What can DNS do besides resolving domain names?

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One http request , What is the typical execution process ？

You can see , The typical process is ：
（1） Client request dns-server, Initiate domain name resolution ;
（2）dns-server Return the Internet corresponding to the domain name ip(1.2.3.4);
（3） Client through the Internet ip(1.2.3.4), Access reverse proxy ;
（4） Reverse proxy through intranet ip(192.168.x.x), Distribute requests to web-server;
（5）web-server The request is processed ;

among , The first step , Convert the domain name to ip The process of , It happens outside the application system , It's through DNS Realized .

Besides domain name resolution , In architecture design , Can also use DNS What do you do ？

One 、 Users visit nearby

DNS Can achieve , The nearest access to the resources the user needs ：
（1） Telecom users want to access a server resource ;
（2） Browser direction dns-server Initiate domain name resolution request ;
（3）dns-server Identify the visitor as a telecom user ;
（4）dns-server Will the telecom room nginx Extranet ip Back to the visitor ;
（5） Visitors visit nearby ;

According to the user ip To return to the nearest server ip, be called “ intelligence DNS”,CDN And the most commonly used in multi machine room and multi living .

Two 、 Reverse agent horizontal expansion

In a typical Internet Architecture , By increasing web-server To expand web Layer performance , But reverse proxy nginx It's still the only access to the whole system , If the system throughput exceeds nginx Performance limit of , Difficult to expand , At this point, we need dns-server With horizontal expansion .

The specific way is ： stay dns-server For the same domain name, you can configure multiple nginx The Internet ip, Every time DNS Resolve request , Polling returns different ip, In this way... Can be achieved nginx Horizontal expansion of , This method is called “DNS polling ”.

3、 ... and 、web-server Load balancing

since “dns polling ” The traffic of the same domain name can be evenly distributed to different nginx, Then it can also be used to do web-server Load balancing of ：
（1） Remove... From the architecture nginx layer ;
（2） Will be multiple web-server The Intranet ip Change to the Internet directly ip;
（3） stay dns-server The Internet corresponding to the domain name ip Polling resolution ;

adopt DNS What are the advantages and disadvantages of load balancing ？

advantage ：

（1） Leverage third parties DNS The implementation of , There is no need to move the server architecture ;
（2） A layer of network requests is missing ;

Insufficient ：

（1）DNS It only has parsing function , Can't guarantee the corresponding Internet ip The usability of , and nginx When acting as a reverse agent , And web-server There is a live detection mechanism between , When web-server Hang up , Ability to automatically migrate traffic ;
（2） When web-server When expansion is needed , adopt DNS The expansion takes effect for a long time , and nginx It's the part of the server that is completely under its own control ,web-server Expansion is more real-time and convenient ;

Because of the above two reasons , Generally, high availability reverse proxy is used in architecture .

summary

In architecture design , Besides domain name resolution ,DNS There are other uses ：
（1） intelligence DNS, According to the user ip To access the server nearby ;
（2）DNS polling , Extend the reverse proxy layer horizontally ;
（3） utilize DNS Implement load balancing ;

I hope you all get something .

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