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Ubuntu - Networking - Ports - Port already in use

Check what is listening on the port.

sudo lsof -ni :53

returns:

COMMAND    PID            USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
systemd-r 1019 systemd-resolve   12u  IPv4  45566      0t0  UDP 127.0.0.53:domain 
systemd-r 1019 systemd-resolve   13u  IPv4  45567      0t0  TCP 127.0.0.53:domain (LISTEN)

sudo ss -lp "sport = :domain"

returns:

Netid                               State                                Recv-Q                               Send-Q                                                             Local Address:Port                                                               Peer Address:Port                               Process                                                                   
udp                                 UNCONN                               0                                    0                                                                  127.0.0.53%lo:domain                                                                  0.0.0.0:*                                   users:(("systemd-resolve",pid=1019,fd=12))                               
tcp                                 LISTEN                               0                                    4096                                                               127.0.0.53%lo:domain                                                                  0.0.0.0:*                                   users:(("systemd-resolve",pid=1019,fd=13))