Which utility should a system administrator use to assess default enabled remotely accessible services before an OS upgrade?

Using a port scanner is the best choice for assessing default enabled remotely accessible services prior to an operating system upgrade. A port scanner is a tool that allows a system administrator to discover open ports on a server or networked device. By scanning the device, the administrator can identify which services are running and accessible from outside the host. This becomes particularly important before an OS upgrade because it helps ensure that any services that may still be required are accounted for and that the upgrade will not unintentionally disrupt them.

In the context of system security, knowing which ports are open and what services are listening on those ports helps the administrator evaluate potential security vulnerabilities that could be exposed due to outdated configurations or services that need to be updated or disabled. Furthermore, this information can assist in planning the upgrade process, ensuring minimal disruption while maintaining network integrity.

Other options, like the Cipher program, facilitate encryption operations but do not provide information on services running on a system. A Telnet client, while useful for connecting to services, does not scan for open ports; it connects to a specified port. Packet sniffers analyze traffic across a network rather than assessing services on the local machine. Thus, a port scanner provides targeted functionality essential for the administrator's tasks before an OS upgrade.