How DNS Works

Domain Name System (DNS) is the default name resolution service used in a Microsoft Windows Server 2003 network. DNS is part of the Windows Server 2003 TCP/IP protocol suite and all TCP/IP network connections are, by default, configured with the IP address of at least one DNS server in order to perform name resolution on the network. Windows Server 2003 components that require name resolution will attempt to use this DNS server before attempting to use the previous default Windows name resolution service, Windows Internet Name Service (WINS).

Typically, Windows Server 2003 DNS is deployed in support of Active Directory directory service. In this environment, DNS namespaces mirror the Active Directory forests and domains used by an organization. Network hosts and services are configured with DNS names so that they can be located in the network, and they are also configured with DNS servers that resolve the names of Active Directory domain controllers.

Windows Server 2003 DNS is also commonly deployed as a non-Active Directory, or standard, Domain Name System solution, for the purposes of hosting the Internet presence of an organization, for example.

DNS Architecture

DNS architecture is a hierarchical distributed database and an associated set of protocols that define:

• A mechanism for querying and updating the database.
• A mechanism for replicating the information in the database among servers.
• A schema of the database.

DNS originated in the early days of the Internet when the Internet was a small network established by the United States Department of Defense for research purposes. The host names of the computers in this network were managed through the use of a single HOSTS file located on a centrally administered server. Each site that needed to resolve host names on the network downloaded this file. As the number of hosts on the Internet grew, the traffic generated by the update process increased, as well as the size of the HOSTS file. The need for a new system, which would offer features such as scalability, decentralized administration, support for various data types, became more and more obvious.

The Domain Name System introduced in 1984 became this new system. With DNS, the host names reside in a database that can be distributed among multiple servers, decreasing the load on any one server and providing the ability to administer this naming system on a per-partition basis. DNS supports hierarchical names and allows registration of various data types in addition to host name to IP address mapping used in HOSTS files. Because the DNS database is distributed, its potential size is unlimited and performance is not degraded when more servers are added.

The original DNS was based on Request for Comment (RFC) 882 (“Domain Names: Concepts and Facilities”) and RFC 883 (Domain Names–Implementation and Specification), which were superseded by RFC 1034 (“Domain Names–Concepts and Facilities”), and RFC 1035 (“Domain Names–Implementation and Specification”). Additional RFCs that describe DNS security, implementation, and administrative issues later augmented the original design specifications.

The implementation of DNS — Berkeley Internet Name Domain (BIND) — was originally developed for the 4.3 BSD UNIX Operating System. The Microsoft implementation of DNS became a part of the operating system in Microsoft Windows NT Server 4.0. The Windows NT 4.0 DNS server, like most DNS implementations, has its roots in RFCs 1034 and 1035.

The RFCs used in Microsoft Windows 2000 and Windows Server 2003 operating systems are 1034, 1035, 1886, 1996, 1995, 2136, 2308, and 2052.

DNS Domain Names

The Domain Name System is implemented as a hierarchical and distributed database containing various types of data, including host names and domain names. The names in a DNS database form a hierarchical tree structure called the domain namespace. Domain names consist of individual labels separated by dots, for example: mydomain.microsoft.com.

A Fully Qualified Domain Name (FQDN) uniquely identifies the hosts position within the DNS hierarchical tree by specifying a list of names separated by dots in the path from the referenced host to the root. The next figure shows an example of a DNS tree with a host called mydomain within the microsoft.com. domain. The FQDN for the host would be mydomain.microsoft.com.

Understanding the DNS Domain Namespace

The DNS domain namespace, as shown in the following figure, is based on the concept of a tree of named domains. Each level of the tree can represent either a branch or a leaf of the tree. A branch is a level where more than one name is used to identify a collection of named resources. A leaf represents a single name used once at that level to indicate a specific resource.

DNS Domain Name Hierarchy

The previous figure shows how Microsoft is assigned authority by the Internet root servers for its own part of the DNS domain namespace tree on the Internet. DNS clients and servers use queries as the fundamental method of resolving names in the tree to specific types of resource information. This information is provided by DNS servers in query responses to DNS clients, who then extract the information and pass it to a requesting program for resolving the queried name. In the process of resolving a name, keep in mind that DNS servers often function as DNS clients, querying other servers in order to fully resolve a queried name.

How the DNS Domain Namespace Is Organized

Any DNS domain name used in the tree is technically a domain. Most DNS discussions, however, identify names in one of five ways, based on the level and the way a name is commonly used. For example, the DNS domain name registered to Microsoft (microsoft.com.) is known as a second-level domain. This is because the name has two parts (known a

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DNS的工作原理

域名系统（DNS）是Microsoft Windows Server 2003网络中使用的默认名称解析服务。DNS是Windows Server 2003 TCP / IP协议套件的一部分，默认情况下，所有TCP / IP网络连接都配置有至少一个DNS服务器的IP地址，以便在网络上执行名称解析。在尝试使用以前默认的Windows名称解析服务Windows Internet名称服务（WINS）之前，需要名称解析的Windows Server 2003组件将尝试使用此DNS服务器。

通常，部署Windows Server 2003 DNS以支持Active Directory目录服务，在这种环境中，DNS命名空间将镜像组织使用的Active Directory林和域。网络主机和服务配置了DNS名称，以便它们可以位于网络中，并且还配置了解析Active Directory域控制器名称的DNS服务器。

例如，Windows Server 2003 DNS也通常被部署为非Active Directory或标准的域名系统解决方案，用于托管组织的Internet存在。

DNS架构

DNS架构是分层分布式数据库和一组相关的协议，定义如下：

bull; 查询和更新数据库的机制。

bull; 在服务器之间复制数据库中的信息的机制。

bull; 数据库的架构。

互联网是由美国国防部以研究为目的建立的小型网络，DNS起源于互联网的早期阶段。通过使用位于中央管理服务器上的单个HOSTS文件来管理此网络中的计算机的主机名，每个网站需要下载这个文件解析网络上的主机名。随着互联网上主机数量的增加，更新过程生成的流量以及HOSTS文件的大小逐渐增加，对提供可扩展性、分散管理和各种数据类型支持等功能的新系统的需求变得越来越明显。

1984年推出的域名系统成为这个新系统。使用DNS，主机名可以驻留分布在多个服务器之间的数据库中，从而减少任何一个服务器上的负载，并提供以每个分区为基础管理此命名系统的能力。 DNS支持分层名称，并允许将除主机名之外的各种数据类型注册到HOSTS文件中使用的IP地址映射。因为DNS数据库是分布式的，所以它的潜在大小是无限制的，并且当添加更多的服务器时性能不会降低。

原始DNS基于RFC 1034（“域名 - 概念和设施”）取代的请求注释（RFC）882（“域名：概念和设施”）和RFC 883（域名 - 实现和规范）和RFC 1035（“域名实现和规范”）。描述DNS安全性，实施和管理问题的其他RFC在后期增加了原始设计规范。

DNS – 伯克利网络域名（BIND）的实现最初是为4.3 BSD UNIX操作系统开发的。微软实现的DNS成为Microsoft Windows NT Server 4.0中的操作系统的一部分。像大多数DNS实现一样，Windows NT 4.0 DNS服务器的根源在于RFC 1034和1035。

在Microsoft Windows 2000和Windows Server 2003操作系统中使用的RFC是1034,1035,1886,1996，1995,2136,2308和2052。

DNS域名

域名系统的实现包含各种类型的数据的分层和分布式数据库，包括主机名和域名。DNS数据库中的名称形成一个称为域名命名空间的分层树结构，域名由以点分隔的单个标签组成，例如：mydomain.microsoft.com。

通过指定从引用主机到根目录的路径中以点分隔的名称列表完全限定域名（FQDN），唯一地标识DNS分层树中的主机位置。下图显示了在microsoft.com中具有名为mydomain的主机的DNS树的示例。主机的FQDN将是mydomain.microsoft.com。

了解DNS域命名空间

DNS域名空间，如下图所示，是基于一个名为domain的树的概念。树的每个级别都可以表示树的树枝或树叶。分支是一个级别，其中使用多个名称来标识命名资源的集合。叶表示在该级别使用一次的单一名称，以指示特定资源。

DNS域名层次结构

上图显示了Internet如何为互联网根服务器分配自己的DNS域名命名空间树的一部分权限。 DNS客户端和服务器使用查询作为将树中的名称解析为特定类型资源信息的基本方法。此信息由DNS服务器提供给DNS客户端的查询响应，然后他们提取信息并将其传递给请求程序以解析查询的名称。在解析名称的过程中，请记住，DNS服务器通常用作DNS客户端，查询其他服务器完全解析查询的名称。

如何组织DNS域名命名空间

在树中使用的任何DNS域名在技术上都是一个域，然而，大多数DNS讨论都是以五种方式之一来识别名称，这些名称根据名称的常用级别和方式。例如，注册到微软（microsoft.com）的DNS域名称为二级域名，这是因为名称有两个部分（称为标签），表示它位于树的根或顶部下方的两个级别。大多数DNS域名都有两个或更多的标签，每个标签在树中指示一个新的级别，名称中使用期间分隔标签。

用于通过命名空间中的功能来描述DNS域名的五个类别在下表中描述，以及每个名称类型的示例。

DNS域名的类型

DNS和互联网域名

互联网域名系统由互联网上的名称注册机构管理，负责维护由组织、国家或地区分配的顶级域名。 这些域名遵循国际标准3166。保留一些供组织使用的许多现有缩写词，以及用于国家或地区的两个字母和三个字母的缩写如下表所示。

一些DNS顶级域名（TLD）

资源记录

DNS数据库由资源记录（RR）组成， 每个RR标识数据库中的特定资源，DNS中有各种类型的RR。本节提供有关资源记录的通用结构的信息，RR在本文档后面的“DNS中的资源记录”中有更详细的论述。

下表提供了有关常见RR结构的详细信息。

常见的DNS资源记录

分发DNS数据库：区域文件和委派

DNS数据库可以划分为多个区域，区域是DNS数据库的一部分，其中包含属于DNS命名空间的连续部分所有者名称的资源记录。区域文件在DNS服务器上维护，单个DNS服务器可以为托管配置零个，一个或多个区域。

每个区域都被锚定在被称为区域的根域的特定域名上，区域包含有关以区域的根域名结尾的所有名称的信息。如果DNS服务器加载了包含该名称的区域，那么DNS服务器被认为是权威的。任何区域文件中的第一条记录是“开始管理”（SOA）RR。SOA RR将该区域的主DNS名称服务器标识为该区域内数据的最佳信息源，并作为处理区域更新的实体。

区域内的名称也可以被委派到不同的DNS服务器上托管的区域。委托是将一部分DNS命名空间的责任分配给由单独实体拥有的DNS服务器的过程。这个单独的实体可以是您公司内的一个组织，部门或工作组。这种委托由NS资源记录表示，该资源记录指定该区域的权威委托区域和DNS名称。跨多个区域的委派是DNS的原始设计目标的一部分。

委托DNS命名空间的主要原因包括：

bull; 需要将DNS域的管理委托给组织内的多个组织或部门。

bull; 需要在多个DNS服务器之间分配维护一个大型DNS数据库的负载，以提高名称解析性能，并创建DNS容错环境。

bull; 需要通过将其包含在适当的域中来允许主持人的组织归属。

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