Active users
A service on port 11 that lists who is currently logged in to a host.
Postel, J., "Active users", STD 24, RFC 866, DOI 10.17487/RFC0866, May 1983, <https://www.rfc-editor.org/info/rfc866>.
@misc{rfc866,
series = {Request for Comments},
number = 866,
howpublished = {RFC 866},
publisher = {RFC Editor},
doi = {10.17487/RFC0866},
url = {https://www.rfc-editor.org/info/rfc866},
author = {J. Postel},
title = {{Active users}},
pagetotal = 1,
year = 1983,
month = may,
} RFC text
Network Working Group J. Postel
Request for Comments: 866 ISI
May 1983
Active Users
This RFC specifies a standard for the ARPA Internet community. Hosts on
the ARPA Internet that choose to implement an Active Users Protocol are
expected to adopt and implement this standard.
A useful debugging and measurement tool is an active users service. An
active users service simply sends a list of the currently active users
on the host without regard to the input.
An active user is one logged in, such as listed in SYSTAT or WHO.
TCP Based Active Users Service
One active users service is defined as a connection based application
on TCP. A server listens for TCP connections on TCP port 11. Once a
connection is established a list of the currently active users is
sent out the connection (and any data received is thrown away). The
service closes the connection after sending the list.
UDP Based Active Users Service
Another active users service service is defined as a datagram based
application on UDP. A server listens for UDP datagrams on UDP port
11. When a datagram is received, an answering datagram is sent
containing a list of the currently active users (the data in the
received datagram is ignored).
If the list does not fit in one datagram then send a sequence of
datagrams but don't break the information for a user (a line) across
a datagram. The user side should wait for a timeout for all
datagrams to arrive. Note that they are not necessarily in order.
User List Syntax
There is no specific syntax for the user list. It is recommended
that it be limited to the ASCII printing characters, space, carriage
return, and line feed. Each user should be listed on a separate
line.
AI ELI5
On this page
§1An Active Users Service
RFC 866 defines the Active Users service. The RFC calls it a useful debugging and measurement tool that simply sends a list of the currently active users on the host without regard to the input.
An active user is one who is logged in, such as listed in SYSTAT or WHO. Hosts that choose to implement the protocol are expected to adopt this standard.
It is a machine that tells you the names of everyone logged in, no matter what you ask it.
§2The TCP and UDP Services
Over TCP, a server listens on TCP port 11. Once a connection is established it sends the list of active users and discards any data it receives. The service then closes the connection.
Over UDP, a server listens on UDP port 11 and answers each received datagram with a list of active users, ignoring the received data. If the list does not fit in one datagram, the server sends a sequence of datagrams but must not break a user's line across two of them. The user side should wait for a timeout for all datagrams to arrive, and they are not necessarily in order.
If the list is too long for one note, the machine sends several notes. Each name stays whole on one note, and the notes may arrive mixed up.
§3User List Syntax
There is no specific syntax for the user list. The RFC recommends limiting it to the ASCII printing characters, space, carriage return and line feed, with each user listed on a separate line.
The list is plain text with one person per line, like names on a roll call.