Thanks to a number of people --
Bob Franklin, Geoff ("IT"), Simon Hobson, John Logsdon (also several private talks), on linux-users;
Adam Allen, Guy Eastwood, Jim Rippon, MJ Ray, Wayne Stallwood, "Ten", Maz Walker, Keith Watson, on ALUG;
it is now done!
As more than a few pointed out, the trick (and I hope this solution will be useful to others in the same situation) is that the ADSL router needs to know how to talk back to the originating internal network. So it needs to have a static route to this configured on the router itself.
To make it explicit:
A: 192.168.0.1 eth0 default gateway -> C = 192.168.0.6 B: 192.168.0.7 eth0 default gateway -> C = 192.168.0.6
C: 192.168.0.6 eth0 default gateway -> ADSL = 192.168.1.1 C: 192.168.1.3 eth1
ADSL: Own address = 192.168.1.1, expects to be talking to 192.168.1.3
Now the following is at least partially specific to the BT Voyager 205 ADSL Router, since it refers to details of its Web interface config pages accessed via
1. Go into "Advanced" & login as administrator 2. Go to "IP Routing" -> "IP Route" and then you see an "IP Address Table" (slightly pruned).
Destn Netmask NextHop IFName RtType RtOrigin 0.0.0.0 0.0.0.0 62.3.82.17 ppp-0 Indirect Dynamic 62.3.82.17 255.255.255.255 xx.yy.zz.pqr ppp-0 Direct Dynamic xx.yy.zz.pqr 255.255.255.255 127.0.0.1 lo-0 Direct Dynamic 127.0.0.0 255.0.0.0 127.0.0.1 lo-0 Direct Dynamic 192.168.1.0 255.255.255.0 192.168.1.1 eth-0 Direct Dynamic 192.168.1.1 255.255.255.255 127.0.0.1 lo-0 Direct Dynamic 192.168.1.2 255.255.255.255 127.0.0.1 lo-0 Direct Dynamic 212.23.3.11 255.255.255.255 62.3.82.17 ppp-0 Indirect Dynamic 212.23.6.35 255.255.255.255 62.3.82.17 ppp-0 Indirect Dynamic
(where "xx.yy.zz.pqr" is my "public" Zen IP addressi, and 62.3.82.17 is Zen's gateway for this connection. The "212..." addresses are Zen nameservers.)
192.168.1.1 is the router's internal ethernet interface, and 192.168.1.2 is its USB interface. It looks to 192.168.1.{3,4,5,...} for outside connections via ETH/USB and in fact I set up C's eth1 to be 192.168.1.3 which works fine.
So as we can see there is of course nothing to with with 192.168.0.0 in there.
At this stage there is an "Add" button at the bottom which brings up a form for entering the details of a new route. So I entered "192.168.0.0" for Destinatiom, left "255.255.255.0" as it was for the Netmask, and entered "192.158.1.3" (the address of eth1 as seen from machine C) as the "Gateway/NextHop". This resulted in the following line:
192.168.0.0 255.255.255.0 192.168.1.3 eth-0 Indirect Local
appearing in the abpve table, just above the "192.168.1.0" line.
Click "Apply" and then "Close" on the subsequent window, and it's done!
Thanks to all who joined in, and get me pointingin the right direction.
Ted.
-------------------------------------------------------------------- E-Mail: (Ted Harding) Ted.Harding@nessie.mcc.ac.uk Fax-to-email: +44 (0)870 094 0861 Date: 27-Oct-05 Time: 11:50:41 ------------------------------ XFMail ------------------------------
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Ted.Harding＠nessie.mcc.ac.uk