DNS (Domain Name System, the domain name system) is one for managing and resolving a domain name and IP address correspondence relationship technology, is capable of accepting a user input domain name or IP address, and then automatically finds the matching (or has a mapping relation) the IP address or domain name, the domain name is about to be resolved to an IP address (analytical positive), or domain name resolves to the IP address (reverse lookup)
the DNS DNS service uses a hierarchical tree structure similar to domain names and IP addresses of record the correspondence relationship, thus forming a distributed database system 
to ensure the normal operation of the Internet, there are the following three types DNS server.
1, the primary server: unique within a particular area, is responsible for maintaining the correspondence between domain names and IP addresses in the region.
2, from the server: obtaining from the master to the correspondence between the domain name and IP address is maintained in case the primary server goes down and the like.
3, the cache server: get through queries to other DNS servers correspondence between domain names and IP addresses, domain names and save frequently queried information to the local server, in order to improve the efficiency of repetitive queries.
DNS DNS service has recursive queries and iterative queries in two ways in the implementation of user-initiated domain name queries.
BIND (Berkeley Internet Name Domain, the Berkeley Internet Name Domain) service is the most widely used worldwide, most secure and efficient DNS service program. In the Linux system, the name of the bind service program is named
bind service program has the following three more crucial documents.
1, a main configuration file (/etc/named.conf): only 58 lines, and after removal of the annotations and blank lines, practical and effective parameters only about 30 lines, which define the operating parameters for the service program bind.
2, the profile region (/etc/named.rfc1912.zones): to save the name and IP address corresponding to the location relationship. Similar to the book catalog, to a specific location for each domain and the corresponding IP address belongs to, when you need to view or modify, in accordance with relevant documents can be found in this location.
3, the configuration data file directory (/ var / named): the directory names and IP addresses used to store a correspondence between the real profile data.
location area configuration file bind service program (/etc/named.rfc1912.zones) to save the correspondence between domain names and IP addresses of the DNS for the IP address being analytical parameters and the domain name resolves to the IP address counter Figure 13-3 and 13-4, respectively, to the resolution parameters shown 
bind service routine encryption mechanisms TSIG (RFC 2845) provides support to ensure the security of information transmission region between the domain name server DNS. TSIG is the use of the enciphering way to protect the transmission information area (Zone Transfer).
DNS cache server (Caching DNS Server) is a domain name is not responsible for data maintenance of the DNS server, the user is frequently used to resolve domain names and IP addresses of record stored locally on the host, so as to enhance the efficiency of the next resolution. Cache servers are generally used within the enterprise network, designed to reduce network users query the DNS time consuming.
Using DNS services to resolve separation technology, allowing readers are located in different geographic scope of access through the same web site, but from a different server to get the same data 
in this chapter course notes as follows
