Saturday, September 4, 2010

dns propagation - why rescuing a dropped site takes time

Domain Name System (dns) is quite involved, and the first time people encounter issues is perhaps when their website disappears from the internet.

An extract from Wikipedia:

As a noteworthy consequence of this distributed and caching architecture, changes to DNS records do not propagate throughout the network immediately

So your website has disappeared and you want it back as soon as possible, but how will that happen? and when will the problem be completely fixed?

Firstly a little explanation....

I have a domain and in order for people to see the site I must have created it, hosted it, registered the domain, AND have a publicly available properly formed dns record.

The last bit is rarely considered specifically, as another company will usually do that on your behalf. However if you have issues with dns, then you should take a keener interest in that last bit about the dns record.

http -> domain lookup -> NS records -> nameserver -> A record -> IP -> homepage

Okay that is a highly simplified, and somewhat abstracted, flow diagram of what happens in order for your homepage to appear.

Dropped sites:

By 'dropped sites', what I mean is that the Domain Name System process, for your site, has fail somewhere in the portion:
...-> domain lookup -> NS records -> nameserver -> A record -> ...

and the result is that your web browser responds with:
Problem occurred while loading the URL

If the problem is that no dns record is known to the Domain Name System, then whoever manages your domain and/or nameserver will have to create a new dns record or fix the existing dns record.

Even if your provider does that right now, then it might take a day or two for those records to propagate, and your website and mail to start working totally how they should.

This is not a failing of the Domain Name System, it is a distributed system designed to scale massively, and to use multiple listing servers to spread the load.
The downside of this is that things take time to propagate to all corners of the vast internet*

( *In particular broadband providers maintain their own dns caches and so that is sometime an additional delay for correct records to be picked up on users PC )

Analogy: Dropping out of dns is a bit like forgetting to renew your yellow pages/thompson directory entry for your business. When you do get the new entry created in yellow pages, it takes time before the new printed catalogue gets to all your customers.

Dns is a technical area, and there are a whole host of consultants and specialists, who can assist you with diagnosing issues.

There are also online tools to help you check your domain, these work well, but may not be the whole story.

The piece which is not so easy to quickly pick up, is being able to speak with technical confidence to your provider, in order to press for a satisfactory outcome.

1 comment:

Anonymous said...

A second and more annoying variant of 'dropping out' of the internet is where your site is visible but your mail has stopped arriving.

Your dns record contains several pointers, the most common of which 'A record' is what is used for web broswers to get to your homepage.

A different pointer 'MX record' is the public information telling other mail servers where to send your email.

Having a good 'A record' but a bad or missing 'MX record' means everything looks great in a web browser, but your company mail is stuck at the sorting office!

There are other variants also, but what I should probably reinforce, is that there are literally hundreds of millions of sites, all with great dns records.

So the system works (although this is of little comfort for the few sites with dns records that are broken or missing)