UDP: no destination port in the `type callback`

[hannesm]

when implementing a DNS client, I would like to register the same callback on several ports. I can do so, but inside of the callback I cannot distinguish which port was written to (thus I cannot verify that the remote IP, remote port, local IP, local port are a good combination of datagrams to accept).

for now, only the source IP address is checked against an expected one, is this sufficient?

there's some asymmetry in the type callback, including a src and src_port, and a dst, but no dst_port.