Re: PATCH: duplicate IP detection

Rogier Wolff (R.E.Wolff@BitWizard.nl)
Fri, 15 Oct 1999 00:29:18 +0200 (MEST)


kuznet@ms2.inr.ac.ru wrote:
> Hello!
>
> > Attached is a patch to passively detect duplicate IP conflicts. I sent it to
> > l-k two weeks ago and got feedback from Rogier Wolff who seemed to like it.
>
> Duplicate address detection does not work in this way.
> Correct way is described f.e. in RFC-2131.
> You will not find there explanation why your scheme is wrong,
> but you may get this information from IPv6 addrconf.
>
> Shortly: ARP packets with your IP source are legal and common phenomenon.
>
> Even more: on switched networks you get you own packets back
> in several exemplars frequently.

The RFC you mention, says that before using an address the machine may
ARP for its own IP address. This is the normal "gratuitous ARP" that
started this discussion.

We've decided that "refusing to use the given IP" is not good, and
that at most we can print a warning, and continue. Fine.

But if, as you say, we can get to see our own ARP packets back, we'll
respond, and notice a conflict too. (*) I'd say that getting your own
packets back unexpectedly is always a VERY BAD situation.

If you're right the patch may need to be extended and check if the
hardware address is one of our own. On the other hand, if we get a
message every minute or so when the network is spewing our own packets
back at us isn't all THAT bad.

Roger.

(*) client broadcast: DHCP request
server reply: DHCP reply: use x.y.z.w
client broadcast: ARP who has x.y.z.w
client recieves arp.
client replies: arp reply: x.y.z.w is at my:hw:ad:dr:es:ss
client recieves reply.
client sends: DHCP Nack:
client sends: DHCP request: start all over.....

-- 
** R.E.Wolff@BitWizard.nl ** http://www.BitWizard.nl/ ** +31-15-2137555 **
*-- BitWizard writes Linux device drivers for any device you may have! --*
------ Microsoft SELLS you Windows, Linux GIVES you the whole house ------

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