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what would cause missing reverse DNS entries?

hi everyone
having a strange problem with some clients missing reverse DNS records. The forward entries are there. Scavenging is turned on but this happens with only a few random clients. We recreate the reverse DNS entry manually to fix. Any idea what would cause missing reverse DNS entries ? Not sure what other information to provide, other than we are not seeing any errors in the logs.
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Darius Ghassem
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Do the affected clients have anything in common that the other clients don't (OS version, static vs. DHCP address, etc)?
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ok so we have multiple remote sites, each with domain controllers. Each remote site is on a different network, for example 192.168.x.0 and 192.168.y.0 and 192.168.z.0... All are routed to the core site, with no traffic restrictions on the inside network.

None of the remote DC's have DNS. The DHCP scope at the remote sites points remote clients to our primary DNS server, which is a virtual machine dedicated to only DNS that is located at the core site. The secondary DNS server is our Primary domain controller, which is a physical box, also at our core site. So as mentioned the clients at the remote sites get DNS numbers that are for DNS servers located at the core site, which is the way it was set up when I got here.

At one site, I checked the DHCP lease pool DNS options and they are not what dariusg recommended. Currently they are set to the following
-"enable dns dynamic updates according to the settings below" is checked and the radio button "dynamically update DNS A and PTR records only if requested by the DHCP clients" is selected.
- "Discard A and PTR records when lease is deleted is checked
- "Dynamically update DNS A and PTR records for DHCP clients that do not request updates (for example, clients running Windows NT 4.0" is UNchecked.

Not sure what the impact will be for changing those settings to what DariusG recommended.  I will try to look that up, any additional advice is appreciated, thanks in advance.
No impact except it will register the records the way you want.
Secondly really should look at redesigning your DNS infrastructure within your network as the DCs at the remote sites aren't even being used most likely I bet most all clients are authenticating to the main DC at the HQ office
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thanks I will make the changes you recommended and report back
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so we made the changes and still getting some missing entries. Seems like its getting scrubbed by scavenging which is turned on.