RE: Router stops routing after changing MAC Address

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

On Mon, 13 Mar 2006, Greg Scott wrote:

> Yup.
>
> I had a situation 2 weeks ago where a customer connected a system to the
> Internet with an IP Address he should not have used.  And the little
> Cisco router on the frontend dutifully recorded it in its ARP cache -
> forever, with no TTL!  This took down their webmail for most of a day
> until we finally had to cycle the power on that nasty little Cisco 678.
>
> Bigger routers do it too.  I've had several situations over the years
> where I replaced an older firewall with a newer one with the same IP
> Addresses.  All the internal servers find it soon enough.  But I've
> waited literally hours for the routers to finally purge their ARP caches
> so they would see my replacement systems - often with the customer
> looking over my shoulders getting more and more nervous by the minute.
>
> And sometimes the routers are not accessible - you can't cycle them even
> if you had permission.  Consider the cases of bridged DSL service -

Bzzzzst... Not! There are not any MAC addresses associated with any
of the intercity links, usually not even in WANs!  MAC is for
Ethernet! Once you go to fiber, ATM, T-N, etc., there are no
MAC addresses. That's why there are bridges and routers, you
got to "connect" your tiny time-slot to your LAN and that
first device contains the MAC address that all your other stuff
talks to.

> where the real router could be on the other side of the country.  Try
> calling an ISP and asking the tech on the other end to purge an ARP
> cache on a router.  So the same IP Addresses but different MAC
> addresses, all you can do is wait for the passage of (lots of) time.
> That happened to me in my own network once.  I accidently took down my
> email server for something like 4 hours one time when I got careless.
>
>> Indeed, there is a large onus on the software doing the MAC
>> override to make sure it does not break the required uniqueness.
>> Just as if one were using locally administered MAC addresses.
>
> Yes.  My 12:34:56 OUI scheme will work for this project but it is
> definitely not good for the long term.  I really really hope I have to
> spend some money with the IEEE soon to support lots and lots of
> rollouts.  :)
>
> - Greg Scott
>
>
>
> -----Original Message-----
> From: Rick Jones [mailto:rick.jones2@hp.com]
> Sent: Monday, March 13, 2006 3:50 PM
> To: linux-os (Dick Johnson)
> Cc: Greg Scott; Chuck Ebbert; linux-kernel; netdev@vger.kernel.org; Bart
> Samwel; Alan Cox; Simon Mackinlay
> Subject: Re: Router stops routing after changing MAC Address
>
> > Anyway, if the device fails, you have
>> routers and hosts ARPing the interface, trying to establish a route
>> anyway.
>
> But only after what may be a much longer time than the customer is
> willing to accept or able to configure.  I know of a number of HA
> situations where the "new" device is given the "old" MAC just to avoid
> that speicific situation of ARP caches not being updated except after
> quite some time.  Not necessarily on the end-systems, the issue can be
> with intermediate devices (routers).
>
> And if one has to work with static ARP entries to deal (however
> imperfectly) with ARP poisioning or whatnot...
>
> Indeed, there is a large onus on the software doing the MAC override to
> make sure it does not break the required uniqueness.  Just as if one
> were using locally administered MAC addresses.
>
> rick jones
> -
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Cheers,
Dick Johnson
Penguin : Linux version 2.6.15.4 on an i686 machine (5589.54 BogoMips).
Warning : 98.36% of all statistics are fiction, book release in April.
_


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