Active users
also
- std-24
Extracted elements (9)
The Active Users service is described as a useful debugging and measurement tool. The protocol deliberately ignores all client input to keep the service simple and one-directional.
UDP datagrams carrying the user list are not necessarily delivered in order; the client side should wait for a timeout to collect all datagrams before processing the complete list.
If the user list does not fit in a single UDP datagram, the server MUST send a sequence of datagrams without breaking a single user's entry (a line) across datagrams.
It is recommended that the user list be limited to ASCII printing characters, space, carriage return, and line feed, with each user listed on a separate line.
The Active Users service sends a list of currently active users on the host (those logged in, as listed by SYSTAT or WHO) without regard to any input received.
The Active Users protocol is assigned well-known port 11 for both TCP and UDP by this specification.
RFC 866 contains no security considerations section. The service exposes a list of currently logged-in users to any host that connects or sends a UDP datagram, with no authentication or access control, which constitutes an information disclosure risk by modern standards.
TCP-based Active Users service listens on TCP port 11. After a connection is established, the server sends the active user list and closes the connection; any data received from the client is discarded.
UDP-based Active Users service listens on UDP port 11. Upon receiving any datagram (its content is ignored), the server sends one or more answer datagrams containing the active user list.