DHCP not updating DNS records when using DDNS

Kyle Johnson kjohnson at fixertec.net
Fri Jan 11 12:58:08 UTC 2013

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
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