DNS protocol and its complete DNS query process

DNS Overview of the agreement

DNS（Domain Names System）, The domain name system , The Internet is a service , Is to carry on the domain name and corresponding IP Address translation server

simply ,DNS Equivalent to a translator , Be responsible for translating the domain name into ip Address

• IP Address ： A long string of numbers that uniquely mark computers on the network
• domain name ： It's a string of names separated by dots Internet The name of a computer or computer unit , Used to locate and identify the computer during data transmission

A query

• recursive query ： If A request B, that B As the receiver of the request, you must give A The answer you want
  
• Iterative query ： If the recipient B There is no requester A The exact content needed , The receiver B Will tell the requester A, How to get this content , But I don't ask myself
  
  Generally, we Local DNS The server The way to send a request is recursive query , Because we only need to make one request , Local DNS The server We will return the final request result . and Local DNS The server towards Other domain name servers The process of the request is Iterative query , Because every time Domain name server Only the results of a single query are returned , The next level of query consists of Local DNS The server Do it yourself .

The query process

The process of domain name resolution is as follows ：

• First Search the browser's DNS cache , Maintain a domain name and IP Table of addresses
• If you don't hit , Continued to Search for operating system DNS cache
• If you don't hit , The operating system sends the domain name to the local domain name server , The local domain name server uses recursion to query its own domain name DNS cache , If the search is successful, the result
• If the local domain name server DNS Cache miss , The local domain name server reports to the superior The domain name server performs iterative queries
  1. Local domain name server towards Root domain server Initiate request , The root domain name server returns the address of the top-level domain name server to the local server
  2. Local domain name server Get this Top-level domain server After the address of , Make a request to it , Get the address of the domain name server
  3. Local domain name server according to Domain name server Send a request to the address of , Finally, we get the corresponding IP Address
• Local domain name server What will be obtained IP Address Return to operating system , At the same time, I will IP The address is cached
• operating system take IP Address returned to browser , At the same time, I will IP Address cache start
• thus , The browser gets the corresponding IP Address , And will IP Address cache start

The flow is shown in the following figure ：