Take the 2-minute tour ×  I've noticed several repeats of the
following error message in /var/log/auth.log on a server:  Aug
10 09:10:16 hostname sshd[661]: error: connect_to port 25:
failed.  I've changed the actual IP address, they are external
addresses that often belong to mail servers.  The part I don't
understand is who exactly is trying to connect to those
addresses and what sshd has to do with it. The sshd is running
on port 22, there is nothing running on port 25 on that server.
What exactly does this line mean, who is initiating the
connection and why is sshd involved?  share|improve this
question  3 Answers 3  You can reproduce this by setting up SSH
dynamic port forwarding:  man ssh:   -D [bind_address:]port
application protocol is then used to determine where to connect
to from the remote machine.  Cur‐          root can forward
privileged ports.  Dynamic port forwardings can also be
specified in the configu‐          ration file.           IPv6
addresses can be specified by enclosing t