[TCP/IP] Explain the specific role of DNS in detail

This article refers to [Graphic TCP/IP]

In the previous article, I learned that IP is used to identify the host in a certain network area, so we usually do not enter the IP to visit when we visit the website. For example, you visit Baidu, generally enter: www.baidu.comthen you will not always similar input 124.23.1.12this IP to access it

Put yourself in it and think about it. If you want users to remember your website, you can't let them remember a large number of numbers. What's more, when IPv6 is used in the future, there are more numbers to remember, so the best way is to give these IPs A name, we call it a domain name , and when we visit these domain names, the specific IP to access must be left to DNS . This article will also introduce the specific function of DNS in detail.

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1. Domain name

At the beginning of the article, I briefly introduced what a domain name is, so let’s take a look at what a domain name looks like

For example, our common www.baidu.com, baidu.comthat is, Baidu official website domain name, which baiduis a name for themselves, when Baidu application for the domain name, .comis indicated by the domain name server belongs to, where we will this domain name is called top-level domain , you can also call it a domain name

Some domain names, for example, example.cn.comcalled secondary domain , example.mail.cn.comcalled three domain ......

Generally speaking, it can be understood that there are several domain names, which are .called several-level domain names.

Now available in all domain names are managed by a server, for example .comby the com服务器administration, .cnthe cn服务器management

Then these domain name servers are managed by a root server, so their hierarchical relationship is like a tree structure, as shown in the following figure:

Figure, com服务器within a cnserver, and cn服务器within and manages a mail服务器such a hierarchy can be used to get three domain names example.mail.cn.comcorresponding IP address of the

Therefore, we can call the domain names of all nodes at level 2 of the tree structure as the first-level domain names ; call the domain names of all nodes at level 3 as the second-level domain names ; and the domain names of all nodes at level 4 Is the third-level domain name , and so on

2. DNS

DNS (Domain Name System) is actually a system that manages the effective relationship between domain names and IP addresses, which is equivalent to a large database

When our host requests a webpage through a domain name, it will first send a request to the DNS server we set, and then the DNS server will send a request to the nearest root server, and the root server will return the IP access address of the first-level domain name server to DNS Server, then the DNS server will access the corresponding domain name server according to this IP address. If the domain name has multiple levels, the domain name server will then return the IP address of the next level domain name server to the DNS for its in-depth access

We look at access to example.mail.cn.coma simplified process diagram

In order to improve efficiency, the DNS server also provides a technology called cache , which is used to store the IP of the domain name server that has been accessed for a short time. Therefore, the real order of the DNS server is not to send a request to the root server first, but to query first. Whether there is the IP address you want to obtain in the cache, if there is, return it directly to our host; if not, follow the above process to request

Suppose we have just visited the request example.mail.cn.comwhen, then again, this time requesting the domain name corresponding URL can be read directly from the cache real IP address, the process as shown:

In fact, the cache time of the DNS server will not be particularly long, because if you cache the IP after the first visit, and then the original IP address is changed at this time, then every time we visit the old IP address, it will continue Error, so the cache time is shorter to avoid such problems

Three, conclusion

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