Linux Home Networking, Part 5 - page 3
Dialing into the Internet
Work in steps when setting up a serial link to the Internet for a network. First set up pppd and make sure that the local PC can access the Internet using the link. Add the logging support if you are having problems. Do not include the firewall support. Disable it entirely if you are having problems. Make sure the on demand support is working properly before proceeding.
Once the link to the ISP is working then it is time to add the masquerading support. Kill and restart the pppd deamon to make sure that any changes to the script files are taken into account. Again, check that the local PC can access the Internet. If not, there is a problem with the ipchains support that has been added. Use the ipchains list function to see what the current settings are to make sure they are what you expected, including the addition of the masquerading rule after a connection is made.
Finally try accessing the Internet from a PC on the local network. If this is not possible, take one step back and make sure the PC can ping the server with the pppd daemon.
Getting this configuration working is usually not too difficult. As with most Linux applications, once configured, it runs forever.