[Gllug] arp problem?

[Casper Gasper]

On 11/06/06, Adrian McMenamin <adrian at mcmen.demon.co.uk> wrote:
> On Sun, 2006-06-11 at 21:21 +0100, Casper Gasper wrote:
> > On 11/06/06, Ian Northeast <ian at house-from-hell.demon.co.uk> wrote:
> > > 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 with *some* devices, it's how we used to get an address onto
> > > Axis print servers. These interpret a ping sent to their MAC as an
> > > instruction to set the IP address to the one in the packet. This is
> > > enough to get onto the box with telnet/HTTP and configure the netmask,
> > > gateway etc.
> > >
> >
> >   Interesting -- never heard of that before!  Technically though, it's
> > not quite the same thing; the box is taking the ip address from the
> > first ping packet it receives, as opposed to being able to respond
> > without an ip address (or an incorrect one), which is what I'm arguing
> > won't work.
> >
>
> Well the box (the Dreamcast) does respond. So how does that work?
> (Serious question)
>

  The _only_ way this will work is if the ip number you're pinging is
tied to the interface.  Maybe it's getting the ip address from the
first ping packet (never heard of that technique before, as I said),
but it won't work without an ip address, and it won't work if the ip
address is different from what you've pinged, regardless of what you
have in the arp table.  As I've said before, try it on another
machine, create a new arp entry for the Dreamcast with a different ip
address, you'll see.

Casper.
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