"deny duplicates;" not working?

Sten Carlsen stenc at s-carlsen.dk
Tue Jan 8 19:06:20 UTC 2013

On 08/01/13 19:51, Neff, Glen wrote:
> We have proprietary network devices that use rely on DHCP for 
> configuration.  We're using a standard 72 hour lease time, and we rely 
> on the IPs to not change, so long as these devices aren't offline for 
> much more than a reboot.
> Static assignments by MAC address are not feesible for our purposes.
> The problem that we are encountering is that periodically these 
> devices must have their firmware updated and these updates are causing 
> the UID sent in the DHCP request to be changed.
The main piece of information to identify a client is actually the UID, 
so with a new UID -> a new client, obviously it can not get the same IP, 
that is reserved for the old client.
You may have luck looking into classes and subclasses. Possibly with 
"spawn". I have never used these features but I have a feeling they may 
actually do what you need.
Use the hardware = MAC to create your subclass and that could be 
independant from the UID - I think.
> Here's an example of three entries taken from the dhcpd.leases file on 
> the server: http://pastebin.ca/2299463
> The first entry was was generated by a reboot early in the day.
> The second entry was "control case" reboot made before we updated the 
> firmware on the client device.  Notice how it's identical to the 
> first, with the exception of the times.
> The third entry is after the firmware update.  Notice that the MAC 
> address remains the same, however the UID has changed, so the server 
> issued it a new IP.
> In consulting dhcpd.conf(5) I found "deny duplicates;" which sounded 
> like a magic bullet workaround, so I added it to the subnet 
> declaration in dhcpd.conf file, but the problem remains the same (in 
> tests with other of the same type devices and firmware upgrade scenario).
> The server version is 4.2.4, running on FreeBSD v8.2-RELEASE.  It's 
> built via the ports system: isc-dhcp42-server-4.2.4_1
> There is a DHCP-relay involved - it's configured on the VLAN interface 
> of a Cisco Nexus 7010, running NX-OS v 5.2(3a).
> Any insite would be appreciated.
> -G
> /*
>  * Glen R. J. Neff
>  * USD Lab Operations Infrastructure Team
>  * glen.neff at emc.com
>  *
>  * EMC^2 == E^2
>  */
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Best regards

Sten Carlsen

No improvements come from shouting:


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