How To Monitor System Logins

A fundamental component of authentication management is monitoring the system after you have configured your users.

Luckily, modern Linux systems log all authentication attempts in a discrete file. This is located at "/var/log/auth.log":

sudo less /var/log/auth.log

May  3 18:20:45 localhost sshd[585]: Server listening on 0.0.0.0 port 22.
May  3 18:20:45 localhost sshd[585]: Server listening on :: port 22.
May  3 18:23:56 localhost login[673]: pam_unix(login:session): session opened fo
r user root by LOGIN(uid=0)
May  3 18:23:56 localhost login[714]: ROOT LOGIN  on '/dev/tty1'
Sep  5 13:49:07 localhost sshd[358]: Received signal 15; terminating.
Sep  5 13:49:07 localhost sshd[565]: Server listening on 0.0.0.0 port 22.
Sep  5 13:49:07 localhost sshd[565]: Server listening on :: port 22.
. . .

How To Use the "last" Command

Usually, you will only be interested in the most recent login attempts. You can see these with the "last" tool:

last

demoer   pts/1        rrcs-72-43-115-1 Thu Sep  5 19:37   still logged in   
root     pts/1        rrcs-72-43-115-1 Thu Sep  5 19:37 - 19:37  (00:00)    
root     pts/0        rrcs-72-43-115-1 Thu Sep  5 19:15   still logged in   
root     pts/0        rrcs-72-43-115-1 Thu Sep  5 18:35 - 18:44  (00:08)    
root     pts/0        rrcs-72-43-115-1 Thu Sep  5 18:20 - 18:20  (00:00)    
demoer   pts/0        rrcs-72-43-115-1 Thu Sep  5 18:19 - 18:19  (00:00)

This gives a formated version of the "/etc/log/wtmp" file.

As you can see, in the first and third line, it shows that the user is still logged into the system. Otherwise, the total time logged into the system during a session is given by a set of hyphen-separated values.

How To Use the "lastlog" Command

If you would like to look at this situation from a different angle, you can view the last time each user on the system logged in.

This information is provided by accessing the "/etc/log/lastlog" file. It is then sorted according to the entries in the "/etc/passwd" file:

lastlog

Username         Port     From             Latest
root             pts/1    rrcs-72-43-115-1 Thu Sep  5 19:37:02 +0000 2013
daemon                                     **Never logged in**
bin                                        **Never logged in**
sys                                        **Never logged in**
sync                                       **Never logged in**
games                                      **Never logged in**
. . .

You can see the latest login time of every user on the system.

Notice how the system users will almost all have "**Never logged in**". We saw earlier how these accounts do not have password authentication set up, so this is the expected value.

Conclusion

User authentication on Linux is a relatively flexible area of system management. There are many ways of accomplishing the same objective with very simple tools.

It is important to understand where the system keeps information about logins so that you can monitor your server for changes that do not reflect your usage.