Domain Name System Explained

In order to understand just how a mail server is supposed to tie to a particular domain as an approved mail exchange server, a high level understanding of the Domain Name System and how it operates is helpful. Once you understand what the DNS is all about, it is easier to understand where your mail server fits in the grand scheme, and why it is so important to ensure that the servers are "well-behaved."

What is the Domain Name System?
The Domain Name System (DNS) is a huge distributed database with the sole function of resolving domain names. With over 16 billion requests a day, this database works harder than any other anywhere in the world, and all it does is convert domain names to IP addresses. At the apex of the DNS are the 13 root servers, labeled A-M. These servers are located on 80 locations in 34 countries throughout the world, most of them outside the United States. Using Anycast technology, several servers can maintain one of the root servers authoritatively. This type of redundancy helps manage all the domain name requests that occur worldwide.

The DNS, from the root server zone files to the end user can be viewed as a large corporation. Tiny issues and problems can be resolved clear down at the user, or employee level, with larger issues being passed up the chain, clear to the CEO, or root name server level. Each DNS server in the chain is responsible for a particular zone, which is like a department or sub-zone of the root zone. Each server will resolve requests if it has the information stored or cached. If a request for a particular domain occurred recently, it could be resolved through the cache, which is a history of previously resolved requests, or stored memory, of that information. The caching can occur at any level in the DNS from the individual web browser to the root server. If a request cannot be resolved at that level, the query moves up the chain until a DNS computer can supply the information.

Resolving Domain Requests
The requests that are made resolve domain names to IP addresses, for example when a user types www.arialsoftware.com, the name is converted to our IP address, 204.17.32.94. Since humans can't read and remember IP addresses easily, domain names are used for most of the websites on the Internet. Computers however can't resolve domain names, and must connect to remote computers using the IP address. The DNS resolves domain names to IP addresses authoritatively, meaning that the results are accurate.

Many local requests are handled at the web browser level, where the first simple caching occurs. When someone visits a website, the resolved IP address for a particular domain is stored in a local cache for quick retrieval when the user goes back. No query needs to be made to the next level of name server until the cache expires. The next level of the DNS is the most active; most of the minor details are handled at what is called the recursor level.

The root servers are queried less frequently, since the information they store seldom changes. The root servers manage the IP addresses for the top-level domains (TLD), .org, .com, .net etc. Like the CEO of a corporation, the root level operations manage the strategic direction of the entire organization and delegate to the next level of DNS the lesser operations. If the request goes unresolved by the local cache or the recursor cache, the query is passed up to check for changes at the next level, and so on up the chain. The root name server may change only a few times per year, but sub domain name servers change more frequently the lower they appear in the hierarchy.

Use Proper DNS Entries
This is where it becomes imperative to link your domain with a well-behaved domain name server. Typically this is one managed by a web host, or even in a company IT department. A particular name server may resolve a number of domain names, and will have a larger cache from where it can find more information if necessary. If a query was made from a different computer to a particular domain, that cached information can be pulled up for any new request made by a different entity. This caching keeps the load off of the upper tiers of the domain name system.

"DNS also distributes the responsibility for assigning domain names and mapping them to IP networks by allowing an authoritative server for each domain to keep track of its own changes, avoiding the need for a central registrar to be continually consulted and updated."

These are typically text records appended to the DNS and list the appropriate information. This is information that needs to be provided to the DNS server by you, or your representative.

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