DNS query Process

From: http://blog.csdn.net/cz_hyf/archive/2006/02/19/603022.aspx

If a user node wants to obtain the corresponding "dpt1
The IP address of. SEU. edu. cn. The processing of this node is as follows:
1) send a "Answer" request to the local DNS server. To improve work efficiency, use the connectionless datagram service. If the local DNS server knows the corresponding dpt1
. Seu. edu. if the local DNS server knows the corresponding SEU. edu. the IP address of the DNS in CN, then access SEU. edu. cn dns, get dpt1
. SEU. edu. cn
Otherwise, if the local DNS server knows the IP address of the DNS corresponding to edu. cn, access
DNS to obtain the IP address of the DNS System of SEU. edu. cn. Otherwise, if the local DNS server knows the IP address of the DNS System of the corresponding CN, access the DNS of the CN and obtain
The IP address of the DNS System of edu. cn; otherwise

2) The local DNS system sends a "Answer" request to the root DNS system. The root DNS system returns the IP address of the "cn" DNS system;

3) The local DNS system accesses the DNS of CN to obtain the IP address of the DNS System of edu. CN;

4) The local DNS system accesses the DNS of edu. CN and obtains the IP address of the DNS System of SEU. edu. CN;

5) The local DNS system accesses the DNS of SEU. edu. cn to obtain dpt1.
The IP address of the dns system of. SEU. edu. CN;

6) The local DNS system returns the corresponding dpt1 to the user node.
The IP address of. SEU. edu. CN.

Many root-node DNS servers are configured on the Internet for mutual backup. user queries are conducted based on the principle of nearby access. To avoid forwarding a request that you cannot find to the root server and using the cache of the upper-level server, the DNS server can also set a forwarder. If the DNS itself does not have a corresponding record, query the DNS marked by the forwarder.

To ensure the reliability of the DNS server, one or more DNS servers are configured in each domain, one of which is the primary server and the other is the secondary server. Once the primary server fails, the user node automatically queries the secondary server.

Work
One way to improve performance is that each DNS server and host maintain a caching device that temporarily stores the retrieved domain name/IP address ing information, thus reducing the number of indirect accesses to DNS. Problems
It is the consistency between the cached information and the original information. Therefore, in practice, the information returned by DNS also contains the address ing lifetime (time_to_live), which is measured in seconds. Survival
The address ing information during the period is saved and referenced if necessary.