vpn through linux firewall to nt 4sp6a

I am trying with no success to vpn through a linux firewall to my nt server. I believe I have the rules setup correctly but am unable to finish verifying username and password. fyi-my connection to the internet is a cable modem. Firewall also setupas dhcp server and client. Micorsoft told me I needed the lastest sp6a ( now installed), didn’t seem to help me.