How to monitor failed SSH login attempts on CentOS 5 and 6

Last updated on April 26, 2013 by Dan Nanni

If you are a web hosting administrator, VPS user, or a Linux security technician, you probably need to closely monitor SSH login activities, especially failed login attempts. Linux has Pluggable Authentication Modules (PAM) built-in, offering configurable authorization for Linux applications and services. You can use PAM to monitor failed SSH login attempts, and act on them (e.g., blocking user).

In this tutorial, I will show how to configure PAM to monitor failed SSH login attempts in CentOS 5 and 6 environments. Depending on the CentOS version you are using, PAM configuration is slightly different.

Configure PAM on CentOS 5

To keep track of failed SSH logins on CentOS 5.*, you need to use a PAM module called pam_tally.so. For that, modify /etc/pam.d/system-auth as follows.

$ sudo vi /etc/pam.d/system-auth
auth required pam_tally.so no_magic_root
account required pam_tally.so deny=3 no_magic_root lock_time=300

The above PAM configuration denies SSH access for a user if the user has failed to log in three times. The user becomes unblocked after 300 seconds.

Once PAM is configured, use a command called faillog to monitor the SSH login activity of a specific user (e.g., xmodulo):

$ sudo faillog -u xmodulo
Login       Failures Maximum Latest             On
xmodulo         2        0   04/23/13 14:12:53  192.168.1.5

To reset the counter of failures for a particular user (e.g., xmodulo):

$ sudo faillog -r -u xmodulo

Configure PAM on CentOS 6

To check failed SSH login attempts on CentOS 6.*, you need to use a PAM module called pam_tally2.so. To configure pam_tally2.so, modify /etc/pam.d/password-auth as below.

$ sudo vi /etc/pam.d/password-auth
auth required pam_tally2.so deny=3 onerr=fail unlock_time=300
account required pam_tally2.so

This PAM configuration blocks SSH login for a particular user after three failed login attempts from the user. The user remains blocked for 300 seconds.

Once PAM is configured like above, use a command called pam_tally2 to monitor the SSH login activity of a particular user (e.g., xmodulo).

$ sudo pam_tally2 -u xmodulo
Login           Failures Latest failure     From
xmodulo             2    04/23/13 22:44:45  192.168.1.5

To unblock a particular user (e.g., xmodulo):

$ sudo pam_tally2 -u xmodulo -r

This tutorial demonstrated how you can monitor SSH login activites and act on affected users using built-in PAM modules. You may also want to consider a complementary tool called fail2ban especially if your SSH server is experiencing large-scale brute-force password guessing attacks. Unlike PAM, fail2ban blocks malicious SSH logins at the network layer (via iptables) and can automate banning/unbanning of offending IP addresses.

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