Port 53 (TCP/UDP) — DNS

Domain Name System queries.

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An open recursive resolver can be abused for DDoS amplification. Restrict recursion to your own networks.

Check if port 53 is open

On the server, see what is listening:

sudo ss -tulpn | grep ':53 '

From another machine, test reachability:

nc -zv your-server.example.com 53

With UFW (Ubuntu/Debian):

sudo ufw deny 53

With firewalld (RHEL/AlmaLinux/Rocky):

sudo firewall-cmd --permanent --remove-port=53/tcp && sudo firewall-cmd --reload