How to Find Out What Service Is Listening on a Specific Port of a Ubuntu Server Without Process ID

networkingportprocessUbuntu

I have decided to post this question even though there are a lot of similar questions, but none of them answered mine.

  1. I periodically check the ports my server listens to.
  2. My ubuntu OS by the output of lsb_relase -a:
Distributor ID: Ubuntu
Description:    Ubuntu 20.04.3 LTS
Release:    20.04
Codename:   focal
  1. I run the command netstat -tulpn4 and the output shows an unknown port:
tcp        0      0 127.0.0.53:53           0.0.0.0:*               LISTEN      580/systemd-resolve 
tcp        0      0 0.0.0.0:62176           0.0.0.0:*               LISTEN      900/sshd: /usr/sbin 
tcp        0      0 0.0.0.0:1122            0.0.0.0:*               LISTEN      1158/sshd: username    
tcp        0      0 0.0.0.0:3333            0.0.0.0:*               LISTEN      1158/sshd: username    
udp        0      0 0.0.0.0:51820           0.0.0.0:*                           -                   
udp        0      0 127.0.0.53:53           0.0.0.0:*                           580/systemd-resolve 
udp        0      0 X.X.X.X:1194           0.0.0.0:*                           870/openvpn
  1. I have used many tools and guides to find out why my server is listening on port 51820 without finding an answer.
  2. I have tried:
    • sudo lsof -i :51820 – and got an empty output.
    • sudo netstat -ltnp | grep -w ':51820' – and got an empty output.
    • sudo fuser 51820/udp – and got an empty output.
    • sudo netstat -peanut | grep ":51820" – the output is:
udp        0      0 0.0.0.0:51820           0.0.0.0:*                           0          26262      -                   
udp6       0      0 :::51820                :::*                                0          26263      -
  • sudo ss -nlp | grep 51820 the output is:
udp     UNCONN   0        0                                             0.0.0.0:51820                                             0.0.0.0:*                                                                                                     
udp     UNCONN   0        0                                                [::]:51820                                                [::]:*
  1. I have looked at the following answers and did not find a solution:

After searching the web about port 51820 I found out it is the "wireguard" I have installed that`s listening on that port, but the question remains the same as in any command I have tried I could not find that out.

Best Answer

You're running operating system that implements Wireguard as a kernel module. Your kernel is listening on this port, and since there's no process owning that socket no PID is reported by netstat.

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