Sunday, March 23, 2014

Failed login attempts : how to setup and clear

Under Linux operating system you can use the faillog command to display faillog records or to set login failure limits. faillog command displays the contents of the failure log from /var/log/faillog database file. It also can be used for maintains failure counters and limits. If you run faillog command without arguments, it will display only list of user faillog records who have ever had a login failure.

PAM Settings

I found that under RHEL / CentOS Linux 5.x, you need to modify /etc/pam.d/system-auth file. You need to configure a PAM module Otherwise faillog command will never display failed login attempts.

PAM Configuration To Recored Failed Login Attempts module maintains a count of attempted accesses, can reset count on success, can deny access if too many attempts fail. Edit /etc/pam.d/system-auth file, enter:

# vi /etc/pam.d/system-auth
Modify as follows:

auth required no_magic_root
account required deny=3 no_magic_root lock_time=180

  • deny=3 : Deny access if tally for this user exceeds 3 times.
  • lock_time=180 : Always deny for 180 seconds after failed attempt. There is also 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. Otherwise the account is locked until the lock is removed by a manual intervention of the system administrator.
  • magic_root : If the module is invoked by a user with uid=0 the counter is not incremented. The sys-admin should use this for user launched services, like su, otherwise this argument should be omitted.
  • no_magic_root : Avoid root account locking, if the module is invoked by a user with uid=0
Save and close the file.

How Do I Display All Failed Login Attempts For a User Called vivek?

Type the command as follows:
# faillog -u vivek
Login       Failures Maximum Latest                   On
vivek           3        0   12/19/07 14:12:53 -0600  64.11.xx.yy

Taks: Show Faillog Records For All Users

Type the following command with the -a option:
# faillog -a

Task: Lock Account

To lock user account to 180 seconds after failed login, enter:
# faillog -l 180 -u vivek
# faillog -l 180

Task: Set Maximum Number of Login Failures

The -m option is allows you to set maximum number of login failures after the account is disabled to specific number called MAX. Selecting MAX value of 0 has the effect of not placing a limit on the number of failed logins. The maximum failure count should always be 0 for root to prevent a denial of services attack against the system:
# faillog -M MAX -u username
# faillog -M 10 -u vivek

How do I Reset The Counters Of Login Failures?

The -r option can reset the counters of login failures or one record if used with the -u USERNAME option:
# faillog -r
To reset counter for user vivek, enter:
# faillog -r -u vivek
On large Linux login server, such as University or government research facility, one might find it useful to clear all counts every midnight or week from a cron job.
# crontab -e
Reset failed login recover every week:
@weekly /usr/bin/faillog -r
Save and close the file.

$ man faillog

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