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.