utmp(4) Kernel Interfaces Manual utmp(4)
NAME
utmp, wtmp, btmp - user login record format
SYNOPSIS
DESCRIPTION
These files, which hold user and accounting information for such commands as and (see last(1), who(1), write(1), and login(1)), have the
following structure as defined by
#define UTMP_FILE "/etc/utmp"
#define WTMP_FILE "/var/adm/wtmp"
#define BTMP_FILE "/var/adm/btmp"
#define ut_name ut_user
struct utmp {
char ut_user[8]; /* User login name */
char ut_id[4]; /* /etc/inittab id(usually line#)*/
char ut_line[12] /* device name (console, lnxx) */
pid_t ut_pid; /* process id */
short ut_type; /* type of entry */
struct exit_status
short e_termination; /* Process termination status*/
short e_exit; /* Process exit status*/
} ut_exit; /* The exit status of a process*/
/* marked as DEAD_PROCESS.*/
unsigned short ut_reserved1; /* Reserved for future use*/
time_t ut_time; /* time entry was made*/
char ut_host[16]; /* host name,if remote*/
unsigned long ut_addr; /* host Internet addr, if remote*/
};
/* Definitions for ut_type */
#define EMPTY 0
#define RUN_LVL 1
#define BOOT_TIME 2
#define OLD_TIME 3
#define NEW_TIME 4
#define INIT_PROCESS 5 /* Process spawned by "init" */
#define LOGIN_PROCESS 6 /* getty process awaiting login */
#define USER_PROCESS 7 /* A user process */
#define DEAD_PROCESS 8
#define ACCOUNTING 9
#define UTMAXTYPE ACCOUNTING /* Max. legal value of ut_type */
/* Special strings or formats used in the "ut_line" field */
/* when accounting for something other than a process */
/* No string for the ut_line field can be more than */
/* 11 chars + a NULL in length */
#define RUNLVL_MSG "run-level %c"
#define BOOT_MSG "system boot"
#define OTIME_MSG "old time"
#define NTIME_MSG "new time"
File contains a record of all users logged onto the system. File contains bad login entries for each invalid logon attempt. File contains
a record of all logins and logouts.
Note that and tend to grow without bound, and should be checked regularly. Information that is no longer useful should be removed periodi-
cally to prevent it from becoming too large. Also note that and are not created by the programs that maintain them. Thus, if these files
are removed, record-keeping is turned off.
FILES
AUTHOR
and were developed by HP and the University of California, Berkeley.
SEE ALSO
last(1), login(1), who(1), write(1), acctcon(1M), fwtmp(1M), utmpd(1M), getut(3C), getuts(3C), getutx(3C).
STANDARDS CONFORMANCE
TO BE OBSOLETED utmp(4)
Check Out this Related Man Page
UTMP(5) File formats 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 sys-
tem, because not all programs use utmp logging.
Warning: utmp must not be writable, 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.
The file is a sequence of entries with the following structure declared in the include file (note that this is only one of several defini-
tions around; details depend on the version of libc):
#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 ACCOUNTING 9
#define UT_LINESIZE 12
#define UT_NAMESIZE 32
#define UT_HOSTSIZE 256
struct exit_status {
short int e_termination; /* process termination status. */
short int e_exit; /* process exit status. */
};
struct utmp {
short ut_type; /* type of login */
pid_t ut_pid; /* pid of login process */
char ut_line[UT_LINESIZE]; /* device name of tty - "/dev/" */
char ut_id[4]; /* init id or abbrev. ttyname */
char ut_user[UT_NAMESIZE]; /* user name */
char ut_host[UT_HOSTSIZE]; /* hostname for remote login */
struct exit_status ut_exit; /* The exit status of a process
marked as DEAD_PROCESS. */
long ut_session; /* session ID, used for windowing*/
struct timeval ut_tv; /* time entry was made. */
int32_t ut_addr_v6[4]; /* IP address of remote host. */
char pad[20]; /* Reserved for future use. */
};
/* Backwards compatibility hacks. */
#define ut_name ut_user
#ifndef _NO_UT_TIME
#define ut_time ut_tv.tv_sec
#endif
#define ut_xtime ut_tv.tv_sec
#define ut_addr 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 terminated by ' ' 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 a utmp record, because there is no assigned terminal. Letting it create one will result in errors, such as 'fin-
ger: cannot 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 asso-
ciated 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), init(1), and some
versions 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 condi-
tions 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. UT_UNKNOWN seems to be
a Linux invention. SYSV has no ut_host or ut_addr_v6 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.
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. Debian systems include a patched libc5 which uses the new utmp format. The problem still exists with wtmp since
it's accessed directly in libc5.
RESTRICTIONS
The file format is machine dependent, so it is recommended that it be processed only on the machine architecture where it was created.
BUGS
This manpage is based on the libc5 one, things may work differently now.
SEE ALSO
ac(1), date(1), getutent(3), init(8), last(1), login(1), updwtmp(3), who(1)
1997-07-02 UTMP(5)