d02 ex03 who.php

Summary

Document information

Author(s):	Joachim Metz <[email protected]>
Abstract:	This document contains information about the utmp login records format.
Classification:	Public
Keywords:

License

Copyright (C) 2018, Joachim Metz <[email protected]>.
Permission is granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.3 or any later version
published by the Free Software Foundation; with no Invariant Sections, no
Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included
in the section entitled "GNU Free Documentation License".

Revision history

Version	Author	Date	Comments
0.0.1	J.B. Metz	May 2018	Initial version.
0.0.2	J.B. Metz	June 2018	Additional information about extensions (utmpx).

1. Overview

The utmp login records format is used by the btmp, utmp and wtmp files.

utmp; contains a current status of the system, system boot time (used by uptime), user logins and logouts
wtmp; contains a historical status of the system
btmp; contains failed login attempts

There are multiple utmp file formats:

Linux libc5 utmp
Linux libc6 utmp
Mac OS X utmp
Mac OS X 10.5 utmpx

2. Linux libc6 utmp

A Linux libc6 utmp file consists of:

one or more records

Characteristics	Description
Byte order	litte-endian
Date and time values	POSIX timestamp with microseconds in UTC
Character strings	ASCII strings are Single Byte Character (SBC) or Multi Byte Character (MBC) string encoded with th system codepage. Though technically maybe incorrect, this document will use term (extended) ASCII string. On modern versions of Linux the system encoding will be UTF-8.

2.1. Record

The Linux libc6 utmp recordd is 384 bytes in size and consists of:

Offset	Size	Description
0	4	Type of login Contains a signed integer See section: Types of login
4	4	Process identifier (PID)
8	32	Terminal Contains an encoded string
40	4	Terminal indentifier
44	32	Username Contains an encoded string
76	256	Hostname Contains an encoded string
332	2	Termination status Contains a signed integer
334	2	Exit status Contains a signed integer
336	4	Session Contains a signed integer
340	4	Timestamp Contains a signed integer
344	4	Microseconds Contains a signed integer
348	16	IP address Contains a packed IPv4 or IPv6 address
364	20	Unknown (reserved for future usage)

Value	Identifier	Description
0	EMPTY UT_UNKNOWN	Record is empty and does not contain valid information
1	RUN_LVL	Record represents a change in system run-level
2	BOOT_TIME	Time of system boot
3	NEW_TIME	Time after system clock change
4	OLD_TIME	Time before system clock change
5	INIT_PROCESS	Process spawned by init
6	LOGIN_PROCESS	Main process for user login session
7	USER_PROCESS	Process spawned by user
8	DEAD_PROCESS	Terminated process
9	ACCOUNTING	Unknown (Not implemented)

3. Mac OS X 10.5 utmpx

A Mac OS X 10.5 utmpx file consists of:

file header
records

Characteristics	Description
Byte order	litte-endian
Date and time values	POSIX timestamp with microseconds in UTC
Character strings	ASCII strings are Single Byte Character (SBC) or Multi Byte Character (MBC) string encoded with th system codepage. Though technically maybe incorrect, this document will use term (extended) ASCII string.

3.1. File header

The Mac OS X 10.5 utmpx file header is 628 bytes in size and consists of:

Offset	Size	Value	Description
0	8	"utmpx-1.00"	Signature
8	288	0	Unknown (empty values)
296	2	10	Type of login Contains a signed integer See section: Types of login
298	330	0	Unknown (empty values)

3.2. Record

The Mac OS X 10.5 utmpx record is 628 bytes in size and consists of:

Offset	Size	Description
0	256	Username Contains an encoded string
256	4	Terminal indentifier
260	32	Terminal (tty name or line) Contains an encoded string
292	4	Process identifier (PID)
296	2	Type of login Contains a signed integer See section: Types of login
298	2	Unknown The runtime size of the type of login is short, could this be the upper 2 bytes used for storage?
300	4	Timestamp Contains a signed integer
304	4	Microseconds Contains a signed integer
308	256	Hostname Contains an encoded string
564	16 x 4 = 64	Unknown (reserved for future usage)

Note	The first record contains little information except for the the boot time for PID 1.

Value	Identifier	Description
0	EMPTY	Record is empty and does not contain valid information
1	RUN_LVL	Record represents a change in system run-level
2	BOOT_TIME	Time of system boot
3	OLD_TIME	Time before system clock change
4	NEW_TIME	Time after system clock change
5	INIT_PROCESS	Process spawned by init
6	LOGIN_PROCESS	Main process for user login session
7	USER_PROCESS	Process spawned by user
8	DEAD_PROCESS	Terminated process
9	ACCOUNTING	Unknown
10	SIGNATURE	Signature, value used in file header
11	SHUTDOWN_TIME	TIme of system shutdown

Note	That the order of OLD_TIME and NEW_TIME is reversed to that of Linux

Appendix A: References

Title:	Wikipedia: utmp
URL:	https://en.wikipedia.org/wiki/Utmp

Title:	utmp man page
URL:	http://man7.org/linux/man-pages/man5/utmp.5.html

Title:	utmp header file
URL:	/usr/include/utmp.h

Title:	utmpx header file
URL:	/usr/include/utmpx.h

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