phy::RawSocket incompatible with Medium::Ip

[toshipiazza]

RawSocketDesc creates an AF_PACKET socket with libc::SOCK_RAW socket type in case of any medium

smoltcp/src/phy/sys/raw_socket.rs

Lines 21 to 40 in fa7fd3c

	let protocol = match medium {
	#[cfg(feature = "medium-ethernet")]
	Medium::Ethernet => imp::ETH_P_ALL,
	#[cfg(feature = "medium-ip")]
	Medium::Ip => imp::ETH_P_ALL,
	#[cfg(feature = "medium-ieee802154")]
	Medium::Ieee802154 => imp::ETH_P_IEEE802154,
	};
	let lower = unsafe {
	let lower = libc::socket(
	libc::AF_PACKET,
	libc::SOCK_RAW | libc::SOCK_NONBLOCK,
	protocol.to_be() as i32,
	);
	if lower == -1 {
	return Err(io::Error::last_os_error());
	}
	lower
	};

However, this doesn't work when Medium::Ip is passed to phy::RawSocket and to RawSocketDesc, since the incoming packets have an L2 frame prepended but Interface expects packets to start from the IP header

According to PACKET(7) man page

The socket_type is either SOCK_RAW for raw packets including the link-level header or SOCK_DGRAM for cooked packets with the link-level header removed.

For Medium::Ip we should set socket_type = libc::SOCK_DGRAM | libc::SOCK_NONBLOCK and protocol = libc::ETH_P_IP so the resulting packets start with the IP header

[toshipiazza]

I guess it's unclear if this is expected or desired to work, since the docs for Medium::Ip suggest that Medium::Ip is an abstraction designed for TUN devices...

If this is not intended to work it may make sense to assert_ne!(medium, Medium::Ip)

[Dirbaio]

yes, it's intended to work. Can you send a PR?

[toshipiazza]

This may be harder to fix than how I initially thought. When sending a packet on a socket created with AF_PACKET + SOCK_DGRAM it looks like you need to specify the L2 destination address in a struct sockaddr_ll to sendto(). This doesn't work well with the Medium::Ip abstraction/design