Re: Patch for route.c

H. Peter Anvin (
Thu, 10 Apr 1997 09:22:11 -0700 (PDT)

> Alan, the network topography has to be controlled by the routers, not the
> hosts. The hosts have no way to accomplish anything by modifying their
> routing tables using 'routed` or 'gated`. No matter what they do, the
> hosts will end up sending their data out the only path to the network,
> their single Ethernet card.

Yes, but what do they set the target MAC address to? An IP datagram
crossing an Ethernet is sent from one host to another, *not* onto the
network as a broadcast. Thanks God.

> When I ^X this message, it will go the hardware address of my Cisco Router.
> None of the other 600-odd nodes on the Local Area Network will even know.
> The Cisco Router doesn't really "know" that it has a route to you. It only
> knows that the IP Address received during the ARP was not for my domain.
> It therefore responds with its hardware address. It will forward anything
> it gets out the T1 link.

ARP does not receive IP addresses. ARP handles IP to MAC address
translation for local networks. Whether or not the network is local
is determined by the formula:

(local_address ^ remote_address) & netmask =?= 0

> One of the advantages of Linux is it allows the default route to be
> specified as `route add default dev eth0` without a named gateway. Any
> gateways that are still alive can then route packets out of the local
> area.

This kind of operation is called proxy-ARP, and is usually considered
a bad idea.