Port 53 — DNS

Port 53 — TCP/UDP

DNS name resolution over UDP and TCP, including TCP for zone transfers and some larger responses.

What is port 53 used for?

DNS queries and authorized zone transfers.

Transport protocol

TCP/UDP

Security precautions

Avoid open recursive resolvers and restrict DNS zone transfers.

Should we open this port in the firewall?

Allow this port only when a required service uses it. Apply least privilege: restrict authorized sources, destinations and interfaces, and log connections where appropriate.

How do I check this port?

Test-NetConnection SERVER -Port 53

Test-NetConnection SERVER -Port 53 and nc -vz SERVER 53 check TCP only. If the service uses UDP, reproduce a real application exchange and inspect logs or a packet capture for UDP 53; a TCP success does not validate UDP.

Port-to-service mappings are common conventions. Applications can use a different configured port; confirm the actual service and transport.