You may not know much about the Domain Name System - yet - but whenever you use the Internet, you use DNS . Every time you send electronic mail or surf the World Wide Web, you rely on the Domain Name System.
You see, while you, as a human being, prefer to remember the names of computers, computers like to address each other by number. On an internet, that number is 32 bits long, or between zero and four billion or so. That's easy for a computer to remember, because computers have lots of memory ideal for storing numbers, but it isn't nearly as easy for us humans. Pick ten phone numbers out of the phone book at random, and then try to remember them. Not easy? Now flip to the front of the book and attach random area codes to the phone numbers. That's about how difficult it would be to remember ten arbitrary internet addresses.
This is part of the reason we need the Domain Name System. DNS handles mapping between host names, which we humans find convenient, and internet addresses, which computers deal with. In fact, DNS is the standard mechanism on the Internet for advertising and accessing all kinds of information about hosts, not just addresses. And DNS is used by virtually all internetworking software, including electronic mail, remote terminal programs such as telnet , file transfer programs such as ftp , and web browsers such as Netscape Navigator and Microsoft Internet Explorer.
Another important feature of DNS is that it makes host information available all over the Internet. Keeping information about hosts in a formatted file on a single computer only helps users on that computer. DNS provides a means of retrieving information remotely, from anywhere on the network.
More than that, DNS lets you distribute the management of host information among many sites and organizations. You don't need to submit your data to some central site or periodically retrieve copies of the "master" database. You simply make sure your section, called a zone , is up to date on your name servers . Your name servers make your zone's data available to all the other name servers on the network.
Because the database is distributed, the system also needs the ability to locate the data you're looking for by searching a number of possible locations. The Domain Name System gives name servers the intelligence to navigate through the database and find data in any zone.
Of course, DNS does have a few problems. For example, the system allows more than one name server to store data about a zone, for redundancy's sake. But inconsistencies can crop up between copies of the zone data.
But the worst problem with DNS is that despite its widespread use on the Internet, there's really very little documentation about managing and maintaining it. Most administrators on the Internet make do with the documentation their vendors see fit to provide, and with whatever they can glean from following the Internet mailing lists and Usenet newsgroups on the subject.
This lack of documentation means that the understanding of an enormously important internet service - one of the linchpins of today's Internet - is either handed down from administrator to administrator like a closely-guarded family recipe, or relearned repeatedly by isolated programmers and engineers. New administrators of domains suffer through the same mistakes made by countless others.
Our aim with this book is to help remedy this situation. We realize that not all of you have the time or the desire to become DNS experts. Most of you, after all, have plenty to do besides managing a domain or a name server: system administration, network engineering, or software development. It takes an awfully big institution to devote a whole person to DNS . We'll try to give you enough information to allow you to do what you need to do, whether that's running a small domain or managing a multinational monstrosity, tending a single name server or shepherding a hundred of them. Read as much as you need to know now, and come back later if you need to know more.
DNS is a big topic - big enough to require two authors, anyway - but we've tried to present it as sensibly and understandably as possible. The first two chapters give you a good theoretical overview and enough practical information to get by, and later chapters fill in the nitty-gritty details. We provide a roadmap up front, to suggest a path through the book appropriate for your job or interest.
When we talk about actual DNS software, we'll concentrate almost exclusively on BIND , the Berkeley Internet Name Domain software, which is the most popular implementation of the DNS specs (and the one we know best). We've tried to distill our experience in managing and maintaining a domain with BIND into this book - a domain, incidentally, that is one of the largest on the Internet. (We don't mean to brag, but we can use the credibility.) Where possible, we've included the real programs that we use in administration, many of them rewritten into Perl for speed and efficiency.
We hope that this book will help you get acquainted with DNS and BIND if you're just starting out, let you refine your understanding if you're already familiar with them, and provide valuable insight and experience even if you know 'em like the back of your hand.
This book deals with the new 8.1.2 version of BIND as well as the older 4.9 versions. While 8.1.2 is the most recent version as of this writing, it hasn't made its way into many vendors' versions of UNIX yet, partly because 8.1.2 has only recently been released, and many vendors are wary of using such new software. We will also occasionally mention other versions of BIND , especially 4.8.3, because many vendors continue to ship code based on this older software as part of their UNIX products. Whenever a feature is available only in the 4.8.3, 4.9, or 8.1.2 version, or there is a difference in the behavior of the versions, we try to point out which version does what.
We use nslookup , a name server utility program, a great deal in our examples. The version of nslookup we use is the one shipped with the 8.1.2 BIND code. Older versions of nslookup provide much, but not quite all, of the functionality in the 8.1.2 nslookup . We have tried to use commands common to most nslookup s in our examples; when this was not possible, we tried to note it.