I have been thinking about how much people are thinking about DNS and I came across the Google Zeitgeist project (http://www.google.com/intl/en/press/zeitgeist/index.html). Basically this is an interface to understand what people are using the Google search engine for. Specifically, I was poking around Insights for Search and queried a few terms related to DNS. The information is fascinating. The most interesting part I noticed is the number of searches and the countries they are coming from. Again, I find this stuff fascinating. We beat the drum each day for DNS and most people never give it a thought, much as it should be, but if you are reading this you probably have a bit more interest. DNS searches have actually decreased over the past few years. Maybe people are more educated? Less concerned? However, DNS attacks are on the rise that is certain.

In our last TechTalk event we had a great number of participants and fielded a lot of questions. There was some good discussion about DNSSEC implementation. Based on what we discussed - you should plan to have your DNSSEC implementations done by the end of 2011, at the latest. Also there were lots of questions about reverse DNS. Reverse DNS is just like DNS but specifically for the IP addresses, for example when you want to know what an IP address points to you would do a reverse DNS query.

The questions were focused on how admins setup a reverse DNS. Reverse DNS is typically maintained by the organization who “owns” the IP address(s) or block. In their DNS server they create records for each of their IP address that point to hostnames. Many times those host names will be generic, which is fine. For certain things, especially email, having the hostname come back as generic can create a problem. For example, when you email server attempts to send a message to another server (the receiving server), nine times out of ten, the receiving server will do a reverse DNS lookup on the IP address of the sending server, if the hostname returned is not related to your email zone or if there is no reverse DNS record the receiving server may reject the message. Some email servers can get particularly persnickety about this.

So make sure your reverse DNS ducks are in a row. One of the easiest ways to verify all of your DNS settings is to run a DNSreport at DNSstuff.com. You first need to get a free 21-day trial account to have access to all tools.

Email is one of the most important communication mediums to date. We have become dependent on it and since it works so well, we have high expectations that it will work when we need it. But, how does it really work, and what if any role does DNS play in email. First as we discussed in the past, every communication on the Internet depends on DNS, including email. So let’s dig in to what we need to know about DNS and email.

When a user sends an email they are not actually sending the email directly to the recipient. They are, rather, asking their server to take the message and deliver it for them. Once the server accepts the message it tries to deliver it. Here is what, in the simplest manner, the server does. It looks at the envelope and finds the address of the recipients mailbox, for example parisi@yada123.com.

The server then looks at the domain part of the address, everything to the right of the @ sign, yada123.com{dot} and does a DNS lookup. In effect the server asks what are the MX records for yada123.com{dot}? (By the way MX stands for Mail eXchanger, i.e. the server/host responsible/able to receive email for a domain) If DNS is working for that zone, in this example, it should receive an answer of yada123.com{dot} – what – huh? Didn’t we just ask for yada123.com’s address? What is the deal? Well we did, but we asked for the MX records for the domain. As a rule MX records point to A records. So we got back an A record. A records are also called host records, and A records resolve to IP addresses, cool, feels like we are getting closer.

What does the server do now? It needs to ask a second question of the DNS, please (servers are very polite) give me the IP address associated with the A record yada123.com{dot}. In this example the result returned should be 70.120.44.51 – finally, “whoo hoo” an IP address - we can do something with that! The mail server attempts to make a TCP/IP connection to 70.120.44.51 and does so using the SMTP protocol. If all goes well, we won’t get into SMTP details here; the receiving server accepts the message and, again in the simplest of examples, puts it in your mailbox.

Let's move on. Let's suppose you want to put up a web site on your new domain name? You find a suitable, reliable web hosting company, buy an account and visit their control panel and create your web site, even if it is just a simple page with "Hello, world" on it.

Now you have a choice to make - you need to setup your DNS settings. For this example, let's say your DNS is currently hosted at GoDaddy, you can continue with that and go and add records there or you could move your DNS to your web hosting company, it's your choice (dedicated DNS hosting is available from EasyDNS, DNS Made Easy and DynDNS - all excellent companies with good infrastructure). If you want to stay with GoDaddy for DNS, you will need to change a few things, basically, remove your entries from pointing to parking servers to point to real servers. There is nothing inherently wrong with staying with GoDaddy, they have reasonable controls and they seem to work well. But, being IT people we never can leave well enough alone. So let's use our own DNS server, at our hosting company. We will need to use their control panel to add all of our DNS records to the new name server. Minimally, we need to create the following records:

As you start to digest this you will see that this is a very simple zone file. We will be digging into this specific example and what decisions are important and could cause pitfalls.

Many DNS issues result from gaps in our knowledge – typically because we do not work with DNS all that much. So I thought it would be good to give you an overview of what happens from the beginning. The premise here is you want to implement a new DNS domain name. Let’s pick something to experiment with, yada123.com, first we need to see if the domain name is available and if so, we need to register it. For this example we are going to use GoDaddy, but any registrar should be fine. Go to www.godaddy.com. Find the search field on the form, GoDaddy’s home page is a bit cluttered but the search is there. Type in yada123.com and hit enter or click go. Congratulations we see that yada123.com is available. We need to buy the domain name. Complete the purchase process and once done, you will have a new domain name.

Now what actually happened here? Once you paid for your domain. GoDaddy added a record to its database, and being that it is a registrar, it sends that information to the root servers for the domain you registered, in this example, the {dot}com domain. The root servers are a collection of servers distributed all over the world that keep the initial pointers to the sub domains for which it is responsible. For example, there are root servers which hold the pointers to the servers responsible for the next level zones. When you read a domain name you read from right to left, and all DNS zones begin with a period, which indicates the root, sometimes people do not type the period when referring to domains but it is implicitly there. So for example, we look at yada123{dot}com{dot} we see that we can start at the {dot} zone, the root, and ask those servers “where do I go to get information on the {dot}com zone?”, the root servers will return a list of server which can be asked about {dot}com zones. These are the servers that GoDaddy has to notify that it has a new domain that has been registered and that the nameservers for that domain are NS57.DOMAINCONTROL.COM and NS58.DOMAINCONTROL.COM, for this example.

Ok, so here is the deal, you open a browser right now and type in www.yada123.com, what happens? Your browser checks its internal DNS cache to see if there is an entry for yada123.com, since there is not, the browser then asks the local resolver if it has an entry for yada123.com, the local resolver says no, but says please hold, it (the local resolver) then asks the DNS server it has been configured with if it knows anything about yada123.com, that DNS server checks its cache, finding nothing, it asks the root servers for the {dot}com servers, it gets the answer then asks the {dot}com servers what they know about the yada123{dot}com zone, if the changes have propagated to the server you asked it should return the SOA record (Start of Authority) for yada123{dot}com, which is NS57.DOMAINCONTROL.COM and NS58.DOMAINCONTROL.COM. Ok now we know which server we have to ask about the www record for yada123{dot}com. Next, the DNS server opens a connection to one of those two servers (NS57 or NS58) and asks for the www record in zone yada123{dot}com – what is the answer? If the record is an A record it will return an IP address. If it is a CNAME (canonical name) it will return another record to lookup, ultimately resulting in an IP address. Now wait, what is the IP address that we are going to get in this specific case? When we registered with GoDaddy, they created a placeholder for the domain and it will direct browsers to a “parking” page hosted by GoDaddy.

So we can begin to see some of the pitfalls. There is a gap between the time you register and the time it takes GoDaddy to submit that zone to the zones root servers. Also, when you then go into GoDaddy site and change your nameservers (aka SOA) to some other server that will take some time as well. Further, if you tried to browse to your site, yada123{dot}com, your browser will cache the last answer it got, your resolver will cache the last answer it got and your DNS server will cache the last answer it got for some amount of time. So once you make a change on GoDaddy, don’t expect to see it for some time. For example, GoDaddy, by default has a TTL (time-to-live) on their name servers (NS) records of 1 hour. So it can take up to an hour for that TTL to go by causing the cache to expire and causing a new lookup to occur. When you need to know what the world thinks you can run our DNSreport tool and find out what is happing right now (One of the things that DNSreport does is never caches data so that we don’t have a delay problem).