redundant DNS -

Is it possible to have redundant DNS?  What I am thinking of is having two completely different ISP's provide two different Internet connections to an office. The idea is that if one goes down the other will be available.  I am trying to figure out however when that happens how would remote folks looking to access by DNS name be able to as the DNS for that would be assigned to one of the static IP's via a DNS provider. Could I have same DNS name point to two different static IP's?  Could I use a service like DYNDNS to do this?
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Kruno DžoićSystem EngineerCommented:
Some DNS providers offer specific failover services that monitor your connections and always respond to DNS queries with the IP of the working connection(s).

Even if they don't offer this they may still offer round-robin DNS. They basically set up two A records with the same hostname but different IP addresses. When somone requests the A record the DNS server responds with the next address in the pool, in your case alternating between the two addresses. If one of your connections becomes unavailable, the ~50% of users directed to the other IP will not notice. I believe that some implementations resopond with all addresses in the pool and leave the decision of which address to use up to the client.

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You can add dual A entries in your DNS for  IN A  IN A

however it is random selection on which ip will be served, so if one is down it may take a while to pick up the next one.  As it is just a repeat A record DYNDNS can handle it.

lineonecorpAuthor Commented:
Some very interesting reading. Per your comment:  "As it is just a repeat A record DYNDNS can handle it."  Can you provide some further explanation as to what that means?
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Kruno DžoićSystem EngineerCommented:
DynSite ( is doing what You need: checking two network interfaces, updating the IP for both to their corresponding DNS.
Hi ,
i agree with @sweetfa2
lineonecorpAuthor Commented:

Thanks for the link.
I see the following note on features on the link you provided:  
"Manages multiple connections and binding of hosts to a specific connection"

I'm assuming this is the ability to have point simultaneously to two different static IPs?

Have you used this product?

If you're familiar with Border Gateway Protocol, how does this compare with that?
lineonecorpAuthor Commented:
Still responding?
lineonecorpAuthor Commented:
Still on this?
Kerem ERSOYPresidentCommented:

Redundant DNS use is what is advised. Generally t is suggested to have at least two DNS'es having addresses from different subnets possible from different providers. If it is from a single provide it is advised to use their DNS'es in different physical locations.

This is quite possible to assign more than one static IP to your DNS resources. DynDNS is only small home users should not be used for corporate tasks.

Jamie McKillopIT ManagerCommented:

What you are looking for is a service that offers DNS auto failover.

These services will monitor your environment and automatically fail you DNS records over to alternate IPs in the event of a failure.

Chris DentPowerShell DeveloperCommented:
DNS itself has no native fail-over capabilities for A (or CNAME) records. Anything described using DNS is a work-around, and normally involves two components: A (sometimes remote, as in jjmck's suggestion) service to monitor connection status, and a "thing" to change the DNS record in the event of failure.

That's basically what services like DynDNS do, pretty successfully. However, I have to agree with KeremE: I'd have trouble recommending a service like that for anything people might get upset about when it's not available.

> If you're familiar with Border Gateway Protocol, how does this compare with that?

As far as providing redundant connectivity is concerned, BGP is vastly superior. But:

When you talk about implementing BGP what you're really talking about is providing redundant paths into an Autonomous System (AS), the AS would contain your very own IP block (something you have to apply for).

The requirements, both technical and financial, are steep. RIPE (for example) require you to run a staffed NOC (Network Operations Centre, 24x7), to be multi-homed (two or more dedicated connections, from different providers), and probably a host of other requirements that I've probably forgotten. That's just to get the AS, you haven't even started on the complexity of BGP and the enterprise-class network kit you'd need to use it.

Have you considered hosting your site with a provider that already has redundant connectivity? That can range from shared hosting services, to Virtual Private Servers, to Dedicated Servers, and upwards, it only depends on your requirements and budget.

Agree with Chris-Dent. Put the services that have to be highly available to the public in a hosting center that already does this. It will cost less than all of the other things you'd have to do to make it work yourself with a lot less work.

If you need high availability for your users, consider working with your ISP- or a different ISP if the current one can't do this- to supply 2 connections to you. We have had 2 fiber ethernet connections from our provider, which connect to their optical equipment, for 5-6 years and have NEVER had an outage. The fiber connects to redundant equipment on our side.
Jan SpringerCommented:
You don't say what operating system you are running.

If *nix based, I would have two zone files:  one for each ISP's address space.  

Your DNS server would be master.  Use an external (under another AS) or two slave servers.  The configuration file would be specified to notify the slave upon a restart or reload.

Put the preferred zone file into production.  Run a cron job that determines upstream connectivity.  Take care to script a changing SOA.  If the preferred upstream is down, swap out the zone files and restart DNS.

If you are not aliasing both IP addresses on the server, you will need to change the ethernet IP and restart network services.
lineonecorpAuthor Commented:
I am running Windows systems.
Kerem ERSOYPresidentCommented:
When you want to have two separate ISP's to provide you internet service then you can either have different IP ranges for them or you can have them broadcast the same IP range for your but in this case the takeover might get some time.

you'd then put the connections before your Firewall but anyway you'll need to have some kind of smart switch which will discover which line is up and route your traffic accordingly.

When it comes to DNS you can assign both IP's to a resource record so as you'll have a record like this:

www               IN   A x.x.x.x
                       IN   A y.y.y.y

Though DNS servers do not provide high availablity without having a sepecial hardware they work in round robin fashion so when you use your nslookup you'll get x.x.x.x, y.y.y.y and you'll get y.y.y.y, x.x.x.x next time. So it will also distribute load among your lines. So actually you  won't need a service such as dyndns.

But you still want some similar thing to dyndns you can use a hardware load balancer ( you'll need one to use 2 lines from different ISP's anyway). They also have DNS functionality and when set-up they can also serve as DNS servers with a very little cache time (0 seconds to make sure that every time when someone accesses your site actually ask the DNS server for the IP resource instead of caching and it and round robin). So when both lines are alive it will servr x.x.x.x for a request then y.y.y.y for the following request and distribute the load but when one of the lines are off it would detect the failure and serve only the line which is running and it will continue to load balance when the broken line was restored.

So you'd either publish the same DNS information to both ISP's DNS servers and have both IP's from each IP for each of your records or you'll serve your DNS over the load balancer and serve the addresses dynamically according to load and only the live line in case some of the lines are off.


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