Round robin DNS is a method by which a DNS record has more than one value. When a request is made to the DNS server which serves this record, the answer it gives alternates for each request. For instance, if you had a two webserver that you wished to distribute requests between, you could setup your DNS zone thusly:

www     IN   A    1.2.3.4
        IN   A    2.3.4.5

When a query is made to the DNS server it will first give the IP of 1.2.3.4 for the www host. The next time a request is made for the IP of www, it will serve 2.3.4.5. This will alternate back and forth on each subsequent query.

While this is a form of load balancing, it should be noted that if one of the hosts becomes unavailable, the DNS server does not know this, and will still continue to give out the IP of the downed server.

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MX Record

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