From mboxrd@z Thu Jan  1 00:00:00 1970
From: Greg Wilson-Lindberg <gregw@spacex.net>
Subject: Multiple interfaces with the same IP address
Date: Wed, 7 Oct 2009 12:36:58 -0700
Message-ID: <4ACCEDDA.9020508@spacex.net>
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Return-path: <netfilter-owner@vger.kernel.org>
Sender: netfilter-owner@vger.kernel.org
List-ID: <netfilter.vger.kernel.org>
Content-Type: text/plain; charset="us-ascii"; format="flowed"
To: netfilter@vger.kernel.org

At this point I'm just asking for a direction to go to get the information that I need.  While I've 
been writing low level networking software for many years, I've never had to get into routing.

Here's my problem, we have an environment where we have a machine with one ethernet connection to 
the wider network and a number of HDLC cards that connect to other embedded systems that we NAT out 
to the ethernet WAN.

                    ------------------------------
                    |       (172.16.4.9/12) hdlc2|---------\
                    |                            |         |
WAN 10.0.0.0/8 ----| eth0 (10.1.32.x/8)         |         |
                    |                            |         |
                    |       (172.16.4.9/12) hdlc1|------\  |
                    |                            |      |  |
                    |       (172.16.4.9/12) hdlc0|---\  |  |
                    ------------------------------   |  |  |
                                                     |  |  |
                                                     |  |  |
                         machine1a-------------------|  |  |
                     (172.24.127.249)                |  |  |
                     (172.16.129.9)                  |  |  |
                                                     |  |  |
                         machine2a-------------------|  |  |
                     (172.24.0.37)                   |  |  |
                     (172.16.129.17)                 |  |  |
                                                     |  |  |
                         machine3a-------------------|  |  |
                     (172.24.0.41)                   |  |  |
                     (172.16.129.25)               ...  |  |
                                                        |  |
                         machine1b----------------------|  |
                     (172.24.127.250)                   |  |
                     (172.16.129.9)                     |  |
                                                        |  |
                         machine2b----------------------|  |
                     (172.24.0.38)                      |  |
                     (172.16.129.18)                    |  |
                                                        |  |
                         machine3b----------------------|  |
                     (172.24.0.42)                      |  |
                     (172.16.129.26)                  ...  |
                                                           |
                         machine1c-------------------------|
                     (172.24.127.251)                      |
                     (172.16.129.11)                       |
                                                           |
                         machine2c-------------------------|
                     (172.24.0.39)                         |
                     (172.16.129.19)                       |
                                                           |
                         machine3c-------------------------|
                     (172.24.0.43)                         |
                     (172.16.129.27)                       |
                                                         ...

As can be seen from the above diagram, the addresses on the HDLC channels are not on separate 
subnets.  We also have some broadcast addresses that the machines on the HDLC nets will need to 
receive.  We do know all of the addresses for all of the machines on the HDLC nets, but we could end 
up with as many as 30-50 on each net.  Most of the packets coming from the ethernet WAN will go to 
just one of the HDLC nets, but the broadcast packets will need to go to all of them.  Also, the HDLC 
nets do not need to be able to communicate between themselves.

Can anybody at least point me in the proper direction on how I need to attack this problem?

Thanks in advance,

Greg Wilson-Lindberg