By Dr. Philip Baczewski, Associate Director of Academic Computing
Back to the Basics: DNS
The Internet is ubiquitous, right? You just plug into the network and it works. All you need is a computer and a network cable and you're on your way. I guess that there's something that happens behind the wall, but nowadays in organizations like UNT, network service is like phone service. You plug in a device and it just works. Well, most of the time. We treat the Internet like the phone service, but it only works that way because of a lot of hard work and planning by a lot of people.
There is quite a bit of complex hardware and wiring which support the physical part of our Internet network. The physical routing technology has gotten more and more sophisticated over the last ten years. We have seen a movement from 10 megabit/second connections on coaxial cable (10 base-2) to 10 megabit/second connections on telephone-style wiring (10 base-T), to the present-day support of 100 megabit/second connections via that same (in most cases) wiring. That upgrade in the physical network has resulted in Internet which appears to operate faster and support more complex technologies such as streaming audio and video. All of that sophistication can grind to a halt without another important component of Internet networking: Domain Name Services (DNS).
Domain Name Services is one of the most important and most generally unknown elements of Internet networking. What I mean by generally unknown is that unless you are a computing or networking professional, DNS is not something with which you are ordinarily concerned. You may never have heard of it. That's why a bit of explanation about DNS is in order here.
In recent years, my experience has been that many cases of what appears to be a network outage has been caused by problems with DNS. I don't mean that the DNS problems are all because of bad DNS service. The problems can be caused on many levels and by many circumstances. Those circumstances can include trying to use the wrong or no server; trying to access an unregistered address; no direct connection to the DNS server; and the occasional DNS server outage.
What is it that DNS does that makes it so important?
DNS's primary function is to translate Internet addresses from a person-friendly format to a computer-friendly format and vice-versa. We know addresses like www.unt.edu, but that is meaningless to a network router. Network routers use addresses like 188.8.131.52. It's easier for most people to remember words (or abbreviations) than numbers. That's not the only benefit of DNS. DNS allows alphabetic addresses to remain the same even if the numeric equivalent needs to change because of a network upgrade or reconfiguration.
DNS is a bit like a big virtual phone book of Internet servers, for Internet software. When we pick the phone, we don't punch in someone's name but instead dial (an obsolete term) their phone number. If we don't know their phone number, we can look it up in a telephone directory (phone book). All Internet applications have a similar functionality built in. When you enter an alphabetic address in your web browser, your browser software will look up that address in the DNS server your computer is configured to use, and the make a connection to the remote server using the numeric address. DNS is a distributed directory of all registered Internet addresses. If the address we need to reference is not on our network, our DNS server can look up that address on a server which supports that remote network.
So what happens if DNS is down?
You might enter the address www.unt.edu in your web browser, but you would not be able to connect to that page. Your browser would tell you that it could not find that server. If you knew the equivalent numeric address you could enter that in your browser, and the connection would be immediately made. In spite of the initial appearance, the network was not down, but DNS problems can make it appear that the network is down.
Another manifestation of DNS problems is servers which respond slowly. Many servers use a "reverse" DNS process to look up the alphabetic address based upon the numeric address which makes the connection. This is useful for security or tracking (to see where most of the connections originate). If a server is experiencing DNS problems, there may be a delay while that server is waiting for the address lookup to complete. To you, this will appear to be a very slow response. In some cases, a delay will cause other functions (like browser cookies) to time out prevent you from establishing any server connection.
How can can you distinguish network from DNS problems?
One easy way is to try a numeric address where an alphabetic address does not work. If a connection can be made using a numeric address where a alphabetic address failed, you can determine that the Internet network is working at most levels, but that DNS service may be unavailable or not operating efficiently.
On microcomputers, most DNS address are provided automatically by the network, however, if you know an alternate DNS address, you can try changing the server that your computer is talking to in order to determine if the problem is with a particular server or with all DNS servers on your network (part of the network might be down and preventing access to your DNS servers). If you can contact servers on your local network, but not on a remote network, then the remote DNS server may be down. You can test this by trying other addresses on that remote network if you know any.
It is evident by now, that DNS is a critical component of Internet networking. When it works well, the Internet will be easier to use than the telephone. When there are problems, it may appear to you that the whole Internet is broken. Knowing a little about how DNS works can help you describe problems when you are talking to technical support staff.
For example, if a server refuses your connection, that is a problem on that server. If your browser can't find a server you know should be available, this indicates a possible DNS problem. Being able to understand and notice the difference is important. The most common report I've hear over the years is "The <fill in the blank> is down." If you call and say "The Internet is down," that only narrows the possible problems to a few hundred. Being able to narrow down the problem will help find a solution much more quickly.