DNS Resolution Principle

The DNS server is responsible for converting the host name along with the domain name to an ip address.

The resolution of DNS is not what you think, if the entire Internet host name is placed Parsing on your company's DNS server, that
is not exhausted.

First of all, the Internet domain is a tree structure, the root contact is represented by a ".", the following high-level domain (for example: cn, tw, com,
net, org... .), there are second, third or more levels down. For example, www.active.com.cn is the upper layer, com is the second layer, active is the third layer, and www is The host name is up.

DNS is a huge distributed database. It is implemented by the name server providing a specified domain information
. In each zone, there are at least two or more name servers that hold all host authorization information for their zone. Therefore, in order to obtain the IP address of the machine in www.active.com.cn, you must contact the name server in the active.com.cn area.

The specific process is as follows: When the application wants to find the information of www, it contacts the local domain name server to perform a so-called repeated query.
The local domain name server sends a request to the name server of the root domain to query the address of www.active.com.cn; the root name server
does not belong to its own jurisdiction, but belongs to one under cn Domain, it will tell you to contact the name server of a cn area to get more information, and send you a list of addresses of all cn name servers; your local name server will continue to send
Parse the request, and one of them looks at its own zone. The above process will be repeated until the domain name service of the machine that resolves www is found to get the IP address of www.active.com.cn.

In order to further improve the response speed of the query, the name server will store the information it obtains in the local cache. In this way, when the network wants to query the host address of the active.com.cn domain, the name server will directly contact the name server of the domain.
The name server does not store this information permanently, but instead discards it automatically after the TTL (Time To Life) time.

Each name server will have a file that holds the root server information.