DNS redundancy

DNS redundancy

Tuesday, April 26th, 2011

DNS redundancy
A few years ago, when internet was still not as common as it is now, I was on a local radio show. They asked me what a domain was. Surely I could have replied in technical details, but I decided to keep it simple. So I replied: a domain is similar to your address, instead of using latitude and longitude (and perhaps elevation if you have an apartment), all governments use easy to remember addresses. Domains are used in a similar way. Every domain points at a unique location or, in some rare occasions, locations.

In lots of ways the postal system used in different countries can be compared to what DNS is. DNS makes sure your domain points to the right location, the right server, by pointing to a certain IP address. DNS is short for Domain Name System, by the way. Every server has its own unique IP address. To understand why redundancy can be important, you have to know how DNS works. DNS is a real hierarchal system and is not afraid to ask someone else if it doesn’t know the answer. It will try to find an answer by asking higher servers to see if a domain exists and does so in multiple parts. If it does not exist, it will let the user know.
DNS servers are installed at ISP’s, webhosting providers and a lot of other organizations.
Before we continue, although Domain Name System sounds very domain-ish, it is also responsible for pointing your e-mail into the right direction.

But what will happen if the DNS server responsible for giving that answer is unavailable?
That can be a problem. I’d like to emphasize the word ‘can’ here. Because there are a lot of situations in which I would not care about DNS downtime. Just two random situations we deal with each day.

Situation #1
Let’s say we have three servers, one for e-mail, one for web and one for DNS. Of course, you could have one for your databases too. So, what will happen if we cut down DNS? Although the answer depends on a few factors, in the first couple of hours nothing should happen at all. DNS uses caching, which means that every server involved will remember the given answer for a short while. This means that every ISP which has been asked about your domain before your DNS server became unavailable will remember the IP a domain should point to. But, it could still mean that a lot of users cannot find the location of your domain. Not every ISP has your domain cached.
Sounds like a problem to me.
In this case I would always recommend our clients to use a redundant set of DNS servers. If your website is down, you can still receive e-mail. If your e-mail is down, visitors can still browse your website. If your DNS is down, you are unreachable.

Situation #2
So, we’re just starting with a new website and don’t have too much money to spend. We have one server with a control panel like cPanel, Plesk or DirectAdmin and run everything on that server. Suddenly a power failure puts your server down. In this case, I would not care about DNS at all, because if you manage to get DNS up, everything else might be down.
The main concern is: get your server back up. In this case, DNS is less important.
However, there is something which can be a problem. This is called e-mail. A sending e-mail server will respond differently if a domain cannot be found in a DNS server opposed to the receiving e-mail server being unavailable. If only the receiving e-mail server is unavailable, the sending e-mail server will retry a few times.
Again, a redundant set of DNS servers is advised. With redundant I mean in a different C-class network preferably on different physical locations too.

A quick tip:
Use different domain extensions for your DNS servers, for instance, ns1.domain.co.uk and ns2.domain.eu. If for some reason the TLD (like .co.uk) loses its glue records (I’ve seen this happen once with .nl), all domains using your DNS servers might be unavailable. Using two different domain extensions solves this.

Thomas Klaver
Chief Executive Officer

http://www.sensson.net/

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