How to Diagnose and Fix Port 53 DNS Conflicts on Linux

Phenomenon

On Linux, port 53 is used for DNS resolution. Example netstat output shows the systemd-resolve daemon listening on both UDP and TCP on the local loopback interface.

systemd-r 20245 systemd-resolve   13u  IPv4 216305        localhost:domain
systemd-r 20245 systemd-resolve   14u  IPv4 216306        localhost:domain (LISTEN)

Analysis

UDP socket

File descriptor 13 belongs to systemd-resolve.

It is a UDP socket bound to localhost (127.0.0.1) on port 53.

TCP listener

File descriptor 14 belongs to systemd-resolve.

It is a TCP socket in LISTEN state bound to localhost on port 53.

Service explanation

The systemd-resolve daemon provides local DNS resolution. It listens on UDP for standard queries and on TCP for larger or more complex responses.

Solution

To verify which process occupies port 53, run: sudo netstat -nultp If the output confirms that systemd-resolved is using the port, you can reconfigure or disable it:

Stop the service: sudo systemctl stop systemd-resolved Edit the configuration file /etc/systemd/resolved.conf (e.g., with vi) and set the desired options, for example:

[Resolve]
DNS=223.5.5.5
DNSStubListener=no

Save the file and exit the editor.

Re‑create the /etc/resolv.conf symbolic link so that it points to the systemd resolver configuration:

sudo ln -sf /run/systemd/resolve/resolv.conf /etc/resolv.conf

Conclusion

Port 53 is normally occupied by a DNS service such as systemd-resolve. Other DNS servers (e.g., BIND, dnsmasq) may also bind to this port. Understanding the netstat output enables administrators to monitor DNS activity and adjust configurations when necessary.