Finding the process id by port number

Question

I'm trying (with minimal success at present) to setup the Dovecot mail server on my Fedora 24 server. I've installed Dovecot and set the conf file up, all fine. But when I run:

systemctl restart dovecot

After editing the conf file I get this message

Job for dovecot.service failed because the control process exited with error code. See "systemctl status dovecot.service" and "journalctl -xe" for details

Running systemctl status dovecot.service gives me a different error

[root@fedora app]# systemctl status dovecot.service
● dovecot.service - Dovecot IMAP/POP3 email server
   Loaded: loaded (/usr/lib/systemd/system/dovecot.service; disabled; vendor preset: disabled)
   Active: failed (Result: exit-code) since Tue 2016-08-16 15:02:30 UTC; 37min ago
     Docs: man:dovecot(1)
           http://wiki2.dovecot.org/
  Process: 11293 ExecStart=/usr/sbin/dovecot (code=exited, status=89)
  Process: 11285 ExecStartPre=/usr/libexec/dovecot/prestartscript (code=exited, status=0/SUCCESS)

Aug 16 15:02:30 fedora dovecot[11293]: Error: service(imap-login): listen(*, 993) failed: Address already in use
Aug 16 15:02:30 fedora dovecot[11293]: master: Error: service(imap-login): listen(*, 993) failed: Address already in use
Aug 16 15:02:30 fedora dovecot[11293]: Error: service(imap-login): listen(::, 993) failed: Address already in use
Aug 16 15:02:30 fedora dovecot[11293]: master: Error: service(imap-login): listen(::, 993) failed: Address already in use
Aug 16 15:02:30 fedora dovecot[11293]: Fatal: Failed to start listeners
Aug 16 15:02:30 fedora dovecot[11293]: master: Fatal: Failed to start listeners
Aug 16 15:02:30 fedora systemd[1]: dovecot.service: Control process exited, code=exited status=89
Aug 16 15:02:30 fedora systemd[1]: Failed to start Dovecot IMAP/POP3 email server.
Aug 16 15:02:30 fedora systemd[1]: dovecot.service: Unit entered failed state.
Aug 16 15:02:30 fedora systemd[1]: dovecot.service: Failed with result 'exit-code'.

I tried running lsof -i | grep 993 but this yields no processes. Any idea how to fix this?

Quick comment: try `lsof -i:imaps` or `lsof -i:993` instead of the `grep`. Or just `grep imaps`. Looking for 993 probably failed simply because lsof translated it to the matching name in `/etc/services`. — mattdm, Aug 16 '16 at 20:04

score 6 · Accepted Answer · answered Aug 16 '16 at 18:08

netstat is your friend when you're trying to troubleshoot a lot of network-related problems. To find a listening port, I would use netstat -tulpn | grep :<port number>

For example, to find what pids are listening on port 22, I would run:

netstat -tulpn | grep :22
tcp  0  0 0.0.0.0:22    0.0.0.0:*    LISTEN      3062/sshd

That tells me that sshd with pid 3062 is listening on port 22.

score 3 · Answer 2 · answered Aug 16 '16 at 18:33

You might also like a solution using ss that is more systematic and more precise than grepping port numbers.

# ss -t -l 'sport = 22'
State      Recv-Q Send-Q Local Address:Port                 Peer Address:Port
LISTEN     0      128        *:ssh                      *:*
LISTEN     0      128       :::ssh                     :::*

Finding the process id by port number

2 Answers2