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.
ISO Plain Language
On this page
§1An Active Users Service
RFC 866 defines the Active Users service and calls it a useful debugging and measurement tool. It 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 on the ARPA Internet that choose to implement an Active Users Protocol are expected to adopt and implement this standard.
It is a service that reports the names of everyone logged in, whatever you ask it. That makes it handy for checking a remote machine.
§2The TCP and UDP Services
Over TCP, a server listens on TCP port 11. Once a connection is established, a list of the currently active users is sent out the connection, any data received is thrown away, and the service closes the connection after sending the list.
Over UDP, a server listens on UDP port 11 and answers each received datagram with a datagram containing the list, ignoring the data it received. If the list does not fit in one datagram, the server sends a sequence of datagrams but must not break the information for one user, that is one line, across a datagram. The user side should wait for a timeout for all the datagrams to arrive, and they are not necessarily in order.
If the list is too long for one message, the server sends several. Each name stays whole in one message, and the messages may arrive out of order.
§3User List Syntax
There is no specific syntax for the user list. The RFC recommends that it be limited to the ASCII printing characters, space, carriage return and line feed, and that each user be listed on a separate line.
The reply is plain text with one person per line, like a sign-in sheet.