I use the command "chage -d0 userid" to force the user change password ( we use RH 8 ) , but I found there are some restrictions of the new password , eg . the password can't too short , must not a dictionary word , must not the same as the user id , I want to know all these restrictions so that I can tell the users how to change their password , is there other restrictions except the above I memtioned ? where I can know it ? besides , is it possible to change these default restrictions so that the users can more easily to change password ? thx in advance.

Take a look at /etc/login.defs, this file holds information relevant to the login process.

Hope this gets you going agian.