man wtmp/utmp. Most systems give you a 'last' or 'lastlog' userspace tool.


utmp(4) File Formats utmp(4)

NAME utmp, wtmp - utmp and wtmp database entry formats

SYNOPSIS #include <utmp.h> /var/adm/utmp /var/adm/wtmp

DESCRIPTION The utmp and wtmp database files are obsolete and are no longer present on the system. They have been superseded by the extended database contained in the utmpx and wtmpx data- base files. See utmpx(4). It is possible for /var/adm/utmp to reappear on the system. This would most likely occur if a third party application that still uses utmp recreates the file if it finds it miss- ing. This file should not be allowed to remain on the system. The user should investigate to determine which application is recreating this file.

SEE ALSO utmpx(4) SunOS 5.11

22 Feb 1999 utmp(4)


OR


UTMP(5) Linux Programmer's Manual UTMP(5)

NAME utmp, wtmp - login records

SYNOPSIS #include <utmp.h>

DESCRIPTION The utmp file allows one to discover information about who is currently using the system. There may be more users currently using the system, because not all programs use utmp log- ging. Warning: utmp must not be writable by the user class "other", because many system programs (foolishly) depend on its integrity. You risk faked system logfiles and modifications of system files if you leave utmp writable to any user other than the owner and group owner of the file. The file is a sequence of utmp structures, declared as follows in <utmp.h> (note that this is only one of several definitions around; details depend on the version of libc): /* Values for ut_type field, below */




ut_addr_v6[0] This structure gives the name of the special file associated with the user's terminal, the user's login name, and the time of login in the form of time(2). String fields are termi- nated by a null byte ('\0') if they are shorter than the size of the field. The first entries ever created result from init(8) processing inittab(5). Before an entry is processed, though, init(8) cleans up utmp by setting ut_type to DEAD_PROCESS, clearing ut_user, ut_host, and ut_time with null bytes for each record which ut_type is not DEAD_PROCESS or RUN_LVL and where no process with PID ut_pid exists. If no empty record with the needed ut_id can be found, init(8) creates a new one. It sets ut_id from the inittab, ut_pid and ut_time to the current values, and ut_type to INIT_PROCESS. mingetty(8) (or agetty(8)) locates the entry by the PID, changes ut_type to LOGIN_PROCESS, changes ut_time, sets ut_line, and waits for connection to be established. login(1), after a user has been authenticated, changes ut_type to USER_PROCESS, changes ut_time, and sets ut_host and ut_addr. Depending on mingetty(8) (or agetty(8)) and login(1), records may be located by ut_line instead of the preferable ut_pid. When init(8) finds that a process has exited, it locates its utmp entry by ut_pid, sets ut_type to DEAD_PROCESS, and clears ut_user, ut_host and ut_time with null bytes. xterm(1) and other terminal emulators directly create a USER_PROCESS record and generate the ut_id by using the string that suffix part of the terminal name (the characters fol- lowing /dev/[pt]ty). If they find a DEAD_PROCESS for this ID, they recycle it, otherwise they create a new entry. If they can, they will mark it as DEAD_PROCESS on exiting and it is advised that they null ut_line, ut_time, ut_user, and ut_host as well. telnetd(8) sets up a LOGIN_PROCESS entry and leaves the rest to login(1) as usual. After the telnet session ends, telnetd(8) cleans up utmp in the described way. The wtmp file records all logins and logouts. Its format is exactly like utmp except that a null username indicates a logout on the associated terminal. Furthermore, the terminal name ~ with username shutdown or reboot indicates a system shutdown or reboot and the pair of terminal names |/} logs the old/new system time when date(1) changes it. wtmp is main- tained by login(1), init(8), and some versions of getty(8) (e.g., mingetty(8) or agetty(8)). None of these programs creates the file, so if it is removed, record-keeping is turned off.

FILES /var/run/utmp /var/log/wtmp

CONFORMING TO POSIX.1 does not specify a utmp structure, but rather one named utmpx, with specifications for the fields ut_type, ut_pid, ut_line, ut_id, ut_user, and ut_tv. POSIX.1 does not specify the lengths of the ut_line and ut_user fields. Linux defines the utmpx structure to be the same as the utmp structure. Comparison with historical systems Linux utmp entries conform neither to v7/BSD nor to System V; they are a mix of the two. v7/BSD has fewer fields; most importantly it lacks ut_type, which causes native v7/BSD- like programs to display (for example) dead or login entries. Further, there is no con- figuration file which allocates slots to sessions. BSD does so because it lacks ut_id fields. In Linux (as in System V), the ut_id field of a record will never change once it has been set, which reserves that slot without needing a configuration file. Clearing ut_id may result in race conditions leading to corrupted utmp entries and potential security holes. Clearing the abovementioned fields by filling them with null bytes is not required by Sys- tem V semantics, but makes it possible to run many programs which assume BSD semantics and which do not modify utmp. Linux uses the BSD conventions for line contents, as documented above. System V has no ut_host or ut_addr_v6 fields.

NOTES Unlike various other systems, where utmp logging can be disabled by removing the file, utmp must always exist on Linux. If you want to disable who(1) then do not make utmp world readable. The file format is machine-dependent, so it is recommended that it be processed only on the machine architecture where it was created. Note that on biarch platforms, that is, systems which can run both 32-bit and 64-bit applications (x86-64, ppc64, s390x, etc.), ut_tv is the same size in 32-bit mode as in 64-bit mode. The same goes for ut_session and ut_time if they are present. This allows data files and shared memory to be shared between 32-bit and 64-bit applications. This is achieved by changing the type of ut_session to int32_t, and that of ut_tv to a struct with two int32_t fields tv_sec and tv_usec. Since ut_tv may not be the same as struct timeval, then instead of the call: gettimeofday((struct timeval *) &ut.ut_tv, NULL); the following method of setting this field is recommended: struct utmp ut; struct timeval tv; gettimeofday(&tv, NULL); ut.ut_tv.tv_sec = tv.tv_sec; ut.ut_tv.tv_usec = tv.tv_usec; Note that the utmp struct from libc5 has changed in libc6. Because of this, binaries using the old libc5 struct will corrupt /var/run/utmp and/or /var/log/wtmp.



BUGS This man page is based on the libc5 one, things may work differently now.

SEE ALSO ac(1), date(1), last(1), login(1), utmpdump(1), who(1), getutent(3), getutmp(3), login(3), logout(3), logwtmp(3), updwtmp(3), init(8)

COLOPHON This page is part of release 3.55 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.