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Systat
Systat is a legacy network service running on port 11, historically used to provide information about active users logged into a system. Initially part of the suite of early Internet services, it queried the system's status and relayed basic data about logged-in accounts. Today, it is primarily obsolete and carries significant security risks..
The Systat protocol was defined in RFC 866 and ran primarily over TCP and UDP port 11. It was implemented on early UNIX systems as part of the BSD r-services suite to provide basic status information of users currently logged into a machine. This lightweight protocol responds with a plaintext list of logged-in users and their terminal information when queried.
Despite its simplicity, Systat served as an important administrative tool in the early days of network management, allowing system administrators to remotely monitor logged-in sessions without needing full shell access. It helped foster collaborative environments by enabling users to see who else was active on multi-user systems.
However, with the advent of more secure and feature-rich solutions (such as SSH and SNMP) and the protocol's inherent lack of access controls or encryption, Systat has fallen out of favor. Most modern systems disable this service by default due to security concerns and limited practical value.