Subject: Re: illegal network routes and a ponderance
To: der Mouse <mouse@Rodents.Montreal.QC.CA>
From: Pavel Cahyna <>
List: tech-net
Date: 02/19/2003 13:32:30

I have a question which is probably simpler than the rest of this

> *What* kind of routing?
> What would it even _mean_ to have a route pointing to a gateway that's
> not on-net for any configured interface address?  Where would you
> expect such packets to be sent?  I can't see anything sensible to do
> with them.

Why is this (having a route pointing to a gateway that's not on net for any
interface) not supported, even if there is a route for this gateway
pointing at another gateway which is on-net? I. e. could the routing table
be searched recursively, until a directly-reachable gateway is found?

hypothetical example (on a host with address) would be:

root@omega:/root# route show
Routing tables

Destination       Gateway            Flags 
default        UG      UG

Of course you will ask why would anybody want to do this instead
of just having as the default route.

One application that I found useful is the following: the routing code
could at every lookup of gateway add a Source Routing header (or
analogous Routing header for IPv6) prescribing that the packet must go
through this gateway.

So if in the scenario above I did

it would act as 

ping -g

and so on.

Is this possible to implement and not illegal according to the

Reason for this is that right now, the Source Routing must be
implemented by the application.

Thanks	Pavel