Unusual use of ssh

Doug Hardie bc979 at lafn.org
Wed May 21 23:35:31 UTC 2008

I have an unusual situation that I suspect is not practical, but just  
in case...

I have a class C network with a T1 to the internet.  There are a  
number of hosts on that network.  Unfortunately the T1 line is just  
part of a path with several additional links before it gets to the  
upstream ISP.  Some of those links are relatively prone to outages.   
In the same facility, I have a number of WiFi access points that are  
connected through a router to a DSL connection to the internet.  That  
path is completely independent from the T1 and actually goes through a  
completely different set of central offices.

What I have tried to do is to link the DSL router to one of my hosts  
via a separate NIC and address that is on the LAN of the WiFi router.   
So far all is good.  I can ping any of the access points from that  
host just fine.  I have established a pass through port in the DSL  
router for SSH that sends the packets to that host.  Sure enough, ssh  
packets are received by the host.  The problem is that it does not  
respond on the right interface.  The routing table uses a default  
route through the T1.  Thats where the sshd responses are being sent.

Since I have no a priori knowledge what IPs I would have available  
when I need to use this back door, I can't pre-setup the routing  
table.  I need sshd to respond on the same interface it receives the  
packets from.  I don't believe that is possible using IPv4 routing.  I  
think that it is using IPv6 but none of the networks involved support  
that yet.  I don't find any option in sshd to force it to respond on  
the right interface either.  Is there something I have missed?

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