RE: 2 NIC's on same network, possible?

["David Gillett" <gillettdavid () fhda edu>]

  The *effect* of the default route is to match any networks
for which you do not have any other route, but it does this
by matching ALL destinations.  It only fails to match networks
you have another route because it does not get checked for a 
match until all other routes have been considered.
  Routing decisions are made by examining one possibility at 
a time; once a match has been found, the rest of the routes
need not be examined.  So the presence of multiple candidate 
matches doesn't interfere with the process at all.

  Someone else has indicated that some implementations may 
complain about multiple routes to a specific (non-default)
destination network UNLESS they have different weights, but
(a) you weren't that specific, and (b) this simply ensures
that the route table entries always get *ordered* the same 
way.  (In fact, some dynamic route-selection algorithms act 
by modifying the weights of redundant routes to reflect load
distribution -- and that *requires* two or more routes to
exactly the same network.  OSPF, I believe, allows four.)

  The example of the default route is only the most common
counter-example which contradicts your assertion.  You most
certainly CAN have multiple routes "pointing towards the
same network".

David Gillett


> -----Original Message-----
> From: Ansgar Wiechers [mailto:bugtraq () planetcobalt net]
> Sent: July 29, 2003 12:33
> To: security-basics () securityfocus com
> Subject: Re: 2 NIC's on same network, possible?
>
>
> David,
>
> On 2003-07-29 David Gillett wrote:
> > > Correct. Your machine can't have two routes pointing 
> towards the same
> > > network.
> >
> > If you have a default route, you HAVE two routes pointing towards
> > every network for which you have an explicit route.  This cannot be
> > a correct statement without some further qualifiers!
>
> The default route is a fallback which applies to every address/network
> you do *not* have an explicit route for. I do not see how this would
> qualify as "two routes towards the same network".
>
> Regards
> Ansgar Wiechers


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