Re: [LARTC] One machine, two net feeds, outbound route selection

[Peter Rabbitson <rabbit+list@rabbit.us>]

Ben Scott wrote:
> On 10/25/07, Peter Rabbitson <rabbit+list@rabbit.us> wrote:
>> Unfortunately not easy without doing local NAT (from the local interface
>> to another local interface).
> 
>   I thought that might be the case.  I even started to write a rule
> about how the NAT might work... but then I ran into brain pain trying
> to figure out how, because I didn't know when the packets get what
> address/interface info assigned to them, and I didn't know how SNAT
> would interact with the routing tables.  Normally, I do SNAT in the
> POSTROUTING chain, but by then the routing rules have already run,
> right?  So the packet would still be bound for the wrong interface,
> even if the source address is translated.  No?
> 

I was not thorough enough. The NAT is necessary in order to make the 
packet come back through the link/interface you want (because as I noted 
previously you do not have control over the choice of a source address). 
Once this is out of the way the only problem is how to make an already 
routed packet to leave via a different interface. One way to do this is 
the ROUTE target: 
http://netfilter.org/documentation/HOWTO//netfilter-extensions-HOWTO-4.html#ss4.5 
There might also be other ways to do this, but I never investigated, as 
this is a mostly theoretical exercise.

Peter
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