Using DNS for failover using multiple A records

It has recently come to my attention that setting up multiple A records for a hostname can be used not only for round-robin load-balancing but also for automatic failover.

So I tried testing it:

1. I loaded a page from our domain
2. Noted which of our servers had served the page
3. Turned off the web server on that host
4. Reloaded the page

And indeed the browser automatically tried a different server to load the page. This worked in Opera, Safari, IE, and Firefox. Only Chrome failed to try a different server.

But after leaving that server offline for a few minutes and looking at the access logs, I found that the number of requests to the other servers had not significantly increased. With 1 out of 3 servers offline, I had expected accesses to each of the remaining 2 servers to roughly increase by 50%, but instead I only saw 7-10%. That can only mean DNS-based failover does not work for the majority of browsers/visitors, which directly co开发者_StackOverflowntradicts what I had just tested.

Does anyone have an idea what is up with DNS-based web browser failover? What possible reason could there be why automatic failover works for me but not the majority of our visitors?

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What's happening is that the browsers are not doing automatic DNS failover.

If you have multiple A records on a domain then when your nameserver requests the IP for the domain you typed into your browser, it'll request one from the SOA. It could be any of those A records. Then it passes it along.

Some nameservers are 'smart' enough to request a new A record if the one it gets doesn't work and some aren't. So if you set multiple A records then you will have set up a pseudo redundancy failover, but only for those people with 'smart' nameservers. The rest get a toss of the dice on which IP they get and if it works then good, and if not then it will fail to load as it did for you in Chrome.

If you want to specifically test this then you can use your hosts file C:\Windows\system32\drivers\etc\hosts in Windows and /etc/hosts in Linux to specify what IP you want to go with what domain to see if you get a true failover - as what you'll run into in practicality is that DNS servers across the net will cache your domain name resolution based on its TTL. So if/when you get a real failure, that IP will still need to be resolve and be otherwise farmed out to another nameserver.

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Another possible explanation is that, for most public websites, the bulk of traffic comes from bots not from browsers. Depending on the bot it is possible that they aren't quite as smart as the browsers when it comes to handling multiple A records for a domain.

Also, some bots use keep-alives to keep the TCP connections open & make multiple HTTP requests over the same connection. Given that the DNS lookup is only done when a connection is made, they will continue to make requests to the old IP address at least as long as the connection is kept open.

If the above explanation has any weight you should be able to see it in your logs by examining the user agent strings.