Auto lock and unlock user account in Linux

If user entered his password incorrectly several times his account can be locked.

sudo faillog -u khmel
Username Failures Maximum Latest
khmel 7 0 Tue Oct 13 15:28:03 +0200 2009 on 212.18.129.


And even reset of password cannot help:
You should use this command, to reset counts of failures:

sudo faillog -u khmel -r
sudo faillog -u khmel
Username Failures Maximum Latest
khmel 0 0 Tue Oct 13 15:29:39 +0200 2009 on 212.18.129.

You also can have another problem: user can login, but cannot use sudo (because he entered too many times wrong password for sudo)

sudo -s
Password:
sudo: pam_acct_mgmt: 7

Sorry, try again.
Solution is the same

sudo faillog -u phn -r"

Additional information:
pam_tally - this module maintains a count of attempted accesses, can reset count on success, can deny access if too many attempts fail.
/etc/pam.d/system-auth - configuration file to configure attempted login accesses.
For instance on khmel.org:

sudo grep pam_tally /etc/pam.d/system-auth
auth required /lib/security/$ISA/pam_tally.so onerr=fail no_magic_root
account required /lib/security/$ISA/pam_tally.so per_user deny=6 no_magic_root reset

Some explanation:
auth - section that will turn on logging of failed login attempts
account - section that will actually do the account locking
deny=n - deny access if tally for this user exceeds n.
no_magic_root - option ensures that accounts with a UID of 0 are tallied.
reset - option indicates that the login failure count will be reset to 0 if a successful login takes place.
per_user - option allows you to exclude accounts from locking if the accounts have a maximum login failure set explicitly.
unlock_time=n option. It allow access after n seconds after failed attempt. If this option is used the user will be locked out for the specified amount of time after he exceeded his maximum allowed attempts.