TCP, libc::TCP_MAXSEG, u32);
+    } else {
+        sockopt_impl!(GetOnly, TcpMaxSeg, libc::IPPROTO_TCP, libc::TCP_MAXSEG, u32);
+    }
+}
 #[cfg(not(target_os = "openbsd"))]
 sockopt_impl!(Both, TcpKeepCount, libc::IPPROTO_TCP, libc::TCP_KEEPCNT, u32);
 #[cfg(any(target_os = "android",

test/sys/test_sockopt.rs

@@ -70,6 +70,57 @@ fn test_so_buf() {
     assert!(actual >= bufsize);
 }
 
+#[test]
+fn test_so_tcp_maxseg() {
+    use std::net::SocketAddr;
+    use std::str::FromStr;
+    use nix::sys::socket::{accept, bind, connect, listen, InetAddr, SockAddr};
+    use nix::unistd::{close, write};
+
+    let std_sa = SocketAddr::from_str("127.0.0.1:4001").unwrap();
+    let inet_addr = InetAddr::from_std(&std_sa);
+    let sock_addr = SockAddr::new_inet(inet_addr);
+
+    let rsock = socket(AddressFamily::Inet, SockType::Stream, SockFlag::empty(), SockProtocol::Tcp)
+                .unwrap();
+    bind(rsock, &sock_addr).unwrap();
+    listen(rsock, 10).unwrap();
+    let initial = getsockopt(rsock, sockopt::TcpMaxSeg).unwrap();
+    // Initial MSS is expected to be 536 (https://tools.ietf.org/html/rfc879#section-1) but some
+    // platforms keep it even lower. This might fail if you've tuned your initial MSS to be larger
+    // than 700
+    cfg_if! {
+        if #[cfg(any(target_os = "android", target_os = "linux"))] {
+            let segsize: u32 = 873;
+            assert!(initial < segsize);
+            setsockopt(rsock, sockopt::TcpMaxSeg, &segsize).unwrap();
+        } else {
+            assert!(initial < 700);
+        }
+    }
+
+    // Connect and check the MSS that was advertised
+    let ssock = socket(AddressFamily::Inet, SockType::Stream, SockFlag::empty(), SockProtocol::Tcp)
+                .unwrap();
+    connect(ssock, &sock_addr).unwrap();
+    let rsess = accept(rsock).unwrap();
+    write(rsess, b"hello").unwrap();
+    let actual = getsockopt(ssock, sockopt::TcpMaxSeg).unwrap();
+    // Actual max segment size takes header lengths into account, max IPv4 options (60 bytes) + max
+    // TCP options (40 bytes) are subtracted from the requested maximum as a lower boundary.
+    cfg_if! {
+        if #[cfg(any(target_os = "android", target_os = "linux"))] {
+            assert!((segsize - 100) <= actual);
+            assert!(actual <= segsize);
+        } else {
+            assert!(initial < actual);
+            assert!(536 < actual);
+        }
+    }
+    close(rsock).unwrap();
+    close(ssock).unwrap();
+}
+
 // The CI doesn't supported getsockopt and setsockopt on emulated processors.
 // It's beleived that a QEMU issue, the tests run ok on a fully emulated system.
 // Current CI just run the binary with QEMU but the Kernel remains the same as the host.