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1 perry 1 UTMP
2 !!!UTMP
3 NAME
4 SYNOPSIS
5 DESCRIPTION
6 FILES
7 CONFORMING TO
8 RESTRICTIONS
9 BUGS
10 SEE ALSO
11 ----
12 !!NAME
13
14
15 utmp, wtmp - login records
16 !!SYNOPSIS
17
18
19 #include
20 !!DESCRIPTION
21
22
23 The __utmp__ file allows one to discover information
24 about who is currently using the system. There may be more
25 users currently using the system, because not all programs
26 use utmp logging.
27
28
29 __Warning: utmp__ must not be writable, because many
30 system programs (foolishly) depend on its integrity. You
31 risk faked system logfiles and modifications of system files
32 if you leave __utmp__ writable to any user.
33
34
35 The file is a sequence of entries with the following
36 structure declared in the include file (note that this is
37 only one of several definitions around; details depend on
38 the version of libc):
39
40
41 #define UT_UNKNOWN 0
42 #define RUN_LVL 1
43 #define BOOT_TIME 2
44 #define NEW_TIME 3
45 #define OLD_TIME 4
46 #define INIT_PROCESS 5
47 #define LOGIN_PROCESS 6
48 #define USER_PROCESS 7
49 #define DEAD_PROCESS 8
50 #define ACCOUNTING 9
51 #define UT_LINESIZE 12
52 #define UT_NAMESIZE 32
53 #define UT_HOSTSIZE 256
54 struct exit_status {
55 short int e_termination; /* process termination status. */
56 short int e_exit; /* process exit status. */
57 };
58 struct utmp {
59 short ut_type; /* type of login */
60 pid_t ut_pid; /* pid of login process */
61 char ut_line[[UT_LINESIZE]; /* device name of tty -
62
63
64 This structure gives the name of the special file associated
65 with the user's terminal, the user's login name, and the
66 time of login in the form of time(2). String fields
67 are terminated by __'0'__ if they are shorter than the
68 size of the field.
69
70
71 The first entries ever created result from init(8)
72 processing inittab(5). Before an entry is processed,
73 though, init(8) cleans up utmp by setting
74 __ut_type__ to __DEAD_PROCESS__, clearing
75 __ut_user__, __ut_host__ and __ut_time__ with null
76 bytes for each record which __ut_type__ is not
77 __DEAD_PROCESS__ or __RUN_LVL__ and where no process
78 with PID __ut_pid__ exists. If no empty record with the
79 needed __ut_id__ can be found, init creates a new one. It
80 sets __ut_id__ from the inittab, __ut_pid__ and
81 __ut_time__ to the current values and __ut_type__ to
82 __INIT_PROCESS__.
83
84
85 getty(8) locates the entry by the pid, changes
86 __ut_type__ to __LOGIN_PROCESS__, changes
87 __ut_time__, sets __ut_line__ and waits for connection
88 to be established. login(8), after a user has been
89 authenticated, changes __ut_type__ to
90 __USER_PROCESS__, changes __ut_time__ and sets
91 __ut_host__ and __ut_addr__. Depending on
92 getty(8) and login(8), records may be located
93 by __ut_line__ instead of the preferable
94 __ut_pid__.
95
96
97 When init(8) finds that a process has exited, it
98 locates its utmp entry by __ut_pid__, sets __ut_type__
99 to __DEAD_PROCESS__ and clears __ut_user__,
100 __ut_host__ and __ut_time__ with null
101 bytes.
102
103
104 xterm(1) and other terminal emulators directly create
105 a __USER_PROCESS__ record and generate the __ut_id__
106 by using the last two letters of __/dev/ttyp__''%c''
107 or by using __p__''%d'' for __/dev/pts/__''%d''.
108 If they find a __DEAD_PROCESS__ for this id, they recycle
109 it, otherwise they create a new entry. If they can, they
110 will mark it as __DEAD_PROCESS__ on exiting and it is
111 advised that they null __ut_line__, __ut_time__,
112 __ut_user__ and __ut_host__ as well.
113
114
115 xdm(8) should not create an utmp record, because
116 there is no assigned terminal. Letting it create one will
117 result in trouble like: finger: can not stat
118 /dev/machine.dom. It should create wtmp entries, though,
119 just like ftpd(8) does.
120
121
122 telnetd(8) sets up a __LOGIN_PROCESS__ entry and
123 leaves the rest to login(8) as usual. After the
124 telnet session ends, telnetd(8) cleans up utmp in the
125 described way.
126
127
128 The __wtmp__ file records all logins and logouts. Its
129 format is exactly like __utmp__ except that a null user
130 name indicates a logout on the associated terminal.
131 Furthermore, the terminal name ____ with
132 user name ____ or
133 ____ indicates a system shutdown or
134 reboot and the pair of terminal names
135 ____/____ logs the old/new
136 system time when date(1) changes it. __wtmp__ is
137 maintained by login(1), and init(1) and some
138 versions of getty(1). Neither of these programs
139 creates the file, so if it is removed record-keeping is
140 turned off.
141 !!FILES
142
143
144 /var/run/utmp
145 /var/log/wtmp
146 !!CONFORMING TO
147
148
149 Linux utmp entries conform neither to v7/BSD nor to SYSV:
150 They are a mix of the two. v7/BSD has fewer fields; most
151 importantly it lacks __ut_type__, which causes native
152 v7/BSD-like programs to display (for example) dead or login
153 entries. Further there is no configuration file which
154 allocates slots to sessions. BSD does so, because it lacks
155 __ut_id__ fields. In Linux (as in SYSV), the __ut_id__
156 field of a record will never change once it has been set,
157 which reserves that slot without needing a configuration
158 file. Clearing __ut_id__ may result in race conditions
159 leading to corrupted utmp entries and and potential security
160 holes. Clearing the above mentioned fields by filling them
161 with null bytes is not required by SYSV semantics, but it
162 allows to run many programs which assume BSD semantics and
163 which do not modify utmp. Linux uses the BSD conventions for
164 line contents, as documented above.
165
166
167 SYSV only uses the type field to mark them and logs
168 informative messages such as e.g. __
169 __ in the line field. __UT_UNKNOWN__ seems to
170 be a Linux invention. SYSV has no __ut_host__ or
171 __ut_addr_v6__ fields.
172
173
174 Unlike various other systems, where utmp logging can be
175 disabled by removing the file, utmp must always exist on
176 Linux. If you want to disable who(1) then do not make
177 utmp world readable.
178
179
180 Note that the utmp struct from libc5 has changed in libc6.
181 Because of this, binaries using the old libc5 struct will
182 corrupt ''/var/run/utmp'' and/or ''/var/log/wtmp''.
183 Debian systems include a patched libc5 which uses the new
184 utmp format. The problem still exists with wtmp since it's
185 accessed directly in libc5.
186 !!RESTRICTIONS
187
188
189 The file format is machine dependent, so it is recommended
190 that it be processed only on the machine architecture where
191 it got created.
192 !!BUGS
193
194
195 This manpage is based on the libc5 one, things may work
196 differently now.
197 !!SEE ALSO
198
199
200 ac(1), date(1), getutent(3),
201 init(8), last(1), login(1),
202 updwtmp(3), who(1)
203 ----
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