MAC addr takeover

Cary B. O'Brien cobrien at access.digex.net
Sat Oct 17 11:04:57 MDT 1998


> On Sat, 17 Oct 1998, Cary B. O'Brien wrote:
> 
> > One way to tell the switch is to use expect, log in, and clear the arp
> > cache, right?  And there may be an SNMP way of doing the same thing.  And
> > do the switches respond to Gratitous Arp?
> 
> In this case, the switch's ARP cache would probably only contain an
> entry for the machine which you just logged in from, or from your SNMP
> manager. Remember, in a switch's case it is not the ARP cache--it is
> the bridge tables that we're talking about.
> 

Ok, I really want to understand this.  I may have to implement this stuff in the
not-so-distant future.

I said switch, I should have said router.  I'm assuming [1] that you have
a situation where you've got server A (main) and server B  (backup) on the
same lan.  Server B monitors server A.  If it fails to respond server B brings
up the IP address clients use to access the service.  Then it (somehow) clears
the arp cache on the router that connects the lan with the servers to the
outside world.  After this, the outside world can access the 'service' on 
server B at the original IP address, right?

[1] Of course, the lan itself and the router are single points of failure.  Is
this my confusion?  

-- cary





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