Which port is commonly used by zone transfers when TCP is required?

Which port is commonly used by zone transfers when TCP is required?

• A. UDP 53
• B. TCP 53
• C. TCP 80
• D. UDP 443

Answer: (B)

Zone transfers involve copying the DNS zone data from a primary server to backups, and they must arrive reliably in order. DNS uses port 53 for transport, but normal queries typically ride over UDP. When a transfer is needed, the transfer uses TCP on port 53 to ensure reliability, complete delivery, and proper sequencing of the records. The other ports listed correspond to different services (for example, UDP 53 is for standard DNS queries, while 80 and 443 are used for HTTP/HTTPS), so the correct choice is TCP on port 53.

Zone transfers involve copying the DNS zone data from a primary server to backups, and they must arrive reliably in order. DNS uses port 53 for transport, but normal queries typically ride over UDP. When a transfer is needed, the transfer uses TCP on port 53 to ensure reliability, complete delivery, and proper sequencing of the records. The other ports listed correspond to different services (for example, UDP 53 is for standard DNS queries, while 80 and 443 are used for HTTP/HTTPS), so the correct choice is TCP on port 53.