Tip of the Trade: Find password weaknesses before it's too late. john-the-ripper offers three cracking modes to evaluate passwords.
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:
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
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.
This article was first published on ServerWatch.com.
September 08, 2008
When it comes to our computers, the classic principle of innocent until proven guilty is simply too dangerous.