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Addressing and Name Services


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Addressing and Name Services



Names and Their Syntax



DNS Lookup Without Full Name Services



Zone Files and Servers



Zone Files and Resource Records



DNS Database Aging and Updating



Reverse Mapping



Domain Administration



DNS/DHCP Interaction



DNS/Routing Interaction



Looking Ahead

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Designing Addressing Architectures for Routing and Switching

From: Designing Addressing Architectures for Routing and Switching
Author: Howard Berkowitz
Publisher: MTP
More Information

10. Addressing and Name Services

What's in a name?

—William Shakespeare

From another place I take my name / An house of ancient fame.

—Edmund Spencer

Set down my name, sir.

—John Bunyan

The Domain Name System (DNS) is a complex subject, and the details of DNS administration fill books of their own [Albitz&Liu 1997]. Beyond administration, however, are the more subtle and leading-edge interactions of addressing mechanisms with DNS.

This chapter is not intended to make you a DNS administrator. It is, however, intended to give you the background to understand the interactions of DNS with addressing, especially between emerging DNS technologies and addressing. Chapter 3, “Application Topology: Naming Endpoints,” describes the architectural aspects of naming, and this chapter discusses the details of DNS support for that architecture.

Special emphasis is given to the interaction between DNS services and addressing, and the address structure among DNS servers themselves. Increasingly, DNS is likely to have more and more of its data machine generated rather than hand administered. DNS/DHCP interaction, for example, may lead to the generation of DNS names not routinely seen by people.

Names and Their Syntax

It's worth reviewing the rules for name syntax before going into name definition. The general form of a DNS name is a sequence of label fields separated by periods. Fields go from most significant on the right to least significant on the left, the opposite of IP addresses.

There is no strict limitation to the length of a label field, but practical human readability suggests it be capped at 8–16 characters. The total length of a domain name must not exceed 255 characters [RFC 1034], although individual implementations may not support names this long. A name this long would be almost impossible for a person to use, but plausibly could be generated by an automatic name and address management system.

In principle, any eight-bit character other than a period can be used in a label field, but this leads to problems. Although technically any 8-bit character other than a period can be used, getting reliable behavior from the wide range of Internet implementations of DNS names means using only A through Z, a through z, 0 through 9, and hyphen (-).

These are the basic rules of DNS syntax. You also need to understand some of the semantics of DNS names. DNS names represent locations in the domain name space. Each level of the domain tree is represented by a field in the full DNS name. Figure 10.1 shows the basic structure of a DNS name.

Although rarely seen in practice, a domain name that ends in a period (for example, is a complete domain name, called an absolute or a fully qualified domain name (FQDN). The root of the DNS name conceptually follows the rightmost period.

Relative domain names, or partially qualified domain names (PQDNs), do not end with periods. Software has to interpret a partial name, and the most common convention is to interpret it relative to the root. With this convention, you can interpret


There also might be cases in which a partial name entry is concatenated on the left of the local domain name. This is common in implementations based on BIND, the most common DNS software, where the local domain is defined in the boot file that directs how to load zone files.

Figure 10.1. Basic DNS name structure.

To reinforce the use of DNS names, before going into the nuances of server definition, you might want to think about some very simple applications of DNS names, without the complexity of full servers.


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