DNS Round Robin and Netmask Ordering
This video will look at two DNS features, DNS round robin and netmask ordering. Check out http://YouTube.com/ITFreeTraining or http://itfreetraining.com for more of our always free training videos. These features allow for simple load balancing and also provide a method for directing a user to local resources rather than remote resources.
Download the PDF handout http://ITFreeTraining.com/handouts/dns/round-robin.pdf
Round Robin
On a DNS server it is possible to create multiple host records with the same name. In this example, 3 DNS records have been created with the host name of WWW. These DNS host records have the IP Address of SRV1, SRV2 and SRV3. With Round Robin enabled on a DNS server, when multiple DNS records exist with the same name, the DNS Server will cycle through the available DNS records on the DNS server. So the first workstation to request a DNS record of WWW may get the DNS record with the IP Address for SRV1 in it. The second workstation will get the DNS record with the IP Address of SRV2 in it and the 3rd workstation will get the DNS record with the IP Address of SRV3. When the 4th workstation requests the DNS records, the DNS server will simply loop back to the first DNS record or, in other words, it will simply cycle through the available DNS records in the DNS server. Round Robin can be used with simple load balancing. As shown, a third of the requests will go to each server in this example. However, the DNS server does not take into account the load on any of the servers when making this decision. The DNS server also does not consider where the user is located in relation to the server that they are accessing. The user could be accessing a server on the other side of the world rather than a local server.
Netmask Ordering
If Netmask ordering is enabled on a DNS server, the DNS server will look at the IP Address of the client that is performing the DNS query. When multiple DNS records exist on the same host name, the DNS server will respond back with a host name that is in the same network as the client when possible. In some cases there may be multiple DNS records in the same network as the client. If round robin is also enabled, the DNS records will be cycled through that are on that network.
Demonstration
This demonstration will look at how to enable and disabled round robin and netmask ordering as well as how to create DNS records to use with round robin and netmask ordering.
1. To make changes to the DNS server settings for round robin and netmask ordering, open server manager and from the tools menu select the option DNS. When DNS Manager opens, it may prompt you for the DNS server to manage. If it does not or you want to manage a different DNS server later, right click on DNS at the top and select the option “Connect to DNS server”.
2. To create a new host record in DNS, right click on the zone that you want to create the DNS record in and select the option “New Host (A or AAAA)”. To create a second, third, etc. DNS record of the same name keep repeating the procedure using the same name. CName cannot be used for only round robin records. This means that if the IP Address of the host were to change, you would need to change the IP Address in the DNS record. In this example, the following DNS records were created. WWW with IP Address 192.168.2.30, www with IP Address 192.168.2.31 and www with IP Address 192.168.3.5.
3. To configure the DNS server settings, Right click on the DNS server name in DNS manager and select the option properties. In the properties, select the tab advanced. By default the options “Enable Round Robin” and “Enable Netmask Ordering” are enabled. To enabled or disable these settings is a matter of ticking or clearing the tick box.
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References
“DNS Round Robin and Destination IP address selection” http://blogs.technet.com/b/networking/archive/2009/04/17/dns-round-robin-and-destination-ip-address-selection.aspx
“Description of the netmask ordering feature and the round robin feature in Windows Server 2003 DNS” http://support.microsoft.com/kb/842197
by itfreetraining
windows server dns
