What Port Does DNS Use?

DNS uses port 53 over UDP and TCP. It usually uses UDP for quick lookups, switching to TCP when the response is too large (the size of the query packet is more than 512 bytes) or when an issue arises. DNS servers handle both types of requests on port 53.

What Happens When Port 53 Is Blocked?

If port 53 is blocked you can experience these errors:

• DNS resolution fails: Your device can't resolve domain names to IP addresses.
• Internet access breaks: Since most online activity relies on DNS, you'll lose access to websites.
• Apps may not work: Services relying on name resolution will time out or fail.

Firewalls and security software must allow UDP and TCP traffic on port 53 for DNS to function properly.

Comparison Between TPC and UDP