====== Ubuntu - Networking - Ports - Port already in use ======

Check what is listening on the port.

<code bash>
sudo lsof -ni :53
</code>

returns:

<code bash>
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)
</code>

<WRAP info>
**NOTE:**  This shows it is being used by systemd-resolve.
</WRAP>


----

<code bash>
sudo ss -lp "sport = :domain"
</code>

returns:

<code bash>
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))  
</code>

<WRAP info>
**NOTE:**  This shows it is being used by systemd-resolve.
</WRAP>

----

===== Disable any service that is running on this port =====

<code bash>
sudo systemctl stop systemd-resolved
sudo systemctl disable systemd-resolved
sudo systemctl mask systemd-resolved

</code>

<WRAP info>
**NOTE:**  The mask is to prevent this service auto starting on reboot.
</WRAP>

----

To undo what you did:

<code bash>
sudo systemctl unmask systemd-resolved
sudo systemctl enable systemd-resolved
sudo systemctl start systemd-resolved
</code>

<WRAP info>
**NOTE:**  An alternative to the **enable** command is:

<code bash>
sudo systemctl defaults systemd-resolved
</code>

</WRAP>