Heres what I am thinking.

In as seamless and search engine friendly way I want to set up some domain names to point to site on host A but if host A goes down or is unreachable I want visitors to go to site on Host B instead. Host A mirrors to host B in the event of this happening.

Ideally this should occur when a visitor hits the domain name and without them having to go through a splash/door or click on anything.

How could this be done? Be as verbose as you think you need to be please.

Thanks.

If host A and B are on the same physical network, set up host B to regularly monitor host A by pinging it. If host A goes down, have host B bring up a virtual interface with host A's IP address and start serving mirrored content.

If they are not on the same physical network, having host B claim A's IP address will do no good because it will be on the wrong network (i.e. no routing in place). The only way I could think of in this situation would be to use some sort of dynamic DNS setup where the DNS server detects that host A is down and changes the DNS to point to host B.

If you have money to spend, there are plenty of vendors that offer solutions to this problem. Cisco's Local Director (for servers on the same network) or Distributed Director (for servers on geographically separate networks) can do this, as can many other pieces of hardware. The more robust a solution you need, the more money you will need to spend.

Doug

Host A and Host B would be on different networks to maximise chances of second site being up if first fails. I have no physical access to the networks. I am looking at starting a hosting company and I want to make sure there is a backup site if the server goes down for any reason.

Would any of the domain name registrars offer this facilty? I've never seen it mentioned. Are there any companies offer this intermediary service?

Money might not be a problem, depends how much :)

edit > just came across zoneedit (http://www.zoneedit.com) , this is something like what I had in mind. Anyone got experience of this service or pointers to other sites offering similar?

Hmm... is this even any better?

So now, we're relying on a third point of failure that has the capability of totally removing access to both servers!

Is this devil's advocate? I seem to be doing this alot recently...

I dont like the thought of that either. Thats why I'd prefer to find a registrar that does this, that way its an extension of a normal existing set up. Emailed the questions to a couple of registrars, waiting for replies.

how does the zoneedit one work? Does it just ping the ips in the list for the domain(server A and B) and if a ping doesnt get thru it pulls the ip A/B from the dns record? Im sure you could set up a script on your dns servers to do that for you yourself.

The solution is to muck about with DNS. Set your domain's nameservers to Host A and Host B. Use small DNS TTL values (eg 60 seconds) to ensure that incorrect data isn't kept cached for very long.

If you want requests to be split between the two hosts, with all the requests going to a single host if the other is non-responsive, then have each host claim ownership of the site -- ie, have each host give its own IP address (and only that address) in response to queries: Since DNS requests will normally be equally split across the two hosts, HTTP requests will be split, but if one host goes down it will not respond to DNS queries, causing all the requests to go to the second host (after a short delay while the first DNS request times out).

If you want all HTTP requests to go to the first host unless that host is down, have the second host monitor the status of the first; if the first fails to respond, the second host should start to publish its own address in response to DNS queries (but otherwise publish only the first address).

Basically the point is that DNS automatically "routes around failure" by asking different name servers if the first fails to respond; since not responding to HTTP queries is practically equivalent to not responding to DNS queries you can use this feature to route HTTP requests to the correct host.

Of course, notwithstanding all preceeding, this likely isn't a good idea unless you are only going to be serving static content; any sort of dynamic content is likely to get very icky when distributed over a WAN.

There will be some dynamic content in the form of ssi.

Unfortunately I will not have direct access to the dns servers either.

I have never seen anything to do with ttl values etc on domain registrars options for dns for a domain name, how would this be accomplished?. I would definately need all requests to go to host A unless it is down, then go to host B. I am looking at mirroring host A to host B about 1 - 4 times a day, this should be enough.

If I were to set host A's nameservers first and then host B's nameservers second on the domain name would it automatically try the first one and then resolve to the second one? Would the naming convention of the nameservers have any effect, ie - ns1 & 2, ns 3 & 4, etc.

You would need access to the dns record for the domain name in order to change the ttl values... or you would need someone that has access to take care of this... It is not a domain registration issue, but rather a host/dns issue.