[LARTC] routing network through bastian host

[David Koski <david@kosmosisland.com>]

Hello,

This seems to be a common problem but AFAIK, without a common solution.  The
network map is as follows:

    T1 to ISP
        |
        | /30 net
        |
+----------------+
| cisco router   |
+----------------+
        | FastEthernet0/0
        | a.b.c.1/24
        |
 (possible switch/hub here in future)
        |
        | a.b.c.2/24
        | eth0
+----------------+
|                |
|           eth1 |---a.b.c.3/24-public-net-----> lan
| linux box      |
|           eth2 |---192.168.1.0/24-private-net---> lan
|                |
+----------------+

Of course the above does not work as eth0 and eth1 are on the same subnet.  So
far, I can see two possible solutions:

1. reassign the cisco to linux net to a private point to point /30 net.  I have
tried with limited success.

2. reassign the cisco to linux net to a.b.c.1/29 (cisco) and a.b.c.2/29 (linux
box).  This would give me a block of 8 between cisco and linux and would enable
me to add a switch/hub between to feed another host for backup mail relay and
dns server.

Questions:

1. Does a different subnet mask define a different subnet?  Will the "two nics
same net" conflict be resolved by applying a longer mask to one subnet?

2. What is required to route through the linux box in "solution" 2 above?  Is
this a case of "more specific, preferred route" applied?

3. Any relevant RFC's would be helpful.

4. Of course, ANY suggestions welcome.

TIA,
David Koski
david@KosmosIsland.com
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