same ip to the same mac ?

Gene Rackow rackow at mcs.anl.gov
Thu Feb 1 08:50:46 UTC 2007


Simon Hobson made the following keystrokes:
 >Petre Bandac wrote:
 >The algorithm for choosing a lease for a client is (roughly) :
 >
 >Has this client had a lease in it's current subnet before ?
 >   If so, then give it the last address it had
 >
 >Is there an unused address ?
 >   If so then allocate a previously unused address
 >
 >Find a previously used but currently free address and offer that. 
 >This is done on a least recently used basis, and obviously once the 
 >address has been reused like this then the link with it's previous 
 >usage is lost.
 >
 >Now, why do you want the address to keep changing ?

There have been times where I have wished that I could turn off
the history feature of the server.  I had a network for visitor
use.  The lease time was set long enough that the would get the
address for the day, but the next morning would need to request
new.  This net needed to be big enough to handle a couple conference
rooms full of people.

One day just after a rather well attended conf, one of our regular
visitors returned after a 2 week vacation and was rather upset that
he got a different IP address than the one he "always got in past".
Turns out he had been getting the same IP address for over a year
and had assumed that he would always get that address.  He had set
up some things at his home institution to accept connections
from this address.  Since the conf was so well attended, and
his address hadn't been online in over a week the system reused
his past address.  This could have happened several times since he
started coming, but he tended to be arround during the confereces
as well so got his lease before it was needed by others.

You can set all the expectations that you want on things, telling
them that this is only a dynamic address and you can't assume you
will get the same one tomorrow or next week.  If they do get the
same one for a long time, they will assume that is the norm no
matter what you tell them...  If the system had given him different
addresses when he came in, he would have probably come to us
and asked for a static assignment.

Yes, I could write a script to stop the dhcp server at 2am,
then purge the leases file of free addresses and restart.
It would be nice to have a pool option to "disable historical_data".

--Gene


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