rami.info

Download pam_radius ftp://ftp.freeradius.org/pub/radius/ and run the following:
tar -xvf pam_radius*.tar
cd pam_radius*
make
cp pam_radius_auth.so /lib/security

Edit /etc/raddb/server and add the following:
127.0.0.1 secret 1

Under Gentoo edit /etc/pam.d/system-auth and add the following line:

auth sufficient pam_radius_auth.so skip_passwd

After:

auth sufficient pam_unix.so likeauth nullok

Make sure you add the users locally on the machine as well as the radius is just used for the password authentication.

Enjoy.