utmp
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 (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 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. */
of the field.
The first entries ever created result from init(8) pro
cessing 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_PRO
CESS.
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 'finger: 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 indi
cates 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 ter
minal 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 cre
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 infor
mative 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 sys
tems include a patched libc5 which uses the new utmp for
mat. 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)
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