linux tcp connection established on client, invisible on server

Question

on one of my linux hosts, one tcp socket seems to be visible on one side and invisble on the other side. I mean netstat displays this socket when I am on one of the hosts and not when I am on the other (linux too):

# netstat -anp|grep 37674
udp        0      0 169.254.192.2:37674         169.254.192.1:8649          ESTABLISHED 22644/xxxxx
# ssh 169.254.192.1
Last login: Mon Jan 13 15:22:54 CET 2014 from xxxxxx on ssh
# netstat -anp|grep 8649
# 

If I undestand correctly, netstat reads connections from /proc/net/tcp. Apparently, local ip address and ports are given as 0123ABCD:1234 in /proc/net/tcp in second column.

37674(10) = 932A(16)

Thus, I tried to find 932A in /proc/net/tcp, ignoring case but found nothing.

IP address is not truncated as I have no other 169.254.192.1xx host on my network.

This connection is still visible after three hours, thus I don't think it's timeout related.

Answer 1

The output of netstat says UDP. (Look in /proc/net/udp)

This means means 169.254.192.2 has called connect() to 169.254.192.1:8649. UDP is however connectionless, so this info is just recorded locally on a socket on the 169.254.192.2 machine.

Calling connect() on an UDP socket just enables you to call send() on that socket without specifying the destination address for each packet with sendto()/sendmsg() - no actual connection created between the two machines.

If the 169.254.192.1 machine isn't listening/receiving packets on port 8649, it isn't meaningful for the 169.254.192.1 machine to set up a socket that can send packets there though.