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
  2. ^ "systat and netstat". eTutorials.