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Is it possible to have a single process that would have two sockets, one using the AF_INET and one using the AF_INET6 families, while both being bound to the same port number, e.g. 20000?

If the IPv6 socket setsockopts to be IPv6-only, would the kernel correctly direct packets to the correct socket based on the IP protocol version?

2

Yes, it is possible (there may be complications depending on the higher level language and how well it lets you select such details).

#!/usr/bin/env expect
set port 1234
proc v4 {fh caddr cport} { puts $fh v4; close $fh }
proc v6 {fh caddr cport} { puts $fh v6; close $fh }
socket -myaddr 127.0.0.1 -server [list v4] $port
socket -myaddr ::1 -server [list v6] $port
vwait godot

With that running:

% lsof -i -nP | grep 1234
expect 96688 jhqdoe 10u IPv4 0xf7... 0t0 TCP 127.0.0.1:1234 (LISTEN)
expect 96688 jhqdoe 11u IPv6 0xf7... 0t0 TCP [::1]:1234 (LISTEN)
% nc 127.0.0.1 1234
v4
% nc ::1 1234
v6

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