DDNS issues

Simon Hobson dhcp1 at thehobsons.co.uk
Mon Mar 22 21:06:04 UTC 2010

Alex Moen wrote:

>A change from the manufacturer has caused the client IDs to change 
>if the client configuration is changed.

This is the root of your problem - since the client has changed, then 
as far as the DHCP server is concerned it is a completely different 
client. You should "express your dissatisfaction" in their direction 

>   This change causes a couple of things to happen, and this is where 
>my problem lies:
>1. Client requests assigned IP address after provisioning has 
>changed client ID.
>2. DHCP server NAKs based on the changed client ID.
>3. Client and server process DISCOVER, OFFER, REQUEST, and ACK 
>sequence.  Client is given a new IP address.
>4. DHCP server attempts to change the DDNS information to DNS server.
>5. DDNS update fails, with these in the DNS server log file:
>	Mar 18 10:55:46.770 update: info: client 
>updating zone 'rg/IN': update failed: 'name not in use' prerequisite 
>not satisfied (YXDOMAIN)
>	Mar 18 10:55:46.774 update: info: client 
>updating zone 'rg/IN': update failed: 'RRset exists (value 
>dependent)' prerequisite not satisfied (NXRRSET)
>Question: how do I fix this problem?  Can I somehow force the DHCP 
>server to ignore the client ID and process only on the MAC address 
>and Circuit ID?  And, why does the DNS server not accept the change 
>from an authorized DHCP server?  How is a situation like this 
>supposed to be handled?
>I am currently running BIND 9.2.4 and DHCP 3.0.3.  I know these are 
>both very old versions (servers have been up for 1585 days and 1494 
>days respectively), and I plan on upgrading to current releases 
>during tomorrow's maintenance period, but before doing that I would 
>like to know if it will help...

Taking that last bit first, I'm not sure if one proposed change has 
made it in to code yet - the ability to specify the primary key. 
Earlier versions are hard coded to use the client identifier if 
present, and fall back to the MAC address if it isn't. This is 
required behavior by the RFCs although it does cause some problems 
like you are having - the main issue that comes up is machines that 
multiboot between PXE/Linux/other that don't supply a client id, and 
Windows that supplies a client id set to the MAC address. Another 
time it comes up is machines with two interfaces (eg a laptop with 
wired and wireless interfaces).

The DNS server doesn't accept the change because it's how the change 
is presented by the DHCP server - a prerequisite is supplied which 
requires that the entry not already exist before adding it. Note that 
the DHCP server does NOT know the name is not in use - "another" 
client still has it until it's lease times out.

An easier fix may be to change the end time of the old lease to a few 
seconds in the future - then the DHCP server will expire it and 
delete the DNS entries when you restart.

Had you known in advance of the change, then you could have shortened 
the lease times prior to the change being implemented, then the old 
leases would have expired more quickly and client outages would have 
been much reduced. The new clients would also have renewed their 
leases more often, and so picked up the DNS changes more quickly 
after the old lease expired (the server will keep retrying the DDNS 
update each time the client renews).

You may caret o read this thread :
and in particular, this post where David explains exactly how the 
updates are done.
As you'll see, this isn't a new problem !

Simon Hobson

WANTED: "Software CD ROM Kit" for Canon CLBP 360-PS printer (Canon 
part no RH6-3612, or possibly RH6-3810, or RH6-3610 might do). I've a 
dead HD and need this CD so I can replace the disk and re-install the 
printer OS on it.
This is NOT the same thing as the printer drivers to load on the 
computer - there's no problem there.
If anyone knows where I might get hold of one I'd be grateful - 
requests to Canon drew a blank, it's been out of support for years.
Alternatively, if anyone has one of these and would let me image 
their hard disk ...

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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