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Utility last searches through the file /var/log/wtmp (or the file designated by the -f flag) and displays a list of all users logged in (and out) since that file was created in reverse chronological order. Names of users and ttyís can be given, in which case last will show only those entries matching the arguments. Linux version of last also displays IP or DNS, but unless option -a is given DNS names of machines from which user connected are abbreviated.
Lat command also allows to determine set of active user. Which is important for sending announcements about shutdown and similar things. Generally users who did not login for a specific server for two month or more should be disabled. This also can be accomplished by analyzing last command output. you do not need password expiation for that.
Utility lastb produiced the same set of reports as the last commend but for bad logs. As such it has certain security value as it allows to see probed. We can define probes as attempt to login from IP that never has a successful login. Usually the number of unsuccessful login attempts is set to some small number such as five or sever (do not use three, unless you need a stream of helpdesk tickets ;-), so there is possibility of denial of service attack if login is possible from any IP. Restriction of IP login space to specific host is a viable and necessary method of protection from such attack in the current environment.
Unfortunately few people know about lastb existence and even fewer use it. Often it is not even enabled by default and you need manually create the file /var/log/btmp for this command to be operational. It is not created automatically. For example:
# lastb -a lastb: /var/log/btmp: No such file or directory Perhaps this file was removed by the operator to prevent logging lastb info. # touch /var/log/btmp lastb -a btmp begins Mon Jul 1 18:57:15 2013
So there are three main forms of last and lastb commands:
With parameter meaning a user: last root; lastb root In this case information for particular user is printed
The pseudo user reboot logs in each time the system is rebooted. Thus last reboot will show a log of all reboots since the log file was created.
|The pseudo user reboot logs in each time the system is rebooted. Thus, the command last reboot will show a log of all reboots since the log file was created.|
Lastb is the same as last, except that by default it shows a log of the file /var/log/btmp, which contains all the bad login attempts.
last [-R] [-num] [ -n num ] [-adiox] [ -f file ] [ -t YYYYMMDDHHMMSS ] [name...] [tty...]
lastb [-R] [-num] [ -n num ] [ -f file ] [-adiox] [name...] [tty...]
We will discuss linux implementation options
The files wtmp and btmp might not be found. The system only logs information in these files if they are present. This is a local configuration issue. If you want the command last and lastb to be operational both files should be created with a simple touch(1) command (for example, touch /var/log/wtmp). Most distributions create /var/log/wtmp) during the installation, but not /var/log/btmp.
- When last catches a SIGINT signal (generated by the interrupt key, usually Ctrl-C) or a SIGQUIT signal (generated by the quit key, usually Ctrl-\), last will show how far it has searched through the file; in the case of the SIGINT signal last will then terminate.
From the point of view of processing output in scripts, we can distinguish three output formats for last:
With the user name, including pseudo user reboot: last reboot (show only reboot)
The most important is output format last -a in which you can see full DNS name of client which logged to the server:
# last -a user1 pts/3 Mon Jul 1 08:35 still logged in joe.doer.vpn.firma.com user1 pts/3 Fri Jun 28 17:20 - 18:24 (01:03) lap245.firma.net root pts/2 Fri Jun 28 02:31 - 02:31 (00:00) user1 pts/2 Tue Jun 25 14:18 - 16:17 (01:58) lap245.firma.net root pts/3 Wed Jun 12 14:08 - 14:21 (00:13) root pts/3 Wed Jun 12 14:06 - 14:08 (00:01) root pts/3 Wed Jun 12 14:04 - 14:06 (00:02) user1 pts/2 Wed Jun 12 12:59 - 14:22 (01:22) joe.doer.vpn.firma.com user1 pts/2 Wed Jun 12 12:49 - 12:59 (00:10) joe.doer.vpn.firma.com oracle pts/1 Wed Jun 12 09:02 - 17:45 (08:43) 10.20.23.23 root pts/1 Wed Jun 5 17:54 - 17:56 (00:02) nti147.usfp2.basf.net root pts/2 Thu May 30 14:57 - 15:20 (00:23) nti106.firma.com oracle pts/1 Thu May 30 10:37 - 17:10 (06:33) lap719.firma.net user1 pts/2 Wed May 29 16:58 - 19:10 (02:12) joe.doer.vpn.firma.com
user2 pts/1 Fri Oct 14 10:17 - 10:17 (00:00) lap316.firma.com user2 pts/1 Fri Oct 14 10:17 - 10:17 (00:00) lap316.firma.com root ssh:notty Thu Oct 13 12:08 - 12:08 (00:00) srv016.firma.com root ssh:notty Thu Oct 13 12:08 - 12:08 (00:00) srv016.firma.com root ssh:notty Tue Sep 20 16:35 - 16:35 (00:00) lap316.firma.com root ssh:notty Wed Aug 10 17:05 - 17:05 (00:00) lap921.firma.com root ssh:notty Fri Jul 15 09:51 - 09:51 (00:00) lap921.firma.com root ssh:notty Fri Jul 15 09:51 - 09:51 (00:00) lap921.firma.com root ssh:notty Thu Jun 30 17:09 - 17:09 (00:00) lusytp02.firma.com roacle** pts/3 Thu Jun 30 16:58 - 16:58 (00:00) lapl61.firma.com root ssh:notty Thu Jun 30 12:24 - 12:24 (00:00) srv029.firma.com root ssh:notty Thu Jun 30 12:24 - 12:24 (00:00) srv029.firma.com user1 ssh:notty Tue Jun 28 14:22 - 14:22 (00:00) lap921.firma.com
With parameter meaning a user: last root; lastb root
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ABUSE: IPs or network segments from which we detect a stream of probes might be blocked for no less then 90 days. Multiple types of probes increase this period.
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