Which port and protocol are used for a DNS zone transfer?

Which port and protocol are used for a DNS zone transfer?

• A. UDP 53
• B. TCP 53
• C. TCP 22
• D. UDP 80

Answer: (B)

Transferring an entire DNS zone between servers needs a reliable, in-order delivery channel, which TCP provides. DNS uses port 53 for its services, and for zone transfers the transport is TCP on that same port. UDP on port 53 is used for regular DNS queries, but it isn’t reliable enough for transferring the full zone data and can drop packets or lose order, which would corrupt the zone. The other options—TCP on a different port (like 22) or UDP on a non-DNS port (like 80)—don’t apply to how DNS zone data is moved between servers.

Transferring an entire DNS zone between servers needs a reliable, in-order delivery channel, which TCP provides. DNS uses port 53 for its services, and for zone transfers the transport is TCP on that same port. UDP on port 53 is used for regular DNS queries, but it isn’t reliable enough for transferring the full zone data and can drop packets or lose order, which would corrupt the zone. The other options—TCP on a different port (like 22) or UDP on a non-DNS port (like 80)—don’t apply to how DNS zone data is moved between servers.