utmp
Section: File Formats (5)
Updated: 202-0-21
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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 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 I]ut_tv]) */
#define NEW_TIME 3 /* Time after system clock change
(in I]ut_tv]) */
#define OLD_TIME 4 /* Time before system clock change
(in I]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 ([aq][rs]0[aq])
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
[ti]
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, recor-keeping is turned off.
FILES
/var/run/utmp
/var/log/wtmp
VERSIONS
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.
STANDARDS
Linux.
HISTORY
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/BS-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.
NOTES
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 machin-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
3-bit and 6-bit applications (x8-64, ppc64, s390x, etc.),
ut_tv
is the same size in 3-bit mode as in 6-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
3-bit and 6-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;
SEE ALSO
ac(1),
date(1),
init(1),
last(1),
login(1),
logname(1),
lslogins(1),
users(1),
utmpdump(1),
who(1),
getutent(3),
getutmp(3),
login(3),
logout(3),
logwtmp(3),
updwtmp(3)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- FILES
-
- VERSIONS
-
- STANDARDS
-
- HISTORY
-
- NOTES
-
- SEE ALSO
-
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