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 logging.

Warning: utmp must not be writable, because many system programs depend on its integrity. You risk faked system logfiles and modifications of system files if you leave utmp writable to any user.

The file is a sequence of entries with the following structure declared in the include file:

    #define UT_UNKNOWN	0
    #define RUN_LVL		1
    #define BOOT_TIME	2
    #define NEW_TIME	3
    #define OLD_TIME	4
    #define INIT_PROCESS	5
    #define LOGIN_PROCESS	6
    #define USER_PROCESS	7
    #define DEAD_PROCESS	8
    #define UT_LINESIZE	12
    #define UT_NAMESIZE	8
    #define UT_HOSTSIZE	16
    struct utmp {
      short ut_type;	/* type of login */
      pid_t ut_pid;		/* pid of process */
      char ut_line[UT_LINESIZE];	/* device name of tty - "/dev/" */
      char ut_id[2];	/* init id or abbrev. ttyname */
      time_t ut_time;	/* login time */
      char ut_user[UT_NAMESIZE];	/* user name */
      char ut_host[UT_HOSTSIZE];	/* host name for remote login */
      long ut_addr;	/* IP addr of remote host */
    };

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 terminated by '\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 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.

getty(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(8), after a user has been authenticated, changes ut_type to USER_PROCESS, changes ut_time and sets ut_host and ut_addr. Depending on getty(8) and login(8), 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 last two letters of /dev/ttyp%c or by using p%d for /dev/pts/%d. 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.

xdm(8) should not create an utmp record, because there is no assigned terminal. Letting it create one will result in trouble like: finger: can not stat /dev/machine.dom. It should create wtmp entries, though, just like ftpd(8) does.

telnetd(8) sets up a LOGIN_PROCESS entry and leaves the rest to login(8) 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 user name indicates a logout on the associated terminal. Furthermore, the terminal name "~" with user name "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 maintained by login(1), and init(1) and some very of getty(1). Neither 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
Linux utmp entries conform neither to v7/BSD nor to SYSV: 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 configuration file which allocates slots to sessions. BSD does so, because it lacks ut_id fields. In Linux (as in SYSV), 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 and potential security holes. Clearing the above mentioned fields by filling them with null bytes is not required by SYSV semantics, but it allows 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. SYSV only uses the type field to mark them and logs informative messages such as e.g. "new time" in the line field. SYSV has one more field to log the exit status of dead processes. UT_UNKNOWN seems to be a Linux invention. There is no type ACCOUNTING in Linux. SYSV has no ut_host or ut_addr fields. 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.

RESTRICTIONS
The file format is machine dependent, so it is recommended that it be processed only on the machine architecture where it got created.

SEE ALSO
ac(1), date(1), last(1), login(1), who(1), getutent(3), init(8)