If you are connected to a LAN but still want to use PPP on your personal Linux machine , you need to address some issues of the routes packets need to take from your machine to reach your LAN (through your Ethernet interface) and also to the remote PPP server and beyond.
This section does NOT attempt to teach you about routing - it deals only with a simple, special case of (static) routing!
I strongly urge you to read the Linux Network Administrator Guide (NAG) if you are NOT familiar with routing. Also the O'Reilly book "TCP/IP Network Administration" covers this topic in a very understandable form.
The basic rule of static routing is that the DEFAULT route should be the one that points to the MOST number of network addresses. For other networks, enter specific routes to the routing table.
The ONLY situation I am going to cover here is where your Linux box is on a LAN that is not connected to the Internet - and you want to dial out to the Internet for personal use whilst still connected to the LAN.
First of all, make sure that your Ethernet route is set up to the specific network addresses available across your LAN - NOT set to the default route!
Check this by issuing a route command, you should see something like the following:-
[root@hwin /root]# route -n
Kernel routing table
Destination Gateway Genmask Flags MSS Window Use Iface
loopback * 255.255.255.0 U 1936 0 50 lo
10.0.0.0 * 255.255.255.0 U 1436 0 565 eth0
If your Ethernet interface (eth0) is pointing at the default route, (the first column will show "default" in the eth0 line) you need to change your Ethernet initialisation scripts to make it point at the specific network numbers rather than the default route (consult the Net2 HOWTO and NAG).
This will allow pppd to set up your default route as shown below:-
[root@hwin /root]# route -n
Kernel routing table
Destination Gateway Genmask Flags MSS Window Use Iface
10.144.153.51 * 255.255.255.255 UH 488 0 0 ppp0
127.0.0.0 * 255.255.255.0 U 1936 0 50 lo
10.1.0.0 * 255.255.255.0 U 1436 0 569 eth0
default 10.144.153.51 * UG 488 0 3 ppp0
As you can see, we have a host route to the PPP server ( 10.144.153.51) via ppp0 and also a default network route that uses the PPP server as its gateway.
If your set up needs to be more complex than this - read the routing documents already mentioned and consult an expert at your site!
If your LAN already has routers on it, you will already have gateways established to the wider networks available at your site. You should STILL point your default route at the PPP interface - and make the other routes specific to the networks they serve.
When you set up a Linux box on an existing LAN to link into the Internet, you are potentially opening your entire LAN to the Internet - and the hackers that reside there. Before you do this, I strongly urge you to consult your network administrator and site security policy. If your PPP connection to the Internet is used to successfully attack your site, you will at the very least earn the intense anger of your fellow users, network and system administrators. You may also find yourself in very much more serious trouble!
Before you connect a LAN to the Internet, you should consider the security implications of even a DYNAMIC connection - hence the earlier reference to the O'Reilly "Building Internet Firewalls"!