DNS commonly uses which transport protocols?

DNS commonly uses which transport protocols?

• A. ICMP
• B. TCP and UDP
• C. UDP only
• D. TCP only

Answer: (B)

DNS uses both UDP and TCP as transport protocols. For most queries, UDP is used because it’s fast and lightweight—messages fit in a single datagram and no connection is needed, which makes DNS lookups quick. However, not all responses fit in a single UDP packet, and in those cases DNS switches to TCP on port 53 to ensure reliable delivery and to handle larger responses. TCP is also required for zone transfers between DNS servers and for secure or larger responses when DNSSEC is involved. When a UDP response would be truncated, the TC bit signals the client to retry over TCP. ICMP isn’t used as the transport for DNS queries, and saying DNS uses UDP only or TCP only would miss these situations where the other protocol is used.

DNS uses both UDP and TCP as transport protocols. For most queries, UDP is used because it’s fast and lightweight—messages fit in a single datagram and no connection is needed, which makes DNS lookups quick. However, not all responses fit in a single UDP packet, and in those cases DNS switches to TCP on port 53 to ensure reliable delivery and to handle larger responses. TCP is also required for zone transfers between DNS servers and for secure or larger responses when DNSSEC is involved. When a UDP response would be truncated, the TC bit signals the client to retry over TCP. ICMP isn’t used as the transport for DNS queries, and saying DNS uses UDP only or TCP only would miss these situations where the other protocol is used.