Chapter 1. Introduction
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# Chapter 1. Introduction

The Internet Domain Name System (DNS) consists of the syntax to specify the names of entities in the Internet in a hierarchical manner, the rules used for delegating authority over names, and the system implementation that actually maps names to Internet addresses. DNS data is maintained in a group of distributed hierarchical databases.

## Scope of Document

The Berkeley Internet Name Domain (BIND) implements a domain name server for a number of operating systems. This document provides basic information about the installation and care of the Internet Systems Consortium (ISC) BIND version 9 software package for system administrators.

This version of the manual corresponds to BIND version 9.11.

## Organization of This Document

In this document, Chapter 1 introduces the basic DNS and BIND concepts. Chapter 2 describes resource requirements for running BIND in various environments. Information in Chapter 3 is task-oriented in its presentation and is organized functionally, to aid in the process of installing the BIND 9 software. The task-oriented section is followed by Chapter 4, which contains more advanced concepts that the system administrator may need for implementing certain options. Chapter 5 describes the BIND 9 lightweight resolver. The contents of Chapter 6 are organized as in a reference manual to aid in the ongoing maintenance of the software. Chapter 7 addresses security considerations, and Chapter 8 contains troubleshooting help. The main body of the document is followed by several appendices which contain useful reference information, such as a bibliography and historic information related to BIND and the Domain Name System.

## Conventions Used in This Document

In this document, we use the following general typographic conventions:

To describe: | We use the style:
a pathname, filename, URL, hostname, mailing list name, or new term or concept | `Fixed width`
literal user input | `Fixed Width Bold`
program output | `Fixed Width`

The following conventions are used in descriptions of the BIND configuration file:

To describe: | We use the style:
keywords | `Fixed Width`
variables | `Fixed Width`
Optional input | [Text is enclosed in square brackets]

## The Domain Name System (DNS)

This document explains the installation and upkeep of the BIND (Berkeley Internet Name Domain) software package. We begin by reviewing the fundamentals of the Domain Name System (DNS) as they relate to BIND.

### DNS Fundamentals

The Domain Name System (DNS) is a hierarchical, distributed database. It stores information for mapping Internet host names to IP addresses and vice versa, mail routing information, and other data used by Internet applications.

Clients look up information in the DNS by calling a resolver library, which sends queries to one or more name servers and interprets the responses. The BIND 9 software distribution contains a name server, named, and a resolver library, liblwres.

### Domains and Domain Names

The data stored in the DNS is identified by domain names that are organized as a tree according to organizational or administrative boundaries. Each node of the tree, called a domain, is given a label. The domain name of the node is the concatenation of all the labels on the path from the node to the root node. This is represented in written form as a string of labels listed from right to left and separated by dots. A label need only be unique within its parent domain.

For example, a domain name for a host at the company Example, Inc. could be `ourhost.example.com`, where `com` is the top level domain to which `ourhost.example.com` belongs, `example` is a subdomain of `com`, and `ourhost` is the name of the host.

For administrative purposes, the name space is partitioned into areas called zones, each starting at a node and extending down to the "leaf" nodes or to nodes where other zones start. The data for each zone is stored in a name server, which answers queries about the zone using the DNS protocol.

The data associated with each domain name is stored in the form of resource records (RRs). Some of the supported resource record types are described in the section called “Types of Resource Records and When to Use Them”.

For more detailed information about the design of the DNS and the DNS protocol, please refer to the standards documents listed in the section called “Request for Comments (RFCs)”.

### Zones

To properly operate a name server, it is important to understand the difference between a zone and a domain.

As stated previously, a zone is a point of delegation in the DNS tree. A zone consists of those contiguous parts of the domain tree for which a name server has complete information and over which it has authority. It contains all domain names from a certain point downward in the domain tree except those which are delegated to other zones. A delegation point is marked by one or more NS records in the parent zone, which should be matched by equivalent NS records at the root of the delegated zone.

For instance, consider the `example.com` domain which includes names such as `host.aaa.example.com` and `host.bbb.example.com` even though the `example.com` zone includes only delegations for the `aaa.example.com` and `bbb.example.com` zones. A zone can map exactly to a single domain, but could also include only part of a domain, the rest of which could be delegated to other name servers. Every name in the DNS tree is a domain, even if it is terminal, that is, has no subdomains. Every subdomain is a domain and every domain except the root is also a subdomain. The terminology is not intuitive and we suggest reading RFCs 1033, 1034, and 1035 to gain a complete understanding of this difficult and subtle topic.

Though BIND is called a "domain name server", it deals primarily in terms of zones. The "primary" and "secondary" declarations in the `named.conf` file specify zones, not domains. When BIND asks some other site if it is willing to be a secondary server for a domain, it is actually asking for secondary service for some collection of zones.

### Authoritative Name Servers

Each zone is served by at least one authoritative name server, which contains the complete data for the zone. To make the DNS tolerant of server and network failures, most zones have two or more authoritative servers, on different networks.

Responses from authoritative servers have the "authoritative answer" (AA) bit set in the response packets. This makes them easy to identify when debugging DNS configurations using tools like dig (the section called “Diagnostic Tools”).

#### The Primary Server

The authoritative server where the main copy of the zone data is maintained is called the primary (or master) server, or simply the primary. Typically it loads the zone contents from some local file edited by humans or perhaps generated mechanically from some other local file which is edited by humans. This file is called the zone file or master file.

In some cases, however, the zone file may not be edited by humans at all, but may instead be the result of dynamic update operations.

#### Secondary Servers

The other authoritative servers, called the secondary (or slave) servers, load the zone contents from another server using a replication process known as a zone transfer. Typically the data is transferred directly from the primary master, but it is also possible to transfer it from another secondary. In other words, a secondary server may itself act as a primary to a subordinate secondary server.

Periodically, the secondary server must send a refresh query to determine whether the zone contents have been updated. This is done by sending a query for the zone's Start of Authority (SOA) record and checking whether the SERIAL field has been updated; if so, a new transfer request is initiated. The timing of these refresh queries is controlled by the SOA REFRESH and RETRY fields, but can be overridden with the max-refresh-time, min-refresh-time, max-retry-time, and min-retry-time options.

If the zone data cannot be updated within the time specified by the SOA EXPIRE option (up to a hard-coded maximum of 24 weeks), the secondary zone expires and no longer responds to queries.

#### Stealth Servers

Usually, all of the zone's authoritative servers are listed in NS records in the parent zone. These NS records constitute a delegation of the zone from the parent. The authoritative servers are also listed in the zone file itself, at the top level or apex of the zone. Servers that are not in the parent's NS delegation can be listed in the zone's top-level NS records, but servers that are not present at the zone's top level cannot be listed in the parent's delegation.

A stealth server is a server that is authoritative for a zone but is not listed in that zone's NS records. Stealth servers can be used for keeping a local copy of a zone, to speed up access to the zone's records, or to make sure that the zone is available even if all the "official" servers for the zone are inaccessible.

A configuration where the primary server itself is a stealth server is often referred to as a "hidden primary" configuration. One use for this configuration is when the primary is behind a firewall and is therefore unable to communicate directly with the outside world.

### Caching Name Servers

The resolver libraries provided by most operating systems are stub resolvers, meaning that they are not capable of performing the full DNS resolution process by themselves by talking directly to the authoritative servers. Instead, they rely on a local name server to perform the resolution on their behalf. Such a server is called a recursive name server; it performs recursive lookups for local clients.

To improve performance, recursive servers cache the results of the lookups they perform. Since the processes of recursion and caching are intimately connected, the terms recursive server and caching server are often used synonymously.

The length of time for which a record may be retained in the cache of a caching name server is controlled by the Time-To-Live (TTL) field associated with each resource record.

#### Forwarding

Even a caching name server does not necessarily perform the complete recursive lookup itself. Instead, it can forward some or all of the queries that it cannot satisfy from its cache to another caching name server, commonly referred to as a forwarder.

Forwarders are typically used when an administrator does not wish for all the servers at a given site to interact directly with the rest of the Internet. For example, a common scenario is when multiple internal DNS servers are behind an Internet firewall. Servers behind the firewall forward their requests to the server with external access, which queries Internet DNS servers on the internal servers' behalf.

Another scenario (largely now superseded by Response Policy Zones) is to send queries first to a custom server for RBL processing before forwarding them to the wider Internet.

There may be one or more forwarders in a given setup. The order in which the forwarders are listed in `named.conf` does not determine the sequence in which they are queried; rather, named uses the response times from previous queries to select the server that is likely to respond the most quickly. A server that has not yet been queried is given an initial small random response time to ensure that it is tried at least once. Dynamic adjustment of the recorded response times ensures that all forwarders are queried, even those with slower response times. This permits changes in behavior based on server responsiveness.

### Name Servers in Multiple Roles

The BIND name server can simultaneously act as a primary for some zones, a secondary for other zones, and a caching (recursive) server for a set of local clients.

However, since the functions of authoritative name service and caching/recursive name service are logically separate, it is often advantageous to run them on separate server machines. A server that only provides authoritative name service (an authoritative-only server) can run with recursion disabled, improving reliability and security. A server that is not authoritative for any zones and only provides recursive service to local clients (a caching-only server) does not need to be reachable from the Internet at large and can be placed inside a firewall.

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