tun: Enable setting/getting of MTU

Works similarly to getting and setting IP addresses, can pretty much be
copy-pasted for the rest of the settings.

tun/src/unix/linux/mod.rs

@@ -68,6 +68,13 @@ impl TunInterface {
         self.perform(|fd| unsafe { sys::if_set_addr(fd, &iff) })?;
     }
 
+    #[throws]
+    pub fn set_mtu(&self, mtu: i32) {
+        let mut iff = self.ifreq()?;
+        iff.ifr_ifru.ifru_mtu = mtu;
+        self.perform(|fd| unsafe { sys::if_set_mtu(fd, &iff) })?;
+    }
+
     #[throws]
     pub fn ipv4_addr(&self) -> Ipv4Addr {
         let mut iff = self.ifreq()?;
@@ -76,6 +83,15 @@ impl TunInterface {
         Ipv4Addr::from(u32::from_be(addr.sin_addr.s_addr))
     }
 
+    #[throws]
+    pub fn mtu(&self) -> i32 {
+        let mut iff = self.ifreq()?;
+        self.perform(|fd| unsafe { sys::if_get_mtu(fd, &mut iff) })?;
+        let mtu = unsafe { iff.ifr_ifru.ifru_mtu };
+
+        mtu
+    }
+
     #[throws]
     fn perform<R>(&self, perform: impl FnOnce(RawFd) -> Result<R, nix::Error>) -> R {
         let socket = Socket::new(Domain::IPV4, Type::DGRAM, None)?;

tun/src/unix/linux/sys.rs

@@ -16,4 +16,7 @@ ioctl_read_bad!(
 );
 ioctl_read_bad!(if_get_index, libc::SIOCGIFINDEX, libc::ifreq);
 ioctl_read_bad!(if_get_addr, libc::SIOCGIFADDR, libc::ifreq);
+ioctl_read_bad!(if_get_mtu, libc::SIOCGIFMTU, libc::ifreq);
+
 ioctl_write_ptr_bad!(if_set_addr, libc::SIOCSIFADDR, libc::ifreq);
+ioctl_write_ptr_bad!(if_set_mtu, libc::SIOCSIFMTU, libc::ifreq);