A fundamental component of authentication management is monitoring the system after you have configured your users.
We will be exploring these concepts on a Ubuntu 22.04 server, but you can follow along on any modern Linux distribution. You can set up a Ubuntu 22.04 server for this tutorial by following our guide to Initial Server Setup on Ubuntu 22.04.
Modern Linux systems log all authentication attempts in a discrete file. This is located at
/var/log/auth.log. You can view this file using
- sudo less /var/log/auth.log
OutputMay 3 18:20:45 localhost sshd: Server listening on 0.0.0.0 port 22. May 3 18:20:45 localhost sshd: Server listening on :: port 22. May 3 18:23:56 localhost login: pam_unix(login:session): session opened fo r user root by LOGIN(uid=0) May 3 18:23:56 localhost login: ROOT LOGIN on '/dev/tty1' Sep 5 13:49:07 localhost sshd: Received signal 15; terminating. Sep 5 13:49:07 localhost sshd: Server listening on 0.0.0.0 port 22. Sep 5 13:49:07 localhost sshd: Server listening on :: port 22. . . .
When you are finished viewing the file, you can use
q to quit
Usually, you will only be interested in the most recent login attempts. You can see these with the
Outputdemoer 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 provides a formatted version of information saved in another file,
Judging from the first and the third line, users are currently logged into the system. The total time spent logged into the system during other, already closed sessions is provided by a set of hyphen-separated values.
You can also view the last time each user on the system logged in using the
This information is provided by accessing the
/etc/log/lastlog file. It is then sorted according to the entries in the
OutputUsername 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**. Many of these system accounts will not be used to log in directly, so this is normal.
User authentication on Linux is a relatively flexible area of system management. There are many ways of accomplishing the same objective with widely available 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.
Next, you may want to learn how to add and delete system users.
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