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 pro-
grams (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 struc-
ture declared in the include file (note that this is only
one of several definitions 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 '\0' if they are shorter than the size of the
field.
The first entries ever created result from init(8) process-
ing 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, tel-
netd(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 termi-
nal 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
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 allo-
cates 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 cor-
rupted 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 informa-
tive 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 dis-
abled 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 got created.
BUGS
This manpage is based on the libc5 one, things may work dif-
ferently now.
SEE ALSO
ac(1), date(1), getutent(3), init(8), last(1), login(1),
updwtmp(3), who(1)