Newbie convert to Linux networking and have implemented a new server into our office - have some 'anti-linux' bods in the office that are waiting for the system to fail, so any help in the following much appreciated...

I have a new linux box with 3 nics in it. I have a DSL connection via a router. This is on a network. I have set the router to and one nic in the linix box to I then have an internal LAN with an IP address range of I have set one of the other nics to which connects to our LAN whilst the other has an address of which connects to a isdn router with an address of So good. I have set the DNS servers on the linux box to those of my service provider.

I have installed safesquid and set my default gateway on the LAN PCs (XP,2k etc) to I can ping this address and I have configured the proxy address in firefox to this machine. The internet works and flies - great.

However, if I try to ping an internet IP or dns address (i.e. I get no replies. I can ping anything from the linux box. This means I cannot get to my mail server as it is external @ the ISP. If I traceroute to an ip it hits but then fails. If I do the same with a address like then I get a DNS failure. My LAN pcs have the ISP DNS address + an internal address of a Win2K box that does internal DNS.

My IP forwarding is set to 1 on the linux box and on my DSL router I have added a route to via

Im struggling to work out whats going on and I'm sure it is obvious to someone that actually knows what they are doing!

The next problem which I can forsee is routing. All internal traffic needs to stay internal (, all internet traffic needs to go out via the internet and anything else in the range needs to go out via the card which should then pump the traffic through the ISDN router.

I currently have the firewall turned OFF but want to implement this asap. I thought I'd get the basic going first.

Any help VERY much appreciated.