Using 'netstat -tulp' on my server box running under CentOS I have discovered strange listening ports 33363 (TCP) and 35661 (UDP). The thing is that 'PID/Program name' field for these ports contains nothing (dash actually). 'fuser -n tcp 33363' shows nothing. I have checked my box with chkrootkit and rkhunter utilities and it seems everything is O.K. Does anybody know what these sockets can be used for?

2 Answers 2


Have you tried with lsof?

lsof -i TCP:33363
lsof -i UDP:35661
  • Thanks for reply! Like 'fuser' 'lsof' gives an empty output.
    – user14241
    Jul 24, 2009 at 8:35
  • Pretty weird... You can also try to trace the pid and see if it shows something interesting: strace -p <PID>
    – scetoaux
    Jul 24, 2009 at 8:58
  • 1
    What PID, Fran?
    – innaM
    Jul 24, 2009 at 9:24
  • Doh! True, we do not know the pid. Thanks for pointing it out Manni :)
    – scetoaux
    Jul 24, 2009 at 9:26
  • Unfortunately lsof seems to usable only by root. My process is running as user "foo". I am user "foo", can get no results. Stragen. The tool nestat shows the open ports.
    – guettli
    Feb 28, 2017 at 8:02

Did you run netstat -tulp as root?

  • 1
    Could have been the reason for the missing pid. You never know ;-)
    – innaM
    Jul 24, 2009 at 9:22

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