same ip to the same mac ?

Glenn Satchell Glenn.Satchell at
Thu Feb 1 11:03:11 UTC 2007

>To: dhcp-users at
>cc: rackow at
>Subject: Re: same ip to the same mac ? 
>Date: Thu, 01 Feb 2007 02:50:46 -0600
>From: Gene Rackow <rackow at>
>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.

>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".

Hi Gene

I would work around this by creating two classes. One for your
conference 'guests' and one for your regulars. Use whatever mechanism
you can to decide if someone is a regular or a guest and put them in
the appropriate pool. This could be sub-class matching on mac addess
(see dhcpd.conf man page and search for CLASSES).

Make the default lease time for your conference guests short, say a
couple of hours - they will renew during the day as required.

For your regulars give them a much longer lease time, say a few days or
a week. Then they will be able to keep their addresses.

You still have one range of IP addresses but the two different groups
will have different lease times. Should get you most of the way to what
you want.

RFC2131 says that the DHCP server should try really hard to give out
the same IP address to a client that returns to the network. So not
doing this violates the DHCP spec.


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