[Gllug] arp problem?

[Casper Gasper]

On 11/06/06, Adrian McMenamin <adrian at mcmen.demon.co.uk> wrote:
> On Sun, 2006-06-11 at 13:48 +0100, Casper Gasper wrote:
> > On 11/06/06, Adrian McMenamin <adrian at mcmen.demon.co.uk> wrote:
> > > On Sun, 2006-06-11 at 13:12 +0100, Casper Gasper wrote:
> > > > > But the second box doesn't have an IP address, it just has a MAC
> > > > > address. The box is booted with a very simple slave program which simply
> > > > > loads loads the kernel into the right memory location and runs the
> > > > > bootloader.
> > > >
> > > >  If it doesn't have an ip address, why are you attempting to ping it?
> > > >
> > >
> > > Because the arp table ought to tell the kernel which MAC address to aim
> > > for
> >
> >   That won't work, I've thought that in the past but it doesn't.  The
> > arp table entry will send the packets to the machine, but when say a
> > ping packet goes through the TCP/IP stack, it will get discarded
> > because the destination IP address doesn't match the local ip -- or
> > else, forwarded if ip forwarding is enabled on the machine.
> >
> It does work.

 No, it doesn't!  To the extent that it ever worked in the past, it's
cause the machine had the right ip address assigned to it that happens
to be in your arp table.  I've been down this alley before, thinking
it will get me into embedded boxes like routers when I don't know the
ip address.  Don't believe me, try it for yourself -- find a machine
on your subnet you can ping, create an arp entry for the mac address
with a different ip address, and give it a go.

 Casper.
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