Enforcing password security with a multiple-user system can be a hassle — users all too often use inadequate passwords. john-the-ripper (also available via most distros) is a password-cracking tool that enables the identification of vulnerable passwords before someone with nefarious intentions finds the weakness.

The first step is to extract the username/password information from the relevant files, using the provided unshadow tool:

unshadow /etc/passwd /etc/shadow > /tmp/password.db

After that, john has three cracking modes:
# Dictionary mode, which tests passwords based on dictionary words. You can use the provided dictionary or provide your own, and there’s an option to enable “word mangling” rules.
# “Single crack” mode, which uses login names and various /etc/passwd values as password candidates, as well as applying word mangling rules.

Incremental mode, which tries all possible character combinations and will obviously take a very, very long time to run. You can change the parameters for this via the config file.

You can run one at a time (in which case, try “single crack” mode first), or run all of them consecutively with

john /tmp/password.db

To show results, use

john –show /tmp/password.db

unshadow will produce a password database only on systems that use /etc/passwd and /etc/shadow for login. For centralized systems, there’s a Kerberos5 module available, or the supplied unafs utility extracts Kerberos AFS passwords. There’s also a LDAP module.

Also remember that you can limit cracking attempts through measures such as locking out specific IP addresses after multiple failed ssh attempts or limiting the number of times a user can get a password wrong when logging on.