Free Port 53 on Ubuntu¶
Ubuntu commonly runs the systemd-resolved DNS stub on 127.0.0.53:53. If Pi-hole, AdGuard Home, or another local DNS server must bind to port 53, disable only the stub listener and keep systemd-resolved available for upstream DNS information.
Confirm what owns port 53¶
sudo ss -luntp | grep ':53 '
resolvectl status
Do not continue until the output confirms that systemd-resolved is the conflicting listener. A different service needs a different fix.
Disable the stub listener¶
Use a drop-in instead of editing the vendor configuration in place:
sudo mkdir -p /etc/systemd/resolved.conf.d
sudo editor /etc/systemd/resolved.conf.d/disable-stub.conf
Add:
[Resolve]
DNSStubListener=no
When the stub is disabled, /etc/resolv.conf must not keep pointing at the stub file. Point it at the resolver-maintained file containing known upstream DNS servers:
sudo ln -sf /run/systemd/resolve/resolv.conf /etc/resolv.conf
sudo systemctl restart systemd-resolved
Verify before installing the new DNS service¶
sudo ss -luntp | grep ':53 ' || true
readlink -f /etc/resolv.conf
resolvectl status
getent hosts example.com
Port 53 should now be free, /etc/resolv.conf should resolve to /run/systemd/resolve/resolv.conf, and normal DNS lookup should still work.
Install or start the replacement DNS service only after these checks pass. Then repeat the socket and lookup tests.
Undo the change¶
sudo rm /etc/systemd/resolved.conf.d/disable-stub.conf
sudo ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf
sudo systemctl restart systemd-resolved
Confirm that 127.0.0.53:53 is listening again.