1. Computer Networks and Internets 《计算机网络与因特网》课件 林坤辉 2004-02-6

2. PART IV Network Applications Chapter 29 Naming With The Domain Name system 基于域名系统的命名

3. 29.1 Introduction IP addresses are fundamental in TCP/IP. Computers are also assigned symbolic names,Symbolic names are convenient for humans. An application must translate each name into an equivalent IP address before using it for communication.

4. 29.2 Structure of Computer Names The naming scheme used in the Internet is called the Domain Name System(DNS). Domain names are hierarchical. An organization must register with the Internet authority,a unique domain suffix is assigned to each organization. Each organization can choose how many segments to use for computers inside the organization and the those segments represent.

11. 29.3 Geographic Structure In addition to the familiar organizational structure,the DNS allows organizations to use a geographic registration. For example : cnri.reston.va.us

12. 29.4 Domain Names Within An Organization An organization can decide whether to introduce additional hierarchical structure. A small corporation can choose no additional hierarchy, while a large organization can choose many levels. Names within an organization do not need to follow a uniform pattern.

16. 29.5 The DNS Client-Server Model The DNS uses client-server interaction. The entire naming system operates as a large,distributed database. each domain name server contains information that links the server to other domain name servers. Whenever an application needs to translate a name to an IP address,the application become a client of the naming system.

17. The client places the name to be translated in a DNS request message, and sends the request to a DNS server. The server extracts the name from the request, translates the name to an equivalent IP address,and returns the resulting address to the application in a reply message.

19. 29.6 The DNS Server Hierarchy DNS servers are arranged in a hierarchy that matches the naming hierarchy,with each being the authority for part of the naming hierarchy. A root server occupies the top of the hierarchy,and is an authority for the top- level domains.

23. 本地名字服务器和根名字服务器 每台主机应该知道本地名字服务器（ local name server ） 每台本地名字服务器应该知道根名字服务 器（ Root name servers ） 一般是通过 NIC 获得信息并手工配置

24. 根名字服务器列表

27. 29.8 Locality Of Reference and Multiple Servers First, a user tends to look up the names of local computers more often than the names of remote computers. Second, a user tends to look up the same set of domain names repeatedly. Having multiple servers within an organization works well.

28. 29.9 Links Among Servers A single server must be responsible for all computers that have a given suffix. Servers in the domain name system are linked together, making it possible for a client to find the correct server by following links. Each server is configured to know the location of the servers of subparts of the hierarchy. Each server is configured to know the location of a root server.

29. 29.10 Resolving A Name The translation of a domain name into an equivalent IP address is called name resolution. Many operating systems provide name resolver software as a library routine that an application can call. For example: gethostbyname (on UNIX systems) (on UNIX systems)

30. Recursive resolution ( 递归解析 )

31. iterative resolution( 迭代解析 )

34. 29.11 Optimization Of DNS Performance There are two primary optimizations used in the DNS: replication and caching. Each root server is replicated. Each server maintains a cache of names. Locality of reference.

35. 29.12 Types Of DNS Entries Each entry in a DNS database consists of three items: a domain name, a record type, and a value. The domain name system stores a type with each entry. When a resolver looks up a name, the resolver must specify the type that is desired.

36. 每个服务器用资源记录（ Resource Record ） 的集合去实现区域信息。本质上，一个资源记 录是一个名字到值的绑定，或者说是一个 5 元 组，包括以下几个字段： 〈 名字（ Name ）, 值（ Value ）, 类型 （ Type ）, 分类（ Class ）, 生存期 （ TTL ） 〉 名字 name/ 值 value ：主机名字到 IP 地址 资源记录

37. 资源记录中各个字段的含义 类型 type –NS ：值（ value ） 字段给出了运行名字服务器的主 机域名，而该名字服务器知道如何解析特定的域名 –CNAME ：值（ value ）字段给出了特定主机的规范 名字，主要用于定义主机别名 –MX ：值（ value ）字段出了运行邮件服务器的主机 域名，而该邮件服务器知道如何接收解析指定域的 值 分类 Class ：允许其他实体（除 InterNIC 外）定义有用 的记录类型，目前广泛使用的分类是因特网使用的分 类，记为 IN 。 生存期 TTL ：指明资源记录的有效期限

38. 作业 29.14