4.3.2.1. Forcing Strong Passwords
To protect the network from intrusion it is a good idea for system administrators to verify that the passwords used within an organization are strong ones. When users are asked to create or change passwords, they can use the command line application passwd, which is Pluggable Authentication Manager (PAM) aware and therefore checks to see if the password is easy to crack or too short in length via the pam_cracklib.so PAM module. Since PAM is customizable, it is possible to add further password integrity checkers, such as pam_passwdqc (available from http://www.openwall.com/passwdqc/) or to write a new module. For a list of available PAM modules, refer to http://www.kernel.org/pub/linux/libs/pam/modules.html. For more information about PAM, refer to the chapter titled Pluggable Authentication Modules (PAM) in the Red Hat Enterprise Linux Reference Guide.
It should be noted, however, that the check performed on passwords at the time of their creation does not discover bad passwords as effectively as running a password cracking program against the passwords within the organization.
There are many password cracking programs that run under Red Hat Enterprise Linux although none ship with the operating system. Below is a brief list of some of the more popular password cracking programs:
None of these tools are supplied with Red Hat Enterprise Linux and are therefore not supported by Red Hat, Inc. in any way.
John The Ripper — A fast and flexible password cracking program. It allows the use of multiple word lists and is capable of brute-force password cracking. It is available online at http://www.openwall.com/john/.
Crack — Perhaps the most well known password cracking software, Crack is also very fast, though not as easy to use as John The Ripper. It can be found online at http://www.crypticide.com/users/alecm/.
Slurpie — Slurpie is similar to John The Ripper and Crack, but it is designed to run on multiple computers simultaneously, creating a distributed password cracking attack. It can be found along with a number of other distributed attack security evaluation tools online at http://www.ussrback.com/distributed.htm.
Always get authorization in writing before attempting to crack passwords within an organization.