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DHCP/DNS Conflicts

Posted on 2014-03-03
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Last Modified: 2014-03-05
We at times have issues with DNS IP conflicts. The IP that a client actually has in DHCP does not necessarily match what is in DNS. This means stale records in DNS. If I do an ipconfig /registerdns from a client experiencing the mismatch, there is no change to the client's DNS record. I would assume that it is always supposed to replace the existing DNS record, correct?

I know we need to tweak our DHCP lease times and maybe our DNS scavenging. Our clients are mostly desktops that do not change. Please give me your opinion on the ideal settings. Currently our DHCP leases are 1 day, our DNS no-refresh interval is 7 days, and our DNS refresh interval is 7 days.
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Question by:MCSF
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by:Santosh Gupta
ID: 39901591
hi,

1. first delete all A records related to desktop from DNS.
2. set DHCP lease period to 21 days.
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Author Comment

by:MCSF
ID: 39901601
The majority of our DNS records are correct. I would think deleting all desktop A records would create short-term chaos.

Just curious - how did you come up with 21 days?

Should an ipconfig /registerdns always replace the A record or only if there is no A record?
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by:aa-denver
ID: 39901663
I'm assuming this is a windows environment

You can turn on DNS registration on the DHCP server for all clients whether they attempt to register or not.  Have DHCP create, update, and or delete the A and PTR records.    This will start cleaning up DNS for you.

The other thing you can do to help clean up DNS is create a GPO for the clients to enable DNS client registration.  That way clients will also begin registering with DNS.

I would set the leases much shorter so that this all happens in a smaller time frame.  The default windows lease is 8 days.  You could set the lease to 2 days, or even 1 day.  You don't mention how big of an environment this is.  But if it is less than 1000 users or so, that should be fine and not overload DCs.

Then you could turn on DNS scavenging.   When you do turn on DNS scavenging set the lease back to 8 days.  Set the refresh interval to 7 days.  Also set the no-refresh interval to 7 days.  And also set the server scavenging cycle to 7 days.   All of these things come into play when scavenging.  Microsoft has set them that way for a reason.

Here's a reference on tricky things that can happen with scavenging.

https://blogs.technet.com/b/networking/archive/2008/03/19/don-t-be-afraid-of-dns-scavenging-just-be-patient.aspx


http://blogs.technet.com/b/askpfe/archive/2011/06/03/how-dns-scavenging-and-the-dhcp-lease-duration-relate.aspx
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by:aa-denver
aa-denver earned 750 total points
ID: 39901689
I just reread your post.  You have DNS scavenging enabled.  One issue  is the 1 day lease, 7 day refresh, 7 day no refresh, etc.  Unless you are running short of IP addresses, put the lease back to 8 days.   What is the server scavenging interval?  7 days.

If you have to keep the leases at 1 day, set the refresh and no-refresh intervals to 2 days each and the scavenging interval to 2 days.  

Read those two articles that i referenced in my previous post.
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Author Comment

by:MCSF
ID: 39903181
aa-denver - I think I will set the DHCP lease back to 8 days. I will start with a couple scopes and work my way up. I am pretty sure it was set to 1 day because we were running out of IPs several years ago. Our subnets have since expanded, so that is not a problem anymore.

Does anyone know the default behavior of running ipconfig /registerdns from a client? If we run this from a client that has a stale DNS record I would expect it to refresh the stale record, but it does not. Is it supposed to?
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footech earned 750 total points
ID: 39905302
Not necessarily.
Assuming you have zones which allow secure dynamic updates:  If a record with the same name does not exist, it will be created.  If a record with the same name does exist it will be updated if the security on the record allows it.  For records created by the DHCP server, the client may not have the permissions needed to update the record.
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Author Comment

by:MCSF
ID: 39907626
We will work toward an 8 day DHCP lease on all scopes to start and will continue to monitor why our DNS isn't dynamically updating the way we would hope. Thanks for your advice!
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