Before there's too much bandwidth going at cross-purposes to my actual query, here's a summary parhaps a bit more explicit on some issues.
1. Basic LAN: 3 machines A, B, C networked with coax cable over their NICs. (I want to keep this as coax, for various practical reasons).
These are on network 192.168.0.* on eth0 for each machine. (is that the same as 192.168.0.0/24?) and I want to keep this as it is too (otherwise too much unpicking in all sorts of places).
2. ADSL provider is Zen, though the phone line is BT. As part of the deal with Zen I purchased through them a BT Voyager 205 "box". This is both modem (in that it takes the signal from the phone line and comverts it) and router (in that it has a host of configurable internal routing and firewalling thingies).
3. The Voyager 205 has 2 sockets: USB and RJ45. I want to use the RJ45. Hence one machine has to have a second NIC since you can't use both BNC (coax) and RJ45 connections on a single NIC.
4. Hence one machine (say B) gets 2 NICs, and the second NIC is put on net 192.168.1.* as eth1
The router by default has a IP address 192.168.1.1 for the RJ45 (and 192.168.1.2 for the USB), and so the next available address from the router is 192.168.1.3
So I give the second NIC the IP address 192.168.1.3 on eth1 on machine B.
5. I now add a default gateway to B
route add -net default gw 192.168.1.1
(this being the IP address of the router itself) and with this setup I can access the web quite freely from machine B.
6. The other two machines (A and C) each have default gateway B.
7. However, from neither of the other machines can I access the web at all.
8. This contrasts, however, with the behaviour when B is used to dial-up using an ordinary phone modem. Kppp does the business, and when it's done there's a default route on B via PPP through interface ppp0 to the outside world.
With this, and default routes on A and C as above, unchanged, all 3 machines can access the web freely.
What I'm trying to achieve with ADSL and the Voyager is what is already achieved with dialup and the phone-modem!
What puzzles me is that if A and C can route their outside traffic via B with their default routes, and B can pick this up and send it out through the phone-modem, why is it that with ADSL -- while B can send its own traffic out through the DSL modem using B's configuration, it can't cope with the traffic coming in from A anc C? While it can cope with its own traffic?
As far as A anc C are concerned, they're still sending their outward-bound traffic to B since that's what they do with "default" stuff. B is somehow failing to transfer this to the DSL despite being able to transfer its own stuff.
Hoping this makes it clearer!
(By the way, for those who've mentioned iptables: I've checked, and there's no program 'iptables' on B, though there's 'iptables' source code in the kernel sources, and apparently 3 related modules: iptable_filter.o, iptable_mangle.o and iptable_nat.o -- and, in case it's relevant, No, I'm not in a hurry to re-compile the kernel! This issue should be simpler than that).
Best wishes to all, Ted.
-------------------------------------------------------------------- E-Mail: (Ted Harding) Ted.Harding@nessie.mcc.ac.uk Fax-to-email: +44 (0)870 094 0861 Date: 26-Oct-05 Time: 13:10:51 ------------------------------ XFMail ------------------------------