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Load Balancing DC's

Posted on 2009-05-12
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hi AD guys,

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

http://www.experts-exchange.com/Software/Server_Software/File_Servers/Active_Directory/Q_24382147.html

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

However, I just thought of something...

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

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

Was just wondering.
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Question by:kam_uk
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Chris Dent earned 250 total points
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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 domain.com resolves to the IP addresses for all of your DCs (across all sites).

Chris
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by:kam_uk
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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 domain.com name, it would randomly come back with *any* of those DC's?
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by:Chris Dent
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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:

192.168.1.1  255.255.255.0

And the system performing the query had this one:

192.168.1.5  255.255.255.0

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

Chris
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by:kam_uk
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Thanks Chris.

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

Client1 attempts a connection to domain.com and gets the DC at 192.168.1.10
Client2 attempts a connection to domain.com - would it get 192.168.1.10 or another one

or is it

Client1 attempts a conncetionand gets 192.168.1.10
Client1 attempts -another- connection and gets another DC
Client2 attempts a connection and gets 192.168.1.10

Hope that makes sense
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by:Chris Dent
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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 10.0.0.50. And that you had this record set configured in DNS:

host IN A 10.0.0.1
host IN A 10.0.0.2
host IN A 10.40.0.1
host IN A 10.40.0.2
host IN A 172.16.1.1

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:

10.0.0.1, 10.0.0.2, 10.40.0.1, 10.40.0.2, 172.16.1.1
10.0.0.2, 10.0.0.1, 10.40.0.2, 172.16.1.1, 10.40.0.1
10.0.0.1, 10.0.0.2, 172.16.1.1, 10.40.0.1, 10.40.0.2

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.

10.0.0.1, 10.40.0.1, 10.40.0.2, 172.16.1.1
10.0.0.1, 10.40.0.2, 172.16.1.1, 10.40.0.1
10.0.0.1, 172.16.1.1, 10.40.0.1, 10.40.0.2

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:

10.0.0.1, 10.40.0.1, 10.40.0.2, 172.16.1.1
10.40.0.1, 10.40.0.2, 172.16.1.1, 10.0.0.1
10.40.0.2, 172.16.1.1, 10.0.0.1, 10.40.0.1

Chris
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