Which utility should a system administrator use to check remotely accessible services before upgrading an OS?

Using a port scanner is the most effective method for a system administrator to check remotely accessible services prior to upgrading an operating system. This utility helps identify which ports are open on a server, allowing the administrator to confirm that the necessary services are accessible and functioning properly. By scanning the ports, the administrator can get a list of active services and determine if any potential issues may arise from the OS upgrade, as it will identify services that need to be configured or monitored closely post-upgrade.

A cipher program is primarily used for encryption and decryption of files, which wouldn't help in evaluating network services. The telnet client enables manual connections to services on specific ports but is less efficient for scanning multiple ports or services simultaneously. A packet sniffer captures and analyzes traffic on the network, which can provide insights into active services but doesn’t pinpoint which services are listening on each port, making it less practical for a pre-upgrade check. Therefore, a port scanner serves the targeted purpose of assessing accessible services effectively before making significant changes like an OS upgrade.