1.  PAM

PAM is a suite of shared libraries that provide an alternative way providing user authentication in programs. The pam USE flag is turned on by default. Thus the PAM settings on Gentoo Linux are pretty reasonable, but there is always room for improvement. First install cracklib.

# emerge cracklib

auth     required pam_unix.so shadow nullok
account  required pam_unix.so
password required pam_cracklib.so difok=3 retry=3 minlen=8 dcredit=-2 ocredit=-2
password required pam_unix.so md5 use_authtok
session  required pam_unix.so

This will add the cracklib which will ensure that the user passwords are at least 8 characters and contain a minimum of 2 digits, 2 other characters, and are more than 3 characters different from the last password. This forces the user to choose a good password (password policy). Check the PAM documentation for more options.

auth     required pam_unix.so nullok
auth     required pam_shells.so
auth     required pam_nologin.so
auth     required pam_env.so
account  required pam_unix.so
password required pam_cracklib.so difok=3 retry=3 minlen=8 dcredit=-2 ocredit=-2 use_authtok
password required pam_unix.so shadow md5
session  required pam_unix.so
session  required pam_limits.so

Every service not configured with a PAM file in /etc/pam.d will use the rules in /etc/pam.d/other. The defaults are set to deny, as they should be. But I like to have a lot of logs, which is why I added pam_warn.so. The last configuration is pam_limits, which is controlled by /etc/security/limits.conf. See the (/etc/security/limits.conf) section for more on these settings.

auth     required pam_deny.so
auth     required pam_warn.so
account  required pam_deny.so
account  required pam_warn.so
password required pam_deny.so
password required pam_warn.so
session  required pam_deny.so
session  required pam_warn.so