"Setting up a Linux server as a router with two Ethernet network
adapters is more straightforward compared to the dynamic dial-up
connection presented in this article because of two things. The
first is the fixed IP addresses involved. Second, routing and NAT
configuration is static. With a dial-up connection these change.
The IP address for the dial-up connection is normally assigned
using DHCP by the ISP when a connection is made. Likewise, the
routing configuration must be changed to accommodate the new IP
"This article takes a look at how to configure pppd. Pppd is the
Linux point-to-point protocol (PPP) daemon. It provides a network
interface to a serial connection that is typically an analog modem
or possibly an ISDN adapter. PPP is also used on some non-dial-up
connections. More specifically, some ISPs provide DSL and cable
modem connections using PPP. In this case, the connection to the PC
is via an Ethernet connection is the same as using fixed IP
addresses but a protocol called PPP over Ethernet (PPPoE) is used.
PPPoE operates the same way PPP does so we will not look at this in
any more detail but concentrate on pppd. Typically just the
programs are different and some configuration details differ."
"Two items must be addressed for the dial-up connection to be
used. The first is the pppd program that provides the connection.
The second is the ipchains program."