DHCP not updating DNS records when using DDNS

James Michael Keller jmkeller at houseofzen.org
Wed Jan 16 17:09:25 UTC 2013

On 01/11/2013 07:58 AM, Kyle Johnson wrote:
> Hello everyone,
> DHCP isc-dhcpd-4.1.1-P1
> BIND 9.8.2rc1-RedHat-9.8.2-0.10.rc1.el6_3.5
> CentOS 6.3 clients
> I am having an issue.  I am going to step through my scenario below.  
> Please forgive me if I get a few details wrong.
> I create a VM with the hostname foobar.  DHCP gives it a lease, and 
> with DDNS, creates the DNS (A, PTR and TXT) records.  This is good.  
> Now I destroy foobar and recreate him with the same hostname, but put 
> him on a different network (vlan); foobar's MAC address has obviously 
> changed.
> At this point, after firing foobar back up, he will get a DHCP lease, 
> but the DNS records will not be updated because the TXT record (a hash 
> of MAC + hostname?) does not match.
> This seems to be the expected behavior, however it is not the behavior 
> that I want to see.  So now I modify foobar's dhclient.conf and tell 
> it to send an identifier, like so:
> send dhcp-client-identifier "foobar.domain.tld";
> Next I freeze my forward and reverse zones, manually remove any 
> offending entries (A, PTR and TXT), thaw the zones, and then reboot 
> foobar.
> Now his DNS records are created, because there is nothing to conflict 
> with.  This time, the TXT record should match the client's identifier 
> (his hostname, in this case, as sent in dhclient.conf).
> So once again, I shutdown foobar, remove his NIC, add a new NIC 
> (changing the MAC address), and put him on a different network.  After 
> firing him back up, he gets a DHCP lease on the correct network, but 
> still his DNS records are not updated!
> I am imagining that my understanding of DDNS and TXT records is way wrong.
>  1. Are my above assumptions correct?
>  2. Is a TXT record, by default, a hash of MAC + hostname?
>  3. If I specify a dhcp-client-identifier in dhclient.conf, how is the
>     TXT record now created (a hash of just the hostname now)?
>  4. I understand that the above behavior is in place to prevent
>     clients from assuming the hostname of existing clients.  In most
>     environments, this is fine, but in mine, I have enough control
>     over my network to consider that possibility moot.
>  5. Can I achieve my desired results?  I am working in a very dynamic
>     environment and do not want to manually freeze and thaw zones
>     every I need to move a host.
>  6. I did not post any log output as I don't think it is needed at
>     this point.  If it is, please ask, and I will provide.
> Thank you for your time!
> Kyle Johnson
> _______________________________________________
> dhcp-users mailing list
> dhcp-users at lists.isc.org
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You need to set 'update-conflict-detection false' which will allow dhcpd 
to over write the A/PTR/TXT when the TXT hash doesn't match or is 
non-existent (as when windows clients self update).   For multiple NIC 
hosts under the same host name (wired/wireless) this will also allow the 
entry to have the most recent assigned address or updated address if you 
have update optimization set to false as well.



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