What happened when we pinged Yahoo!'s name servers?

You would expect such a large organisation as Yahoo! to have more than just a couple of domain name servers - In fact, on the day we looked (31st January 2006) they were running EIGHT.

First we used our Who-Is tool to get a list of the name servers registered for yahoo.com, as follows:

• ns1.yahoo.com
• ns2.yahoo.com
• ns3.yahoo.com
• ns4.yahoo.com
• ns5.yahoo.com
• ns6.yahoo.com
• ns7.yahoo.com
• ns8.yahoo.com

Then we "pinged" each name server to check that they were responding, and everything went well until we got to ns8.yahoo.com. As you can see from the table on the right, Yahoo!'s name servers numbered 1 to 7 each responded, whereas name server 8 failed to give any response at all.

Fortunately, with 8 name servers running, the loss of one had very little effect. The other 7 remained running (maybe doing a little extra work), but certainly no Yahoo! web sites failed or emails got lost.  

This example shows how large internet companies take DNS seriously, and always ensure that they have several domain name servers running, to avoid any down-time if one should fail.

Similar research shows how many large internet companies follow the same "best practice". Here's a few "famous" domain names we looked up, just out of interest (the number of name servers in use by each company is shown in brackets):

• google.com (5)
• lycos.com (5)
• microsoft.com (5)
• nic.com (3)
  
• bbc.co.uk (4)
  
• linux.com (5)

So, now that we've seen that large companies, responsible for high percentages of daily internet traffic, take DNS seriously and run multiple domain name servers (often in different locations or even countries), is it safe to assume that everyone does this?

The simple answer is NO!

It is a technical requirement that all domains on the internet are served by a minimum of 2 name servers - Modern day software allows us to run a pair of name servers on just one computer, so the simple fact is that there are millions of domains on the internet that rely on just one computer to act as both their name servers - Great, until something goes wrong with that computer.

Taking it a stage further, each domain doesn't have its own unique domain name server - a single name server can, in fact, act for thousands of domains. When it fails, so do all the domains it serves.

Fortunately, a lot of the information is cached, so a temporary "blip" by a name server often means that there are no serious effects, but when one is down for a period, things can start to go seriously wrong.

Now that we've explained a little about how DNS works, and what can go wrong, let's  start to take a look at how YOU can check your own DNS settings for your domain, and how you might improve on them.