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How are the secondary addresses used? Is it application specific?

Note: I'm not asking about round robin dns. That could be done by only returning 1 address at a time.

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It could, but it's not. Typical implementation of round-robin DNS is to return all A records to a client request. Otherwise, you won't get much round-robin; you'd give a single-address response to a massive ISP's caching server, and all of their clients will hit only that address. More info here. – Shane Madden Apr 29 '11 at 16:52
Even though this is a comment, I find this the most useful reply. If it were an answer, I would select it as accepted – Aheho Apr 29 '11 at 19:13
What are you really trying to ask here? You don't understand how RR-DNS works, so a discussion of what other objectives someone might be trying to achieve are probably pretty futile. – symcbean Apr 29 '11 at 22:45

I believe that IS round robin DNS. The DNS server returns all the IPs for services matching that A record. It is up to the client/browser to resolve which IP to use.

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The client should use them in the order provided, and most clients will only use the first one returned. The DNS Server will mix up the order to create the round robin effect. – Chris S Apr 29 '11 at 16:51

If a client fails to connect to the first IP-address, it can try to connect to the next IP-address in the list if more than one IP address is provided. You can see it as a "cheap" fault tolerance solution. And the order of the addresses is altered (round robin) so it also works as a "cheap" load balancing.

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You said that "if a client fails to can try to connect to the next IP address" But in practice do client application actually do this? – Aheho Apr 29 '11 at 19:10
@Aheho: It's up to the client to decide. But all modern browsers does it, even Internet Explorer since Windows XP SP2. See What happens when a Front End goes down? Do we lose traffic? – Jonas Apr 29 '11 at 19:32

For one thing, the DNS protocol is designed to be independent of the data that's actually stored in it. There are many record types that really need multiple answers, like MX (mail server*s*), and NS (name servers) records. Generally you should always have multiple of those if you're running a decent site with redundancy in place.

For addresses, however, there are times that people like returning multiple records. The DNS specifications state that if you get more than one, you should ideally "pick one at random". This is highly helpful for spreading load across multiple machines, though in practice it's not as ideal as you'd like which is why there is a more common practice of IP-based load balancers rather than using multiple answers. Thus, multiple-A or multiple-AAAA (IPv6) are less used than they used to be.

But remember: DNS, the protocol, is designed not to care about the data it is carrying. It will happily deliver multiple records of a particular type even if it is not useful to the end-application.

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