Disable systemd-resolved

disable-systemd-resolved-service.md

Check and see if port 53 is taken:

lsof -i :53

OR

netstat -tulpn | grep ":53 "

Disable and stop the systemd-resolved service:

sudo systemctl disable systemd-resolved.service
sudo systemctl stop systemd-resolved.service

same

On casaos/rpi, the port 53 is occupied by docker-proxy process and not systems. Hence the commands wont work, I'm afraid to kill docker-proxy, what if it takes down casaos? Any expert please help

$ sudo netstat -tulpn | grep ":53 "
tcp        0      0 0.0.0.0:53              0.0.0.0:*               LISTEN      1152/docker-proxy   
tcp6       0      0 :::53                   :::*                    LISTEN      1166/docker-proxy   
udp        0      0 0.0.0.0:53              0.0.0.0:*                           1182/docker-proxy   
udp6       0      0 :::53                   :::*                                1198/docker-proxy 

can confirm it will kill casaos and your wifi in general, if you happen to make the same mistake use this, Run (resolvectl status) (or systemd-resolve --status in older versions of systemd). The output should look like this:

you are running AdGuard Home via Docker and it is, in fact, directly exposing port 53 (TCP and UDP) for all interfaces (0.0.0.0 and ::), so the docker-proxy process appears to be occupying that port.

This is normal and safe in this case, as it is AdGuard Home that is using port 53 through Docker.