Pluggable Authentication Modules (PAM) - some basic tricks on CentOS 6

I've been playing around with PAM on a couple distros recently, and I thought I'd share some quick tips and tricks in setting up a secure CentOS 6 Linux multi-user environment. Whilst these are not bulletproof password policies, they are a step beyond the default distribution configuration and are not too complex that the users would be bugging you, the friendly neighbourhood sysadmin.

As usual, any feedback is appreciated, so drop me a line: noveck@woblag.com. Once it gets past the spam filters, I'll try my best to respond asap.

1. Use PAM to disable the use of null passwords in user Accounts.

vi /etc/pam.d/system-auth

Find line 

password sufficient pam_unix.so md5 shadow nullok try_first_pass use_authtok

Remove/delete nullok option, so the line now reads:
password sufficient pam_unix.so md5 shadow try_first_pass use_authtok

save and close file

2. Use PAM to prevent re-using/recycling passwords .

This example prevents the use of the last 3 passwords.

vi /etc/pam.d/system-auth
find line
password sufficient pam_unix.so md5 shadow nullok try_first_pass use_authtok

Add to end of line
password sufficient pam_unix.so md5 shadow nullok try_first_pass use_authtok remember=3

save and close file

3. Set password minimum length

This example sets the minimum password length to 8 characters.

vi /etc/pam.d/system-auth

find line
password sufficient pam_unix.so md5 shadow nullok try_first_pass use_authtok

Add new line BEFORE
passwd password requisite pam_cracklib.so minlen=8
save and close file

4. Configure server to deny access with multiple incorrect login attempts

This example temporarily denies access after 5 attempts. The temporary lockout time can also be configured for a certain time, which will be set to 1 hour (3600 seconds) in this example.

vi /etc/pam.d/system-auth

Add the following line to end of file
auth required pam_tally.so onerr=fail deny=5 unlock_time=3600

save and close file

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