RE: Router stops routing after changing MAC Address

["Greg Scott" <GregScott@InfraSupportEtc.com>]

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 -
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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