Added Socket::set_multicast_if_v4_n to set interface by index (#458)

[boris-n]

Implementation for Socket::set_multicast_if_v4_n (similar to Socket::join_multicast_v4_n) which allows to set interface by index or address (via struct InterfaceIndexOrAddress), for Linux/Android/Fuchsia/FreeBSD/NetBSD. (Other OSes seem to have no support for this.)

[boris-n]

Added feature = "all" as setting multicast interface by index is not supported on Windows and MacOSX.

[boris-n]

Rebased to current master, waiting for review/approval.

[boris-n]

Rebased to current master (again), waiting for review/approval.

[michaelinoue]

Any thoughts on expanding this to set both an address and an if index? It seems like we're pinned to using either or in this PR, but we could potentially want both (e.g. sending multicast traffic from a particular address on the interface).

[Thomasdezeeuw]

I left a comment, but I'm not sure about this pr. I don't use these APIs and currently don't have the time to look into them properly. For this to be merged it would be nice if someone with experience with the options could confirm they work as expected. This also needs a test.

[Thomasdezeeuw]

This doesn't seem right. The enum variant clearly say Index, but we're using an IPv4 address. I think we shouldn't support the index here and recommend to use the Address instead for NetBSD, but I'm not sure.

[boris-n]

The implementation is correct (although I have to admit it is probably confusing). The NetBSD API seems to be a bit "unusual" for setting the multicast interface by index. Let me cite from the NetBSD man page (https://man.netbsd.org/NetBSD-7.0/ip.4):

The IP_MULTICAST_IF option overrides the default for subsequent transmissions from a given socket:
struct in_addr addr;
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));
where "addr" is the local IP address of the desired interface [...]. An application may also specify an alternative to the default network interface by index:
struct uint32_t idx = htonl(ifindex);
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &idx, sizeof(idx));
where "ifindex" is an interface index as returned by if_nametoindex(3).

So, while Linux, FreeBSD, Android and Fuchsia use a special struct mreqn (which has a separate field for the interface index) if they want to set the multicast interface by index, NetBSD uses a struct in_addr to set the address by IP and an uint32_t (in network byte order.) to set the address by interface index. struct in_addr and uint32_t both are 4 bytes in size, so the kernel cannot distinguish the two cases from the size parameter. Looking into the kernel code shows that IP addresses in subnet 0.0.0.0/8 are interpreted as interface index (see https://github.com/NetBSD/src/blob/trunk/sys/netinet/ip_output.c#L1691). This is the reason why I converted the interface to an address and used this one with the setsockopt call.

But I agree, the implementation is probably easier to understand if the code aligns to the examples given in the NetBSD manual page. I provided a modified version which does so.