Systat or Active Users is a simple Internet protocol ostensibly useful for "debugging and measurement". 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.
- ^ Postel, J. (May 1983). Active Users. IETF. doi:10.17487/RFC0866. RFC 866.
- ^ "systat and netstat". eTutorials.