dhcp deletes ddns entry

Simon Hobson dhcp1 at thehobsons.co.uk
Wed May 30 06:45:03 UTC 2007

Craig wrote:

>We have one machine (Linux: CentOS 4.4; dhclient v3.0.1) that has it's
>ip address dynamically allocated by dhcpd (v3.0.1). But, the DHCPREQUEST
>will trigger the DNS entry (via DDNS) to be deleted.
>At 8:41, the client (CENTOS_CLIENT) appears to do a DHCPREQUEST, at
>which time the dhcpd server seems to verify that the RR does exists and
>then deletes it. (Shouldn't it do an update?)
>Then the next time the client does a DHCPREQUEST (9:45), the RR does not
>exists and is, obviously, added.
>But, in the interim, the client name does not exists in DNS.
>I am running a slightly older version of dhcpd (3.0.1), but I can't find
>anything in the release notes that say this is fixed in 3.0.5. Also,
>some simple web searches did not find anything.

3.0.1 isn't "slightly older", it's ancient ! Upgrade and see if the 
problem is still there.

As to "Shouldn't it do an update?", well no it doesn't work that way. 
The algorithm is something like :

If update required (ie lease database says it hasn't been done)
   if client hasn't said it will do forward updates
     do forward update
   do reverse update

The forward update is done by
if (both A record and matching TXT record exist) then delete A 
record, delete TXT record, create new A record, delete new TXT record.

The reverse update is done by
delete PTR record(s), create PTR record.

It's just simpler to delete any existing record(s) and create new 
ones rather than work through all the permutations of possible 

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