[Gllug] arp problem?

[Richard Cohen]

On 11/06/06, John Winters <john at sinodun.org.uk> wrote:
> On Sun, 2006-06-11 at 12:36 +0100, Adrian McMenamin wrote:
> > On Sun, 2006-06-11 at 03:28 +0100, Casper Gasper wrote:
> > > On 10/06/06, Adrian McMenamin <adrian at mcmen.demon.co.uk> wrote:
> > > > I have a network card (eth0) attached to this (Ubuntu) box which has a
> > > > crossover cable connection to one other device.
> > > >
> > > > I assign the attached device's IP addr using arp - I have this in
> > > > rc.local:
> > > >
> > > > arp -s 192.168.61.55 00:d0:f1:03:12:b9
> > >
> > >   Why do you need to put a static entry in the arp table?
> > >
> > What is the alternative? (Seriously).
>
> The alternative (normal) way is just to let the boxes discover MAC
> addresses using the ARP mechanism.  The conversation goes something like
> this:
>
> 1st box (shouting): Anyone out there got IP address XX.XX.XX.XX?
> 2nd box:            Yup.  That's me.
>
> Both boxes then cache the MAC address of the other in their arp caches
> and the subsequent conversation continues much more quietly.
>
> It's only if you're doing something a bit odd that you need to pre-load
> the arp cache with MAC addresses.

There are two reasons which come to mind to load a static value into
the ARP cache:

You're running a system without a hard drive, say, a Dreamcast, which
has a kernel which picks up its own IP address at boot time by doing
ARP the other way - "I've got MAC address XXXX - what is my IP
address?"

You're running something off a non-ethernet connection - say, an iPAQ
off a serial line - but you want to connect it into your LAN with a
regular address rather than masquerading - so you publish an ARP
record for your machine's MAC but the iPAQ's IP...

> HTH
> John

Cheers
Richard
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