Add ControlMessage::Ipv6MulticastHopLimit

When sending IPv6 multicast packets with `sendmsg`, Linux does not use
the hop limit set on the socket. Instead, the hop limit has to be
specified for each individual message with ancillary data in a cmsg.

This commit adds the enum variant
`ControlMessage::Ipv6MulticastHopLimit` to specify the limit.

Author: sgasse

src/sys/socket/mod.rs

@@ -1112,7 +1112,7 @@ pub enum ControlMessage<'a> {
     #[cfg_attr(docsrs, doc(cfg(feature = "net")))]
     UdpGsoSegments(&'a u16),
 
-    /// Configure the sending addressing and interface for v4
+    /// Configure the sending addressing and interface for v4.
     ///
     /// For further information, please refer to the
     /// [`ip(7)`](https://man7.org/linux/man-pages/man7/ip.7.html) man page.
@@ -1125,7 +1125,7 @@ pub enum ControlMessage<'a> {
     #[cfg_attr(docsrs, doc(cfg(feature = "net")))]
     Ipv4PacketInfo(&'a libc::in_pktinfo),
 
-    /// Configure the sending addressing and interface for v6
+    /// Configure the sending addressing and interface for v6.
     ///
     /// For further information, please refer to the
     /// [`ipv6(7)`](https://man7.org/linux/man-pages/man7/ipv6.7.html) man page.
@@ -1150,6 +1150,18 @@ pub enum ControlMessage<'a> {
     #[cfg_attr(docsrs, doc(cfg(feature = "net")))]
     Ipv4SendSrcAddr(&'a libc::in_addr),
 
+    /// Configure the hop limit for v6 multicast traffic.
+    ///
+    /// Set the multicast hop limit for this message. The argument is an
+    /// integer between 0 and 255. Other than the default, the value -1 will
+    /// set the hop limit to 1, not the route default. Without this cmsg, UDP
+    /// packets sent with sendmsg have a hop limit of 1 and will not leave the
+    /// local network.
+    /// For further information, please refer to the
+    /// [`ipv6(7)`](https://man7.org/linux/man-pages/man7/ipv6.7.html) man page.
+    #[cfg(any(target_os = "linux"))]
+    Ipv6MulticastHopLimit(&'a libc::c_int),
+
     /// SO_RXQ_OVFL indicates that an unsigned 32 bit value
     /// ancilliary msg (cmsg) should be attached to recieved
     /// skbs indicating the number of packets dropped by the
@@ -1263,6 +1275,8 @@ impl<'a> ControlMessage<'a> {
                       target_os = "openbsd", target_os = "dragonfly"))]
             #[cfg(feature = "net")]
             ControlMessage::Ipv4SendSrcAddr(addr) => addr as *const _ as *const u8,
+            #[cfg(any(target_os = "linux"))]
+            ControlMessage::Ipv6MulticastHopLimit(limit) => limit as *const _ as *const u8,
             #[cfg(any(target_os = "android", target_os = "fuchsia", target_os = "linux"))]
             ControlMessage::RxqOvfl(drop_count) => {
                 drop_count as *const _ as *const u8
@@ -1326,6 +1340,10 @@ impl<'a> ControlMessage<'a> {
                       target_os = "openbsd", target_os = "dragonfly"))]
             #[cfg(feature = "net")]
             ControlMessage::Ipv4SendSrcAddr(addr) => mem::size_of_val(addr),
+            #[cfg(any(target_os = "linux"))]
+            ControlMessage::Ipv6MulticastHopLimit(limit) => {
+                mem::size_of_val(limit)
+            },
             #[cfg(any(target_os = "android", target_os = "fuchsia", target_os = "linux"))]
             ControlMessage::RxqOvfl(drop_count) => {
                 mem::size_of_val(drop_count)
@@ -1365,6 +1383,9 @@ impl<'a> ControlMessage<'a> {
                       target_os = "openbsd", target_os = "dragonfly"))]
             #[cfg(feature = "net")]
             ControlMessage::Ipv4SendSrcAddr(_) => libc::IPPROTO_IP,
+            #[cfg(any(target_os = "linux"))]
+            #[cfg(feature = "net")]
+            ControlMessage::Ipv6MulticastHopLimit(_) => libc::IPPROTO_IPV6,
             #[cfg(any(target_os = "android", target_os = "fuchsia", target_os = "linux"))]
             ControlMessage::RxqOvfl(_) => libc::SOL_SOCKET,
             #[cfg(target_os = "linux")]
@@ -1411,6 +1432,8 @@ impl<'a> ControlMessage<'a> {
                       target_os = "openbsd", target_os = "dragonfly"))]
             #[cfg(feature = "net")]
             ControlMessage::Ipv4SendSrcAddr(_) => libc::IP_SENDSRCADDR,
+            #[cfg(any(target_os = "linux"))]
+            ControlMessage::Ipv6MulticastHopLimit(_) => libc::IPV6_HOPLIMIT,
             #[cfg(any(target_os = "android", target_os = "fuchsia", target_os = "linux"))]
             ControlMessage::RxqOvfl(_) => {
                 libc::SO_RXQ_OVFL