DLZ / ISC DHCP query

Marty Lee marty at maui-systems.co.uk
Tue Apr 1 08:52:26 UTC 2014

Ok, finally managed to get a test rig set up with wireshark and have
now seen more about what’s going on & can see the pre-requisites going
over the wire.

Versions: ISC DHCPD 4.2.6, Bind 9.9.5

DHCPD sends a dynamic update with a pre-req that the name doesn’t exist
Bind replies with a fail, as the name does exist

DHCPD then sends a new dynamic update with a pre-req that the TXT record exists
Bind replies with a success, however:
 - within the packet are 2 updates
  - 1st is to remove the original ‘A’ record
  - 2nd is to add the new ‘A’ record
 - Bind calls the dlz ‘dlz_subrdataset’ but not the ‘dlz_addrdataset’ for the 2nd update record

End result, is that once the TXT record exists, Bind 9.9.5 just tries to delete the A
record from the update and doesn’t create the new one.

So - looks like something is up with the Bind code, so I’m off to have a look at that;
especially now I can play with all of this on a test network and it’s 100% repeatable.



On 27 Mar 2014, at 19:13, Evan Hunt <each at isc.org> wrote:

> On Thu, Mar 27, 2014 at 06:58:35PM +0000, Marty Lee wrote:
>> BTW, doing a manual Dynamic DNS update using nsupdate works fine - the A
>> and TXT records are created without any problem and the A record isn?t
>> then deleted, so it?s something to do with the DHCP server and it?s
>> interaction with Bind.
> I'd run wireshark on the link between dhcp and bind9 to see what
> the update packets look like.  When you tested with nsupdate, did you
> use prerequisites?
> -- 
> Evan Hunt -- each at isc.org
> Internet Systems Consortium, Inc.

Marty Lee                         e: marty at maui-systems.co.uk
Technical Director                v: +44 845 869 2661
Maui Systems Ltd                  f: +44 871 433 8922
Scotland, UK                      w: http://www.maui-systems.co.uk

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