Locking accounts with a number of incorrect login attempts

When a user incorrectly enters a password several times and you want the account to lock for a certain amount of time.  This configuration does NOT work to lock the root account (although settings can be changed to include root in the locked accounts list.

See Security and Hardening Guide for full documentation.
Section 15.4.3 Locking user accounts after too many login failures

The documentation indicates the modification of "/etc/pam.d/login", however, the common-auth file could also be used to limit the number of failures for all methods of authentication. Be aware that the default method for configuring the pam common-* files is done via pam-config.  If custom configuration of the files is done, the pam-config method should be disabled to avoid having the configuration overwritten during an update or upgrade.  For instructions, please see the documentation for  3.6 Manually configuring PAM.

The following is an example for making the change in common-auth:

1. Edit the /etc/pam.d/common-auth file to include:

auth required pam_env.so
auth required pam_unix.so  try_first_pass
auth required pam_tally2.so deny=4 unlock_time=300

Where deny=<number of attempts before lockout>
Where unlock_time=<time in seconds for account to be locked>

2. Edit the /etc/pam.d/common-account to include:

account requisite pam_unix.so
account sufficient pam_localuser.so
account required pam_tally2.so

To check the users that have been locked out and the number of attempts run: pam_tally2

To unlock a specific account manually as root run

pam_tally2 --user=<username> --reset

For more information and other options check the man page of pam_tally2