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Is it possible to set up a failover system such that when a website (external to our organisation) goes down (can't be reached) that the DNS redirects to an internal site?

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If the DNS redirects to an internal site, then the Public still can't access it. You're better off using two different host locations and doing a failover to the other one in case of an emergency. –  ionFish Jun 28 '12 at 21:30
If it is ok to use the internal site, then why wouldn't you just use the internal site all the time? If it breaks, you have control to fix it. –  August Jun 29 '12 at 17:02
@ionFish, I wonder if there is a translation issue. I believe he he is talking about a service hosted in the 'cloud', and then a failback to a version that is hosted in his internal hardware. –  Zoredache Jun 29 '12 at 21:48
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2 Answers

DNS isn't the tool to use to solve this. You can do this with an active/passive load balancer, but if you were going to do this, you might as well just make them both public facing and active/active.

What is the business case for having an internal redirect instead of using a real HA solution?

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You might want to support with facts why you recommend against a solution. –  michele Jun 29 '12 at 16:04
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DNS is a perfectly fine tool to do this.

There are several vendors for this. They periodically poll your service and point your domain name to a backup server address when it fails. Search for "DNS failover", and pick carefully: they will have to run your DNS too.

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Then you'll need to wait for client caches to expire. It will work...eventually, but it's not really highly available if there's downtime while waitin for caches to invalidate. –  MDMarra Jun 29 '12 at 16:33
That's what TTL was made for. Set TTL to 120 seconds, job done. –  michele Jun 29 '12 at 20:54
You're crazy of you think caching resolvers respect TTL well enough for DNS-only failover to be a viable solution. Sure your own servers probably do, but what about clients with Comcast, Verizon, or any other ISP that doesnt care to respect the DNS TTL RFC definitions? –  MDMarra Jun 29 '12 at 20:58
I don't agree that DNS is perfectly fine, DNS does has some serious drawbacks since some ISPs ignore TTLs, but a DNS solution may also be good enough depending on the requirements and budget. –  Zoredache Jun 29 '12 at 21:50
A few ISPs don't respect TTLs. Their customers will pay for it. Bummer. If you have ever set up a HA solution, you know it's complex and expensive out of reach for nearly everyone. A load balancer is just moving the problem to a different address. DNS failover is the way to go in 95% of the cases. –  michele Jun 30 '12 at 0:38
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