Support for sending multicast packets on multiple interfaces with a single socket about socket2

[alexcrichton]

Thanks for the explanation and the thorough issue report! I suspect that the bindings here are pretty simple so I think they'd work well in this crate. In general this crate is "mostly single-line wrappers around system functionality", so as long as it fits that bill it should be good to add.

[bltavares]

Hi @alexcrichton, thanks for the comment. I've started exploring the design space and doing something usable across platforms required many lines of unsafe code.

I've started to do that on a fork of socket2, but as it kept growing I felt like it would be a bit too big for the project. So I've moved those out to a new crate using some of the OS-specific traits to get access to the socket handle, which was great!

I've put the functionality on multicast-socket, and maybe there are a few things that could be added back here, such as socket.set_pktinfo(true), or exposing to_s_addr, which would help me reduce some copy-and-paste.

The actual sendmsg and recvmsg seems to be a bit more complex to be packaged as cross-patform generic code while trying to be a "single-line wrapper" and I wonder if it would be worth extracting out.

Thanks a lot for providing the AsRawFd and AsRawSocket crate, which allowed to access the underlying handler to wrap things on that nice little crate :) I've tested it on Android, Windows, Linux, Mac and even MIPS and I can do mdns discovery now!

Feel free to either close the issue or suggest any of the code to be contributed back that fits project. Thanks for the attention!

[Thomasdezeeuw]

@bltavares what concretely has to be done to solve this issue? In other words what is socket2 missing (on the master branch)?

[bltavares]

I think we could close the issue, as it is already possible to use the AsRawFd to access things from a socket2::Socket, but there are a few things that could be ported over to socket2, but I'm not sure if that is the intention of the crate.

• Expose to_s_addr to avoid copy-and-paste
• Add socket.set_pkt_info(true) that wraps this piece similarly to how socket.set_reuse_address(true) works
• A more generic way to find socket extensions, such as WSARecvMsgExtension on windows
• A socket.bind_multicast which abstracts the differences between platforms
• Expose recvmsg, and sendmsg (and on windows, the equivalents from an extension method WSA*) to be able to make syscalls with more extensions

This is not required to work, more of a list of ideas when using socket2 on this specific use-case. Given the crate can be used through the AsRawFd trait I would be ok to close this issue and only pick some of the ideas that match the crate goal :)

[Thomasdezeeuw]

I think we could close the issue, as it is already possible to use the AsRawFd to access things from a socket2::Socket, but there are a few things that could be ported over to socket2, but I'm not sure if that is the intention of the crate.

* Expose [to_s_addr](https://github.com/bltavares/multicast-socket/blob/d62ec76551b050364bacf4f9540492279644077c/src/win.rs#L443-L449) to avoid copy-and-paste

Is SockAddr::{family, len, as_ptr} sufficient for this?

socket2/src/sockaddr.rs

Lines 65 to 78 in 9c4a404

	/// Returns this address's family.
	pub const fn family(&self) -> sa_family_t {
	self.storage.ss_family
	}
	/// Returns the size of this address in bytes.
	pub const fn len(&self) -> socklen_t {
	self.len
	}
	/// Returns a raw pointer to the address.
	pub const fn as_ptr(&self) -> *const sockaddr {
	&self.storage as *const _ as *const _
	}

* Add `socket.set_pkt_info(true)` that wraps [this piece](https://github.com/bltavares/multicast-socket/blob/d62ec76551b050364bacf4f9540492279644077c/src/win.rs#L443-L449) similarly to how `socket.set_reuse_address(true)` works

A pr for this would definitely be accepted.

* A more generic way to find socket extensions, such as [WSARecvMsgExtension](https://github.com/bltavares/multicast-socket/blob/d62ec76551b050364bacf4f9540492279644077c/src/win.rs#L150) on windows

* A `socket.bind_multicast` which abstracts [the differences between platforms](https://github.com/bltavares/multicast-socket/blob/d62ec76551b050364bacf4f9540492279644077c/src/win.rs#L158-L160)

These would be a bit harder but we can work on it. At the very least we could expose the OS specific functions.

* Expose [recvmsg](https://github.com/bltavares/multicast-socket/blob/d62ec76551b050364bacf4f9540492279644077c/src/unix.rs), and [sendmsg](https://github.com/bltavares/multicast-socket/blob/d62ec76551b050364bacf4f9540492279644077c/src/unix.rs#L184) (and on windows, the equivalents from [an extension method WSA*](https://github.com/bltavares/multicast-socket/blob/d62ec76551b050364bacf4f9540492279644077c/src/win.rs#L306)) to be able to make syscalls with more extensions

We expose Socket::recv_with_flags:

socket2/src/socket.rs

Lines 318 to 328 in 9c4a404

	/// Identical to [`recv`] but allows for specification of arbitrary flags to
	/// the underlying `recv` call.
	///
	/// [`recv`]: Socket::recv
	pub fn recv_with_flags(
	&self,
	buf: &mut [MaybeUninit<u8>],
	flags: sys::c_int,
	) -> io::Result<usize> {
	sys::recv(self.inner, buf, flags)
	}

And Socket::recv_vectored_with_flags:

socket2/src/socket.rs

Lines 363 to 381 in 9c4a404

	/// Identical to [`recv_vectored`] but allows for specification of arbitrary
	/// flags to the underlying `recvmsg`/`WSARecv` call.
	///
	/// [`recv_vectored`]: Socket::recv_vectored
	///
	/// # Safety
	///
	/// `recv_from_vectored` makes the same safety guarantees regarding `bufs`
	/// as [`recv_vectored`].
	///
	/// [`recv_vectored`]: Socket::recv_vectored
	#[cfg(not(target_os = "redox"))]
	pub fn recv_vectored_with_flags(
	&self,
	bufs: &mut [MaybeUninitSlice<'_>],
	flags: i32,
	) -> io::Result<(usize, RecvFlags)> {
	sys::recv_vectored(self.inner, bufs, flags)
	}

Or do you need access to msghdr structure?

[JonathanPlasse]

• There is a similar crate that do this a-ba/udp_sas.