add IpMtu sockopt

Author: ShadowJonathan

CHANGELOG.md

@@ -38,6 +38,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   ([#1857](https://github.com/nix-rust/nix/pull/1857))
 - Added `SockProtocol::Raw` for raw sockets
   ([#1848](https://github.com/nix-rust/nix/pull/1848))
+- added `IP_MTU` (`IpMtu`) `IPPROTO_IP` sockopt on Linux and Android.
+  ([#1865](https://github.com/nix-rust/nix/pull/1865))
 
 ### Changed
 

src/sys/socket/sockopt.rs

@@ -925,6 +925,15 @@ sockopt_impl!(
     libc::IPV6_RECVERR,
     bool
 );
+#[cfg(any(target_os = "android", target_os = "linux"))]
+sockopt_impl!(
+    /// Fetch the current system-estimated Path MTU.
+    IpMtu,
+    GetOnly,
+    libc::IPPROTO_IP,
+    libc::IP_MTU,
+    libc::c_int
+);
 #[cfg(any(target_os = "android", target_os = "freebsd", target_os = "linux"))]
 sockopt_impl!(
     /// Set or retrieve the current time-to-live field that is used in every

test/sys/test_sockopt.rs

@@ -236,6 +236,33 @@ fn test_so_tcp_keepalive() {
     }
 }
 
+#[test]
+#[cfg(any(target_os = "android", target_os = "linux"))]
+#[cfg_attr(qemu, ignore)]
+fn test_get_mtu() {
+    use nix::sys::socket::{bind, connect, SockaddrIn};
+    use std::net::SocketAddrV4;
+    use std::str::FromStr;
+
+    let std_sa = SocketAddrV4::from_str("127.0.0.1:4001").unwrap();
+    let std_sb = SocketAddrV4::from_str("127.0.0.1:4002").unwrap();
+
+    let usock = socket(
+        AddressFamily::Inet,
+        SockType::Datagram,
+        SockFlag::empty(),
+        SockProtocol::Udp,
+    )
+    .unwrap();
+
+    // Bind and initiate connection
+    bind(usock, &SockaddrIn::from(std_sa)).unwrap();
+    connect(usock, &SockaddrIn::from(std_sb)).unwrap();
+
+    // Loopback connections have 2^16 - the maximum - MTU
+    assert_eq!(getsockopt(usock, sockopt::IpMtu), Ok(u16::MAX as i32))
+}
+
 #[test]
 #[cfg(any(target_os = "android", target_os = "freebsd", target_os = "linux"))]
 fn test_ttl_opts() {