Load Balancing DC's

Posted on 2009-05-12
Last Modified: 2012-05-06
hi AD guys,

A little while ago, I raised a questions about applications needed a domain controller to point to for LDAP purposes..

The solution seemed to be to create a DNS entry named and have the applications point to there.

However, I just thought of something...

If I ping (my domain name) then I am returned the address of a DC.

Instead of creating a DNS entry named and pointing to my DC's, do I have this already in the form of ""?

Was just wondering.
Question by:kam_uk
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Accepted Solution

Chris Dent earned 250 total points
ID: 24369848

Yep you do already have it.

The only reason to use a specific name is if you want to reduce the number of DCs that respond. You should see that resolves to the IP addresses for all of your DCs (across all sites).


Author Comment

ID: 24369886
Thanks Chris...and just to confirm, it is strictly round robin only - the closest DC is not taken into account.

For instance, if I am in the UK and have 5 DC's in UK, Germany, US, Japan and Australia (all in the same domain for the sake of argument)..if I used the name, it would randomly come back with *any* of those DC's?
LVL 70

Expert Comment

by:Chris Dent
ID: 24369920

Yep, correct :)

It rotates the order unless NetMask Ordering gets a chance to come into play.

That would happen if a DC had this IP:

And the system performing the query had this one:

It's very limited, if it's not in the same range you don't get a matched response, just standard Round Robin.


Author Comment

ID: 24372652
Thanks Chris.

Out of interest, how does it "rotate the order" that per connecting client, or per per's say I have 10 clients.

Client1 attempts a connection to and gets the DC at
Client2 attempts a connection to - would it get or another one

or is it

Client1 attempts a conncetionand gets
Client1 attempts -another- connection and gets another DC
Client2 attempts a connection and gets

Hope that makes sense
LVL 70

Expert Comment

by:Chris Dent
ID: 24372752

It's a global rotation, memorising order per client would be a lot of work :)

No matter the order, if NetMask Ordering can play it will promote any "close" entries to the top of the list.

You can see it in action using a made up record and NsLookup.

Imagine you had a client on the IP And that you had this record set configured in DNS:

host IN A
host IN A
host IN A
host IN A
host IN A

While you're in the same classful subnet as some of those records (NetMask Ordering defaults to 24 bit subnets, but can be changed to other Classful subnets, 8 bit and 16 bit). You will find that this response order is used when repeatedly queried:,,,,,,,,,,,,

Go a bit slow (a second or two between each query), because it won't rotate the order exceptionally quickly. More likely down to NsLookup behaviour than anything else.

If there was only one record in the same subnet as the client it would always respond with that address. e.g.,,,,,,,,,

If you were to query that set of records from a different machine, one that isn't on any matching range you'd get this behaviour:,,,,,,,,,


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