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Monday, November 18, 2013

DNS Round Robin

DNS Round Robin and NLB are two configurations that can be used to ensure application availability in scenarios where there are no shared storages in use. They are usedful for applications which handle one time requests and need not be handled by a singler server throughout the session. This article aims at explaining the basics of DNS Round Robin technique

DNS Round Robin:

Here the load-balancing act happens at the Name resolution stage. There will be multiple entries in the DNS server for a host name , pointing to application server IPs across which the load should be balanced. For eg: there will be n number of IP addresses associated with a host name . When the first client request a name resolution, the first IP from the list is returned.When a second client request a name resolution, the next IP is returned. Thus we can ensure that the incoming requests for a particular application is equally distributed among the available application servers.

An additional option named netmask ordering can be used, if you want to take in consideration the subnet of the querying client. If this option is enabled, the host IP address that is in the same subnet as the querying client is returned. For eg: Te host entry app.testme.com has two records created with IP address 192.168.10.1 and 192.168.20.2 and the netmask ordering option is enabled. When a client from the subnet 192.168.10.x/24 makes a query, the IP 192.168.10.1 is returned. When a client from the subnet 192.168.20.x/24 makes a query, the IP 192.168.20.2 is returned.

Both the DNS round robin and netmask ordering options are available in the properties of your DNS server. ie from DNS manager console->DNS server name->properties-Advanced. You need to select the option "Enable round robin" and "Enable netmask ordering" and enable them.

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