"deny duplicates;" not working?

Neff, Glen glen.neff at emc.com
Tue Jan 8 18:51:40 UTC 2013

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.

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.


 * Glen R. J. Neff
 * USD Lab Operations Infrastructure Team
 * glen.neff at emc.com
 * EMC^2 == E^2
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