Question about dhcp-client-identifier

Simon Hobson dhcp1 at
Tue Aug 21 23:01:52 UTC 2007

Darren wrote:
>That is true until you have two devices with the same mac address.

Then your network is 'broken'. If it's users doing silly things then 
educate them with a piece of clue-by-four ;-) Mind you, I gather Dell 
managed to ship some duplicates* when Manchester Uni bought a load of 
machines in one batch which caused 'interesting effects' !

>   Then
>it uses the UID and allocates the two devices two different ips.  If
>these two devices are in the same physical network, they can't function
>in this situation anyway, so better to give them both the same IP
>address and ignore the fact that they are two different devices.  Even
>if they are in two different physical networks, this behavior is still
>desirable in our configuration.

In both cases the devices will not work properly, wouldn't it be 
better for them to be very broken so that the problem is less subtle 
and less hard to diagnose ?

* I think it was something like for every 256 MACs (ie first 5 bytes 
the same), there were 257 machines with two ...:00 or ...:ff. Since 
they bought several thousand machines and run a flat network, they 
got a number of duplicate MACs.

There is a patch about that modifies the behaviour regarding 
client-id - it was done to deal with the "PXE (and Linux) doesn't 
sent it but Windows sets it to MAC address" problem that eats up IPs. 
Whether it will help with your problem I don't know.

>Date: Tue, 12 Dec 2006 07:43:57 +0200
>From: Yedidyah Bar-David <didi at>
>To: dhcp-users at
>Subject: Re: status update
>On Mon, Dec 11, 2006 at 04:35:15PM -0500, SCOTT BURGIN wrote:
>  > Hello.  Running V3.0.4 server on RHEL.
>  >
>  > We have a remote site using PXE to image workstations.
>  > What we're seeing is the initial PXE boot (with a certain uid) is
>  > causing a lease to be written to the leases file as active.
>  > After the image is laid down and the machine boots the second time, the
>  > machine sends another Discover using a different uid, getting a
>  > different lease.
>  >
>  > I understand this is correct behavior per RFC, but have read that this
>  > is widely acknowledged to be an implementation PIA.
>  > What's fuzzy to me after searching the archives is what everyone agrees
>  > to be options currently to this dilemna for me.
>  > I've heard of patches, but can't seem to find them...other options ?

Also, I believe it's planned for a future version to allow the admin 
to specify the key - so you could change it from the current (fixed) 
setting of "pick first ( client_id, hardware)" to just "hardware".

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