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RedHat 9 (Linux i386) - man page for utmp (redhat section 5)

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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 system, because not all programs use utmp log-
       ging.

       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 ver-
       sion 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 termi-
       nated by '\0' 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 init-
       tab, 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  '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 indicates 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 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 con-
       figuration 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 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  docu-
       mented 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)
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