raw: allow specifying a filter at creation

[lmb]

Packet filtering on Linux raw sockets suffers from a race condition: packets are queued on a raw socket as soon as it is created. Attaching a filter via SetBPF does not clear the queue, and so the next ReadFrom after SetBPF may return a random packet.

Fix this by not specifying a protocol at socket creation time, which prevents the socket from capturing packets from the get go. We can then attach a filter. Finally, we call Bind which enables capturing packets for the protocol requested by the user.

See [1] for more information on the issue.

1: https://github.com/the-tcpdump-group/libpcap/blob/4f2b0f5befaa75c18a68a6849c759fe94c885727/pcap-linux.c#L7501

[arthurfabre]

Might not matter, but this only works for setting the initial filter. Updating the filter afterwards can still return packets that match the old filter. Maybe we could have a FlushAndSetBPF() function that uses the libpcap approach:

• Set a filter that will never match
• Read everything from the socket
• Set the correct filter