Support ancillary data over `sendmsg`, `recvmsg`

[brunowonka]

Should socket2 support sendmsg/recvmsg including control ancillary data to expose things like IP_TTL and IPV6_PKTINFO.

I took a quick stab at it to figure out how it'd look like. I only did the writing side, I think there's a lot of design work and discussion here and I'm more than half-convinced this should be an enum instead of what I wrote.

I think it'd be interesting to basically provide some safe wrappers around CMSG and the many control data types. Unclear if it's in the interest of the authors of the crate to maintain all of those, however. Also I have 0 experience with Windows here so I didn't even try satisfying it with my prototype, and I'm honestly not even sure where to begin there.

Interested to hear the maintainer's thoughts. I'm willing to iterate on design and implementation here if there is interest.

Note: This is somewhat related to #86 from what I gathered in the discussion over there.

[Thomasdezeeuw]

@brunowonka I don't really have time to review this at the moment, but if you open a draft pr I can start reviewing the code once I have some time. In general I think that if the API surface isn't too big socket2 should be able to support this.

[brunowonka]

Thanks @Thomasdezeeuw, opened #316 to get the conversation started

[leshow]

I don't have an opinion on the code per-say, but I did want to link some other resources I stumbled across that may be helpful:

• quinn udp used in rust QUIC impl, has a cmsg type that may be informative
• nix sendmsg

As an argument for supporting sendmsg/recvmsg, if you want to modify the interface or ip used when sending or receiving a message while bound to ipaddr_any, for instance, those methods are necessary. dnsmasq does this in its DHCP impl. If socket2 is meant to support "advanced configuration options" it seems like something it should be able to do.

edit: it looks like quinn uses sendmmsg/recvmmsg on linux but falls back to just send_to/recv_from on windows? see here

[the8472]

There also is some incomplete work in the standard library

• Tracking Issue for feature(unix_socket_ancillary_data) rust#76915
• https://doc.rust-lang.org/std/os/unix/net/struct.UnixStream.html#method.recv_vectored_with_ancillary

[leshow]

If this is useful for anyone, I extracted some of the code from quinn into a library that has sendmsg/recvmsg for tokio & std https://github.com/leshow/unix-udp-sock

[chrysn]

Reading and writing PKTINFO would also be of great help for std_embedded_nal, as having the local address is mandatory for that interface, to allow QUIC and CoAP implementations. (Right now, that crate goes through nix, but this crate could offer it in a sufficiently abstract way that even Windows can be supported).

[Thomasdezeeuw]

We now support this with MsgHdrMut and MsgHdr, recvmsg and sendmsg.