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Diffstat (limited to 'man/man5/utmp.5')
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diff --git a/man/man5/utmp.5 b/man/man5/utmp.5 new file mode 100644 index 0000000..2f3c07e --- /dev/null +++ b/man/man5/utmp.5 @@ -0,0 +1,348 @@ +.\" Copyright (c) 1993 Michael Haardt (michael@cantor.informatik.rwth-aachen.de), +.\" Fri Apr 2 11:32:09 MET DST 1993 +.\" +.\" SPDX-License-Identifier: GPL-2.0-or-later +.\" +.\" Modified 1993-07-25 by Rik Faith (faith@cs.unc.edu) +.\" Modified 1995-02-26 by Michael Haardt +.\" Modified 1996-07-20 by Michael Haardt +.\" Modified 1997-07-02 by Nicolás Lichtmaier <nick@debian.org> +.\" Modified 2004-10-31 by aeb, following Gwenole Beauchesne +.TH utmp 5 2024-05-02 "Linux man-pages (unreleased)" +.SH NAME +utmp, wtmp \- login records +.SH SYNOPSIS +.nf +.B #include <utmp.h> +.fi +.SH DESCRIPTION +The +.I 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. +.P +.B Warning: +.I 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 +.I utmp +writable to any user other than the owner and group owner of the file. +.P +The file is a sequence of +.I utmp +structures, +declared as follows in +.I <utmp.h> +(note that this is only one of several definitions +around; details depend on the version of libc): +.P +.in +4n +.EX +/* 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 + \fBinit\fP(1)) */ +#define BOOT_TIME 2 /* Time of system boot (in \fIut_tv\fP) */ +#define NEW_TIME 3 /* Time after system clock change + (in \fIut_tv\fP) */ +#define OLD_TIME 4 /* Time before system clock change + (in \fIut_tv\fP) */ +#define INIT_PROCESS 5 /* Process spawned by \fBinit\fP(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 (\fBgetsid\fP(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] +.EE +.in +.P +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 +.BR time (2). +String fields are terminated by a null byte (\[aq]\e0\[aq]) +if they are shorter than the size +of the field. +.P +The first entries ever created result from +.BR init (1) +processing +.BR inittab (5). +Before an entry is processed, though, +.BR init (1) +cleans up utmp by setting \fIut_type\fP to \fBDEAD_PROCESS\fP, clearing +\fIut_user\fP, \fIut_host\fP, and \fIut_time\fP with null bytes for each +record which \fIut_type\fP is not \fBDEAD_PROCESS\fP or \fBRUN_LVL\fP +and where no process with PID \fIut_pid\fP exists. +If no empty record +with the needed \fIut_id\fP can be found, +.BR init (1) +creates a new one. +It sets \fIut_id\fP from the inittab, \fIut_pid\fP and \fIut_time\fP to the +current values, and \fIut_type\fP to \fBINIT_PROCESS\fP. +.P +.BR mingetty (8) +(or +.BR agetty (8)) +locates the entry by the PID, changes \fIut_type\fP to +\fBLOGIN_PROCESS\fP, changes \fIut_time\fP, sets \fIut_line\fP, and waits +for connection to be established. +.BR login (1), +after a user has been +authenticated, changes \fIut_type\fP to \fBUSER_PROCESS\fP, changes +\fIut_time\fP, and sets \fIut_host\fP and \fIut_addr\fP. +Depending on +.BR mingetty (8) +(or +.BR agetty (8)) +and +.BR login (1), +records may be located by +\fIut_line\fP instead of the preferable \fIut_pid\fP. +.P +When +.BR init (1) +finds that a process has exited, it locates its utmp entry by +.IR ut_pid , +sets +.I ut_type +to +.BR DEAD_PROCESS , +and clears +.IR ut_user , +.IR ut_host , +and +.I ut_time +with null bytes. +.P +.BR xterm (1) +and other terminal emulators directly create a +\fBUSER_PROCESS\fP record and generate the \fIut_id\fP by using the +string that suffix part of the terminal name (the characters +following +.IR /dev/ [pt] ty ). +If they find a \fBDEAD_PROCESS\fP for this ID, +they recycle it, otherwise they create a new entry. +If they can, they +will mark it as \fBDEAD_PROCESS\fP on exiting and it is advised that +they null \fIut_line\fP, \fIut_time\fP, \fIut_user\fP, and \fIut_host\fP +as well. +.P +.BR telnetd (8) +sets up a \fBLOGIN_PROCESS\fP entry and leaves the rest to +.BR login (1) +as usual. +After the telnet session ends, +.BR telnetd (8) +cleans up utmp in the described way. +.P +The \fIwtmp\fP file records all logins and logouts. +Its format is exactly like \fIutmp\fP except that a null username +indicates a logout +on the associated terminal. +Furthermore, the terminal name \fB\[ti]\fP +with username \fBshutdown\fP or \fBreboot\fP indicates a system +shutdown or reboot and the pair of terminal names \fB|\fP/\fB}\fP +logs the old/new system time when +.BR date (1) +changes it. +\fIwtmp\fP is maintained by +.BR login (1), +.BR init (1), +and some versions of +.BR getty (8) +(e.g., +.BR mingetty (8) +or +.BR agetty (8)). +None of these programs creates the file, so if it is +removed, record-keeping is turned off. +.SH FILES +.I /var/run/utmp +.br +.I /var/log/wtmp +.SH VERSIONS +POSIX.1 does not specify a +.I utmp +structure, but rather one named +.I utmpx +(as part of the XSI extension), +with specifications for the fields +.IR ut_type , +.IR ut_pid , +.IR ut_line , +.IR ut_id , +.IR ut_user , +and +.IR ut_tv . +POSIX.1 does not specify the lengths of the +.I ut_line +and +.I ut_user +fields. +.P +Linux defines the +.I utmpx +structure to be the same as the +.I utmp +structure. +.SH STANDARDS +Linux. +.SH HISTORY +Linux utmp entries conform neither to v7/BSD nor to System V; they are a +mix of the two. +.P +v7/BSD has fewer fields; most importantly it lacks +\fIut_type\fP, 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 \fIut_id\fP fields. +.P +In Linux (as in System V), the \fIut_id\fP field of a +record will never change once it has been set, which reserves that slot +without needing a configuration file. +Clearing \fIut_id\fP 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. +.P +.\" mtk: What is the referrent of "them" in the following sentence? +.\" System V only uses the type field to mark them and logs +.\" informative messages such as \fB"new time"\fP in the line field. +System V has no \fIut_host\fP or \fIut_addr_v6\fP fields. +.SH 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 +.BR who (1), +then do not make utmp world readable. +.P +The file format is machine-dependent, so it is recommended that it be +processed only on the machine architecture where it was created. +.P +Note that on \fIbiarch\fP platforms, that is, systems which can run both +32-bit and 64-bit applications (x86-64, ppc64, s390x, etc.), +\fIut_tv\fP is the same size in 32-bit mode as in 64-bit mode. +The same goes for \fIut_session\fP and \fIut_time\fP 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 +.I ut_session +to +.IR int32_t , +and that of +.I ut_tv +to a struct with two +.I int32_t +fields +.I tv_sec +and +.IR tv_usec . +Since \fIut_tv\fP may not be the same as \fIstruct timeval\fP, +then instead of the call: +.P +.in +4n +.EX +gettimeofday((struct timeval *) &ut.ut_tv, NULL); +.EE +.in +.P +the following method of setting this field is recommended: +.P +.in +4n +.EX +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; +.EE +.in +.\" .P +.\" Note that the \fIutmp\fP struct from libc5 has changed in libc6. +.\" Because of this, +.\" binaries using the old libc5 struct will corrupt +.\" .IR /var/run/utmp " and/or " /var/log/wtmp . +.\" .SH BUGS +.\" This man page is based on the libc5 one, things may work differently now. +.SH SEE ALSO +.BR ac (1), +.BR date (1), +.BR init (1), +.BR last (1), +.BR login (1), +.BR logname (1), +.BR lslogins (1), +.BR users (1), +.BR utmpdump (1), +.BR who (1), +.BR getutent (3), +.BR getutmp (3), +.BR login (3), +.BR logout (3), +.BR logwtmp (3), +.BR updwtmp (3) |