DHCP Failover Pairs and Option 61 Client Identifier

[David A. Evans]

        I have been fighting an issue with HP printers, DDNS and our ISC 
based fail over pair.   I'm throwing this out on the list to document the 
symptoms as I was unable to find a similar problem documented on this list 
or via google.

        It turns out there is one (1) model of HP printer that ALL give 
back the exact same, non-null, non-unique, option 61 Client Identifier. 
This caused some confusion on the fail over pair as much of the fail over 
communication is based on option 61 being unique.   What we first saw was 
a constant change in theses printers IP's when more than one is on the 
same subnet, as some of the printers kept getting the short lease time 
from the 'backup' DHCP server.  We also saw some of the printers just drop 
off the network all together.  I'm pretty sure this was due to IP 
conflicts.

        Someone that better understands the fail over communication can 
explain this but what caused us to have to figure this out is the DHCP 
fail over pair started deleting the wrong DNS entries.  I'm sure it has to 
do with the fact that one lease was expiring with the non-unique client 
identifier and when that was communicated to the other DHCP server in the 
pair, it matched it to the wrong lease in its database and then sent off 
the wrong DDNS removal message.

        HP is now aware of the issue, however I have yet to hear back on a 
fix for the printer's firmware. 

        So I'm seeing this with this config:
        1. Non-unique Option 61 (root cause and what is broken)
        2. ISC based fail over pair
        3.  Fail over pair set to do DDNS on behalf of the client


David A. Evans
Enterprise IP/DNS Management
Network Infrastructure Tools and Services
Evans_David_A at cat.com
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