The utmps-write program

utmps-write is a command-line generic utmp client for utmps. It sends an arbitrary utmp entry to the utmpd and/or wtmpd daemon. It can be used to test utmps installations.

Interface

     utmps-write [ -u ] [ -w ] [ -U utmpd-socket ] [ -W wtmpd-socket ] [ -t timeout ] [ -T timestamp ] [ -h host ] [ -i ip ] [ -l user ] [ -p pid ] id type line

utmps-write constructs an utmp entry of type type, with identifier id, containing the line line.
Other fields can also be manually filled via options; by default, utmps-write will put in reasonable values.
utmps-write connects to a utmpd and/or a wtmpd instance, and sends them that utmp entry for writing.
It exits 0 on success, or prints an error message on stderr.

The type argument must be symbolic: EMPTY, BOOT_TIME etc. The valid types are the symbolic constants for the ut_type field of the utmpx structure, as documented here or in the utmps/utmpx.h header provided with the utmps package.

Options

-u : add the entry to the utmp database.
-w : add the entry to the wtmp database. At least one of the -u or -w option must be given.
-U utmpd-socket : if the -u option has been given, connect to a utmpd instance listening on utmpd-socket. The default is the compile-time default, /run/utmps/.utmpd-socket or the value given to the --with-utmp-socket configure option.
-W wtmpd-socket : if the -w option has been given, connect to a wtmpd instance listening on wtmpd-socket. The default is the compile-time default, /run/utmps/.wtmpd-socket or the value given to the --with-wtmp-socket configure option.
-t timeout : if the operations have not been completed under timeout milliseconds, exit with an error message. By default, utmps-write will wait forever for an answer from the utmpd or wtmpd daemons.
-T timestamp : spoof the ut_tv field of the utmp entry. timestamp must be given as an absolute TAI64N label in external TAI64N format, prepended with a @ character. By default, ut_tv will contain the time when utmps-write was invoked.
-h host : spoof the ut_host field of the utmp entry. By default, it is empty (all null characters).
-i ip : spoof the ut_addr_v6 field of the utmp entry. ip can be given as an ipv4 or an ipv6 address. By default, it's :: (all null characters).
-l user : spoof the ut_user field of the utmp entry. This can only be done by root, otherwise the utmp or wtmp daemon will refuse to add the entry. By default, the field contains the user's name as obtained by getpwuid().
-p pid : spoof the ut_pid field of the utmp entry. By default, the field contains the pid of the utmps-write process.

Notes

There is an official API to write a complete utmp entry to the utmp or the wtmp database, and this is what utmps-write uses. However, there is no official API to read, and format, a complete utmp entry from the databases; you can read them from instance via util-linux's utmpdump utility, but you need to give it the direct path to the utmp and wtmp files.
The wtmp database can only grow; a user calling utmps-write -w repeatedly can easily make it grow fast and indefinitely, using up all the available disk space. This is a fundamental problem with the design of utmp, and is already achievable without the use of utmps-write. The only solution is for administrators to detect fast-growing wtmp files, and clean them up or archive them.