RE: Caching a sniffer

["David Gillett" <gillettdavid () fhda edu>]

> -----Original Message-----
> From: Shawn Jackson [mailto:sjackson () horizonusa com]
> >   I think this suggestion was that the MAC flood would 
> > trigger the aging, so it keeps pace automatically.  That, in 
> > turn, requires the malicious 
> > host that's flooding bogus MAC addresses to continue doing 
> > so, in order 
> > to try to keep valid MACs from being retained in the table 
> > (long enough 
> > to keep their traffic from flooding and being seen by the sniffer).
>
> So there would need to be a algorithm to detect the MAC flood and
> respond by increasing the age time for records in the table. Once the
> flood stops aging would return to normal. Correct?

  Not really.  I'm a switch, and I've just seen a new source MAC 
address, which I want to insert into my table.  Uh-oh, the table
is full.  Instead of just failing to learn the new address, I'll 
"age out" the oldest address in the table prematurely to make 
space for the new one.  (If I track their ages so I can age them 
out normally, finding the oldest one shouldn't be any problem.)
  I didn't "detect the flood" or alter the normal aging time.  I
just discarded some old information instead of discarding the new
information just received.  (In essence, this means treating the
MAC table like a FIFO cache.)

David Gillett


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