DNS fault tolerance service

I have two web servers replicated , i need a DNS service that watch the servers, if one them down, the service will change the dns records and point to the other server.
What those service called ?? which companies can provide this service ?
LizaMolyAsked:
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Toni UranjekConsultant/TrainerCommented:
DNS will not "watch" your servers.
The cheapest way out is to use Round Robin.
Just add two host or alias records for your website in your external DNS:

www.example.com = IP1
www.example.com = IP2

If one of servers goes down, there will be some problems.

If you need highly available solution you need to implement Network Load Balancing.
footechCommented:
Technically speaking, DNS does not do this, it only provides name resolution.  However, more DNS providers are starting to offer monitoring and failover as an option.  Here's some examples -
https://www.neustar.biz/resources/product-literature/ultra-dns-sitebacker
http://dyn.com/managed-dns/active-failover/
http://www.dnsmadeeasy.com/dns-fail-over-system-monitoring/

I would say these types of services are not 100% effective (i.e. clients could still experience some disruption in service) though because of the effects of DNS record caching and how different ISPs handle that.

A load balancer (sounds like you're describing an active/passive setup) is not subject to the above weakness, but it could be a point of failure itself.  You could have a pair (which monitor each other), and then the WAN link could be a single point of failure.  I think you'll start to see the chain.  You just have to make the determination of what risk you're willing to tolerate and how much redundancy you can afford.  The services from the DNS provider are probably the lowest rung.

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