Round-robin DNS with different subnets

Round-robin DNS with different subnets

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I recently encountered an interesting problem with Windows 2000 DNS that had me puzzled. I set up a round-robin DNS with three entries for the same host using three different IP addresses. I turned on the "Enable Round Robin" option at the DNS server, but when I attempted to use different machines to access the round-robin host name, each machine went to the same server. The "round robin" part of the DNS did not seem to be working.

After hours of troubleshooting this problem, I determined that if I modifed the host entries at the DNS server to all use IP addresses on the same subnet, the DNS server would suddenly start using round-robin DNS and send each machine to a different server!

What was happening was that because my test was using three workstations on the same network as one of the IP addresses of the host in the round-robin DNS, the DNS server was prioritizing the DNS lookup to use the closest IP address available instead of giving it the next one on the list (which is how round-robin DNS is supposed to work).

Microsoft has an article that explains "Network Prioritization" that can be found at http://www.microsoft.com/technet/treeview/default.asp?url=/technet/prodtechnol/winxppro/reskit/prjj_ipa_vitx.asp, but I found my solution to be much easier than what Microsoft recommends in the article, which includes registry changes to clients, etc.

In your DNS snap-in, right-click on your DNS server and choose "Properties." Go to the Advanced tab where you will find the option that turns round-robin DNS on and off. Look for a setting called "Enable Netmask Ordering" and uncheck that option. This will turn off the network prioritization feature of the DNS server and now round-robin DNS will work.

This was first published in October 2003

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