Last week I talked about testing the strength of users’ passwords. Another
way to ensure security is to set a good password policy.
Tip of the Trade: Establishing a good password policy from the start is just as critical to security as testing the strength of passwords already in use. The PAM module pam_cracklib can enforce both length and complexity.
The PAM module pam_cracklib can enforce both length and
complexity. For length, it uses the minlen option. For complexity, it
has options dcredit, ucredit, lcredit, and
ocredit, which refer to digit, upper-case character, lower-case
character, and other character, respectively. A value of -1 for one of these
means “require one character of this type,” and a value of 1 means “give 1
credit for this type.” The credit system involves giving “length credits” for
using non-lowercase characters (so you can have a shorter password than the
minimum length if it uses non-lowercase characters), but this can be confusing
for users, so it may be best to just require certain types of character.
Try the following line in /etc/pam.d/common-password in Debian-type distros or
/etc/pam.d/system-auth in RedHat-type distros:
password requisite pam_cracklib.so retry=3 minlen=10 difok=3 dcredit=-1 ucredit=-1 lcredit=-1
This will set a maximum of three attempts at getting an acceptable password (users
can always rerun passwd to try again); a 10-character minimum length;
a minimum of three characters different from the last password; and a requirement
that the password contain at least one each of digit, lower-case character,
and upper-case character.
» Cracking Passwords
» Aliases and Variables Keep Things Short and Simple
Read All Tips of the Trade
Finally, to make all your users change their passwords regularly, edit the
/etc/login.defs file to set the PASS_MAX_DAYS variable to
the maximum time allowed before changing a password. This affects only new
accounts; use the command chage to affect existing users.