A client packet that specifies a destination port of 53 is requesting which service?

When a client packet specifies a destination port of 53, it is requesting Domain Name System (DNS) services. DNS is a protocol used for resolving domain names into IP addresses, allowing users to access websites using human-readable names rather than numerical IP addresses.

Port 53 is specifically designated for DNS requests, which can occur over both UDP (User Datagram Protocol) and TCP (Transmission Control Protocol). UDP is generally used for typical DNS queries as it allows quicker, connectionless communication, while TCP is utilized for tasks that require greater reliability, such as zone transfers between DNS servers.

Understanding that each of the other services uses different ports clarifies why they are not applicable here. HTTP typically operates over port 80, FTP uses ports 20 and 21, and SSH operates on port 22. Consequently, a destination port of 53 unmistakably indicates a request for DNS services.