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Linux last and lastb commands

Show listing of last logged in users and bad logins

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Output format

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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.

Last vs lastb

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

Three forms of the last command

So there are three main forms of last and lastb commands:

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


Output format

From the point of view of processing output in scripts, we can distinguish  three output formats for last:

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
user1    pts/3        Fri Jun 28 17:20 - 18:24  (01:03)
root     pts/2        Fri Jun 28 02:31 - 02:31  (00:00)
user1    pts/2        Tue Jun 25 14:18 - 16:17  (01:58)
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)
user1    pts/2        Wed Jun 12 12:49 - 12:59  (00:10)
oracle   pts/1        Wed Jun 12 09:02 - 17:45  (08:43)
root     pts/1        Wed Jun  5 17:54 - 17:56  (00:02)
root     pts/2        Thu May 30 14:57 - 15:20  (00:23)
oracle   pts/1        Thu May 30 10:37 - 17:10  (06:33)
user1    pts/2        Wed May 29 16:58 - 19:10  (02:12)

Output format of lastb

user2    pts/1        Fri Oct 14 10:17 - 10:17  (00:00)
user2    pts/1        Fri Oct 14 10:17 - 10:17  (00:00)
root     ssh:notty    Thu Oct 13 12:08 - 12:08  (00:00)
root     ssh:notty    Thu Oct 13 12:08 - 12:08  (00:00)
root     ssh:notty    Tue Sep 20 16:35 - 16:35  (00:00)
root     ssh:notty    Wed Aug 10 17:05 - 17:05  (00:00)
root     ssh:notty    Fri Jul 15 09:51 - 09:51  (00:00)
root     ssh:notty    Fri Jul 15 09:51 - 09:51  (00:00)
root     ssh:notty    Thu Jun 30 17:09 - 17:09  (00:00)
roacle** pts/3        Thu Jun 30 16:58 - 16:58  (00:00)
root     ssh:notty    Thu Jun 30 12:24 - 12:24  (00:00)
root     ssh:notty    Thu Jun 30 12:24 - 12:24  (00:00)
user1    ssh:notty    Tue Jun 28 14:22 - 14:22  (00:00)




/var/log/wtmp /var/log/btmp


Miquel van Smoorenburg, [email protected]

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