[Gllug] Help with setting up a Router

[John Winters]

On Tue, 2002-09-17 at 12:52, Wolfgang Draskovic wrote:
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> 
> Hi
> 
> > All you need to do is set up the box with appropriate addresses on each
> > NIC (and implicit routes) and then a default route pointing out through
> > the outer interface (you'll need to know what the next machine in the
> > chain of steps to the outside world is).
> 
> I don't think it's necessary to know the next hop. The routingtable just 
> holds the local interface

It is necessary for a default route (or indeed, any kind of non-local
routing).

When a machine has a packet to send out to an address it can do one of
two things:

1) If the routing tables indicate that the relevant network is local to
one of the m/c's interfaces then it issues an ARP request on that
interface to try and find the hardware address of the relevant target
machine.  If that succeeds the packet is then sent direct to its target.

2) If the routing tables indicate that the relevant network is *not*
local to any of the interfaces then it forwards the packet on to another
machine (a gateway) for handling.  The most common occurence of this is
with a default route but there may also be other non-local routes to
specific networks in the table.  In either case, the system needs to
know the IP address of the "next hop" machine.  It then proceeds as for
option 1 but using the IP address of the gateway to find out what
hardware address to send to.

John

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