DHCPd problem with PXE/Windows clients - multiple leases

David Rickard david.rickard at bcuc.ac.uk
Wed May 9 10:43:17 UTC 2007


Thanks for the replies.

I did find the patch to change the behaviour - it's definitely a
possible way out of this, although presumably only works on certain
versions of the code. I'm not at all scared of patching and compiling
dhcpd myself tho, so I will quite possibly try it.

I did find (at the suggestion of a colleague) that setting a static
address in the host declaration in the dhcpd.conf makes the PC stay on
the same address regardless, probably because it is always using the MAC
as the identifier. That will actually work out quite nicely for us with
some PCs as we would prefer to have them on static IPs (for firwalls and
things), only they need to be able to roam, so DHCP is desirable for
maintaining the configuration.

Our open-access student computer rooms we have less concern about which
IP which PC is on, so in those instances, the patch would be more
relevant, but I am looking at re-writing my web interface for managing
the hosts to just automagically put the IPs into the config and be done
with it.

I'll give those two options a whirl though.

Many thanks again.


David Rickard
ICT Security Officer
david.rickard at bcuc.ac.uk, 01494 522141 Ext: 3531
Information & Communication Technologies
Buckinghamshire Chilterns University College
Queen Alexandra Road, High Wycombe, BUCKS, HP11 2JZ

-----Original Message-----
From: dhcp-users-bounce at isc.org [mailto:dhcp-users-bounce at isc.org] On
Behalf Of Simon Hobson
Sent: 08 May 2007 16:53
To: dhcp-users at isc.org
Subject: Re: DHCPd problem with PXE/Windows clients - multiple leases

David Rickard wrote:

>We've got three DHCP servers on three sites, running the ISC DHCP
>on SuSE Linux Enterprise Server 10. We also have Zen for Desktops 7
>Hotfix 3) on all three sites running on Netware 6.5.
>My problem is that when the PC boots, it ends up with two leases - one
>for when the PXE runs, and another for when Windows boots!

We knew that was going to be the problem before even opening the mail !

If you looked in the archives, going back as far as a week or two, 
you'd find more than one discussion of this common problem. It's 
caused by Windows using the hardware MAC address as the client 
identifier, while PXE (and Linux, and a few others) don't. The 
client-id is the primary database key, with MAC address used if 
client-id isn't supplied - hence as far as the server is concerned 
these are two different clients.

Solutions include :

1) Don't run Windows, it's a pain in the network ;-)

2) Persuade Windows to do something different - good luck !

3) Configure all clients to use the same client id

4) set up a pool just for PXE clients and give it a short lease time 
(to allow reuse)

5) make sure the pool is big enough for all clients to have two

6) Apply Didis patch that fiddles with the client-id

7) Wait for a future version of the server that allows the admin to 
specify the key to be used.

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