Systat (protocol)

Systat or Active Users is a simple Internet protocol ostensibly useful for "debugging and measurement".^[1] A connection to port 11, by either TCP or UDP, elicits a list of users currently logged into the system. Though it remains an official Internet protocol, its use is considered a security vulnerability.^[2]

References

1. ↑ Postel, J. (May 1983). Active Users. IETF. RFC 866. https://tools.ietf.org/html/rfc866. 
2. ↑ "systat and netstat". eTutorials.