DDNS client: Multiple zone entries

Merton Campbell Crockett m.c.crockett at adelphia.net
Thu Oct 26 02:01:31 UTC 2006

On 25 Oct 2006, at 14:46 , nsvora wrote:

> Hi All,
> While testing the DDNS server configuration with proprietory client, I
> noticed a strange thing. I need your help to understand this.

One way of minimizing the occurrence of multiple host name records is  
to configure DHCP to use "ddns-update-style interim;".  With this  
change, DHCP collects a little more information about the system  
requesting an IP address.  In addition, it adds some additional TXT  
records to the zone file for tracking purposes.  When the lease  
expires it removes DNS entries from both the forward and reverse zone  

The interim style also ensures that the DNS entries are removed on a  
DHCP release.  If you remember to release the DHCP address before  
changing the system's host name.  You will eliminate the problem.   
However, if you forget to release the IP address, you will only have  
"multiple" entries for the unexpired time of the lease.

The interim style does make an attempt to remove the old DNS data but  
it isn't totally reliable.

Among the additional pieces of information maintained in dhcpd.leases  
is the MAC address.  If you change the NIC in the system, you will  
need to know how to use nsupdate to clean up the zone files. :)

Merton Campbell Crockett

> Pre-condition:
> -----------------------
> DHCP and DDNS server are running on a single Red Hat LINUX machine.
> The DHCP server was able to generate the dynamic IP Address as
> specified in the dhcpd.conf.
> DDNS server was able to add the entries in the forward and reverse  
> zone
> files eg : testdns.com and 1.168.192.in-addr.arpa
> Test Condition:
> --------------------------
> I modified the host name and restarted the named service so that
> changes are reflected in the zone files. The forward zone file was
> expected to be updated with the modified host name.
> Results:
> ------------
> Forward zone had two entries viz; one with old host name and other  
> with
> new host name.
> Analysis:
> -------------
> I have found that this is happening because in DDNS client code,
> before updating the server database with modified host name, client is
> checking whether a record with "new" host name exists.
> And server replies in negative for this ( which is correct ) and  we
> add a new record for the new host name. But previous entry with the  
> old
> host name is never deleted from the server.
> This check is done via "Prerequisite" record sent by the DDNS
> client. Since the prerequisite check fails, client doesn't send
> request for deletion of the "old" host name entry.
> Forward zone file : /var/named/chroot/var/named/example.com.zone
> Could you kindly let me know if this behavior is correct to have
> multiple entries in the forward zone on change of host name.
> ( Similar would be observed for reverse zone on change of IP Address
> i.e multiple IP point to same name. )
> Thanks in advance,
> With Regards,
> Nirav

Merton Campbell Crockett
m.c.crockett at adelphia.net

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