utmp, wtmp - login records
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 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 */
#define EMPTY 0 /* Record does not contain valid info
(formerly known as UT_UNKNOWN on Linux) */
#define RUN_LVL 1 /* Change in system run-level (see
init(1)) */
#define BOOT_TIME 2 /* Time of system boot (in ut_tv) */
#define NEW_TIME 3 /* Time after system clock change
(in ut_tv) */
#define OLD_TIME 4 /* Time before system clock change
(in ut_tv) */
#define INIT_PROCESS 5 /* Process spawned by init(1) */
#define LOGIN_PROCESS 6 /* Session leader process for user login */
#define USER_PROCESS 7 /* Normal process */
#define DEAD_PROCESS 8 /* Terminated process */
#define ACCOUNTING 9 /* Not implemented */
#define UT_LINESIZE 32
#define UT_NAMESIZE 32
#define UT_HOSTSIZE 256
struct exit_status { /* Type for ut_exit, below */
short e_termination; /* Process termination status */
short e_exit; /* Process exit status */
};
struct utmp {
short ut_type; /* Type of record */
pid_t ut_pid; /* PID of login process */
char ut_line[UT_LINESIZE]; /* Device name of tty - "/dev/" */
char ut_id[4]; /* Terminal name suffix,
or inittab(5) ID */
char ut_user[UT_NAMESIZE]; /* Username */
char ut_host[UT_HOSTSIZE]; /* Hostname for remote login, or
kernel version for run-level
messages */
struct exit_status ut_exit; /* Exit status of a process
marked as DEAD_PROCESS; not
used by Linux init(1) */
/* The ut_session and ut_tv fields must be the same size when
compiled 32- and 64-bit. This allows data files and shared
memory to be shared between 32- and 64-bit applications. */
#if __WORDSIZE == 64 && defined __WORDSIZE_COMPAT32
int32_t ut_session; /* Session ID (getsid(2)),
used for windowing */
struct {
int32_t tv_sec; /* Seconds */
int32_t tv_usec; /* Microseconds */
} ut_tv; /* Time entry was made */
#else
long ut_session; /* Session ID */
struct timeval ut_tv; /* Time entry was made */
#endif
int32_t ut_addr_v6[4]; /* Internet address of remote
host; IPv4 address uses
just ut_addr_v6[0] */
char __unused[20]; /* Reserved for future use */
};
/* Backward 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 a null byte ('\0') if they are shorter than the size of the field.
The first entries ever created result from init(1)
processing inittab(5). Before an entry is processed,
though, init(1) 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(1) 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(1) 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 following /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 maintained by login(1),
init(1), 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.
/var/run/utmp
/var/log/wtmp
POSIX.1 does not specify a utmp structure, but rather one
named utmpx (as part of the XSI extension), 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.
Linux.
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 configuration 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 System 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.
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;