DNS Not Just UDP

Let me explain.

UDP on port 53 is the default, Simple, fast, lightweight.

Why Simple, fast, lightweight?

Needs only 2 packets :

• 1 packet for query
• 1 packet for response

TCP on port 53

Needs 7 packets :

• 3 packets for the handshake
• 2 packets for the query/response
• 2 packets for connection teardown

When TCP comes into play

• UDP packets can’t be greater than 512 bytes. If a response is bigger, the query switches to TCP.
• Zone transfers need reliability and completeness, so they use TCP.
• If UDP traffic is blocked (e.g., by an ISP), DNS falls back to TCP.