This implementation uses a raw IP socket combined with a BPF packet
filter to receive traffic for one UDP port, without having to bind()
to that port. This enables this Conn to coexist on a machine alongside
a DHCP server.
AF_PACKET is poorly suited to be the main receiving socket of this
implementation, for a variety of reasons: it bypasses netfilter, you
receive duplicate packets when bridges and aggregated links are involved,
and more generally it's a lot of pain just to get a MAC address out of
the link layer.
Next up, I'm going to try using an AF_INET/SOCK_RAW socket as the main
listening socket, which loses a bit of precision in acquiring packets, but
puts more of the kernel and its desirable semantics between us and the wire.
We'll still have to use an AF_PACKET socket to transmit to unconfigured
clients (although the spec gives us an option to sidestep that as well
if we want).
This is a fairly advanced, linux-specific implementation that lets you run a DHCP
listener on a machine that's already running some other DHCP service.
