DNS Troubleshooting

DNS is one of those magical things on the internet. It goes by unnoticed for ages, but when it goes wrong everyone knows about it! A few weeks ago I had decided that I wanted to learn more about DNS, so Amazon came to the rescue and DNS and BIND (yet another O’Reilly book) was pushed through my letter box.

Anyway, as you can probably imagine, the book is pretty boring to read, so it was rather fortuitous (for me, at least) that a couple of my customers have been experiencing DNS problems this week. Indeed, Flickr had some problems a few days ago, and several people in the forums were saying it was a DNS issue. I doubted this, and with my new found knowledge had a look into the problem. A DNS problem it wasn’t, and, after looking a bit closer, I suggested it was a load balancer issue. “Oh no!” said the site admins, “the load balancer is working fine!” Ahem. No it wasn’t. A few hours later, they finally discovered a problem with the load balancer.

So what else can DNS tell you? I’d been having some intermittent problems delivering email to an organisation, but they wouldn’t accept that there was anything wrong with their email. A quick dig MX the-domain.com told be the mail servers that they were using, and after a few short telnet sessions to the SMTP servers, I discovered that in fact three of the four mail servers wouldn’t accept mail for the recipient’s domain. MX records are slightly different to other DNS records as each record has a priority level. The preference mail server was the one that was working ok, but if this was ever unavailable the other servers would be used and it was these that were rejecting mail.

So although DNS is a rather dull subject, it’s definitely worth learning a bit more about the theory behind it, and how to use tools such as dig to tell you more about what is going on. I still have a lot to learn, but will probably post a bit more up here about it in due course. Bet you can’t wait!