DNS
Cnic.org,开放的网络天书！
DNS (Domain Name System, 域名系统), DNS是因特网的一项核心服务,它作为可以将域名和IP地址相互映射的一个分布式数据库，能够使人更方便的访问互联网，而不用去记住能够被机器直接读取的IP数串。
举一个例子，www.cnic.org作为一个域名就和IP地址130.94.122.199相对应.DNS就像是一个自动的电话号码簿，我们可以直接拨打cnic的名字来代替电话号码（IP地址）。DNS在我们直接呼叫网站的名字以后就会将像www.cnic.org一样便于人类使用的名字转化成像130.94.122.199一样便于机器识别的IP地址。
DNS最早于1983年由保罗·莫卡派乔斯 (Paul Mockapetris)发明; 原始的技术规范在882号因特网标准草案 (RFC 882)中发布. 1987年发布的第1034和1035号草案修正了DNS技术规范，并废除了之前的第882和883号草案. 在此之后对因特网标准草案的修改基本上没有涉及到DNS技术规范部分的改动。
DNS implements a hierarchical name space by allowing name service for parts of a name space known as zones to be "delegated" by a name server to subsidiary name-servers. DNS also provides additional information, such as alias names for systems, contact information, and which hosts act as mail hubs for groups of systems or domains.
The present restriction on the length of domain names is 63 characters, excluding the www. and .com or other extension. Domain names are also limited to a subset of ASCII characters, preventing many languages from representing their names and words correctly. The Punycode-based IDNA system, which maps Unicode strings into the valid DNS character set, has been approved by ICANN and adopted by some registries as a workaround.
The DNS system is run by various flavors of DNS software, including:
- BIND (Berkeley Internet Name Domain), the most commonly used namedaemon.
- DJBDNS (Dan J Bernstein's DNS implementation)
- MaraDNS
- NSD (Name Server Daemon)
- PowerDNS
Any IP computer network can use DNS to implement its own private name system. However, the term "domain name" is most commonly used to refer to domain names implemented in the public Internet DNS system. This is based on thirteen "root servers" worldwide, all but three of which are in the United States of America. From these thirteen root servers, the rest of the Internet DNS name space is delegated to other DNS servers which serve names within specific parts of the DNS name space.
An 'owner' of a domain name can be found by looking in the whois database: for most gTLDs a basic WHOIS is held by ICANN, with the detailed WHOIS maintained by the domain registry which controls that domain. For the 240+ Country Code TLDs the position is usually that the registry holds the entire authorative WHOIS for that extension, as part of their many functions.
The current way the main DNS system is controlled is often criticized. The most common problems pointed at are that it is abused by monopolies or near-monopolies such as VeriSign Inc., and problems with assignment of top-level domains.
Some also allege that many implementations of DNS server software fail to work gracefully with dynamically allocated IP addresses, although that is the failure of specific implementations and not failures of the protocol itself.
相关: cybersquatting, dynamic DNS, ICANN, DNSSEC
