same ip to the same mac ?

Simon Hobson dhcp1 at thehobsons.co.uk
Thu Feb 1 11:28:04 UTC 2007


Gene Rackow wrote:

>  >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.

Congratulations, I think this is the first genuine reason for making 
addresses change !

You could suggest to him that he uses dns entries rather than ip 
addresses. He can either use one of the dynamic dns services, or if 
you run ddns he may be able to use that (depending on your setup and 
whether it's publicly visible).


>Yes, I could write a script to stop the dhcp server at 2am,
>then purge the leases file of free addresses and restart.

Couple of other ideas.

One is to have a number of pools and rotate them, eg :

subnet .... {
   pool {
     range a.b.c.50 a.b.c.99 ;
     deny booting ;
   }
   pool {
     range a.b.c.100 a.b.c.149 ;
   }
   pool {
     range a.b.c.150 a.b.c.199 ;
   }
}

clients attempting to renew address in the ppol with "deny booting" 
will be forced to get a new address.


Another method might be to adapt one of the dhcp testing tools to 
create (short) leases (using random client-ids) until the pool is 
exhausted which would have the effect of erasing prior assignments.


>It would be nice to have a pool option to "disable historical_data".

That isn't going to happen, sorry.

Whilst your requirement is reasonable, I think it's the first time in 
the years I've been on the list that it hasn't been "lets p**s off 
our customers by breaking all their connections every two hours 
because stupid management think it will stop them running servers". 
So it's a feature that's only had one reasonable request in years, 
breaks the RFC, and most importantly would require some fairly major 
changes I suspect to the code in order to implement. And all at a 
time when the ISC team are committed to a fairly important project to 
produce an IP6 reference DHCP server.



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