[LARTC] ISP egress filters and routing for multi-homed hosts

[LARTC] ISP egress filters and routing for multi-homed hosts

[Kenneth Porter]

I've got a multi-homed host connected to both a UUNET T1 and a PacBell ADSL 
connection. Both are commercial accounts with small dedicated netblocks. 
The default route is set to use the T1.

I recently added route entries for a couple of low-priority outside hosts 
to use the ADSL connection. I now find that connection *from* those hosts 
to the T1 interface fails. I'm suspecting it's because PBI is 
egress-filtering source addresses not under its control. (Acceptably a Good 
Thing.) For instance, I tried ping and found that the ICMP reply is going 
back through the PBI connection as expected, but the source address is that 
of the T1 interface. (From tcpdump output.)

Is there some way to force those packets to go back out the T1 interface 
while still sending packets initiated from this host through my ADSL 
interface, perhaps with iproute2 or some route cleverness?
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