♥ Foreword
When we often visit some websites, enter an address similar to www.baidu.com in the browser, then enter this address in the browser ---> Baidu server will return us this Baidu page, what is the process in the middle What kind?
With this question in mind, let's analyze the process together.
1. The relationship between domain name and IP
First of all, addresses like 'www.baidu.com' and 'www.sina.com.cn' are collectively called domain names. Then, after using the domain name to access, can the request reach the corresponding server directly?
no!
In this process, the domain name needs to be converted into an IP address. Only by searching through the IP address can the location of the server be found and the server receive the request.
Why do you need to perform this conversion operation between domain names and IPs? Let's answer two questions:
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So why don't we communicate directly with IP addresses?
Because IP addresses are not convenient for users to remember, domain names are more convenient for users to remember and use. For example, www.baidu.com is the domain name of Baidu; www.sina.com.cn is the domain name of Sina, which is very easy to understand and remember;
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Then why not just use the domain name to find the server?
This is because the length of the domain name is not fixed, which is not convenient for computers to process; and the IP address is of fixed length, if it is an IPv4 address, it is 32 bits, and if it is an IPv6 address, it is 128 bits, so it is easier for computers to process it with a fixed length.
So, to sum up, the IP address is for the host, and the domain name is for the user. So we need to convert the domain name and IP address to each other.
2. Convert domain name to IP address
hosts file
At the beginning, people used the hosts file of the computer to convert the domain name and IP.
There is a host file under the path of our computer C:\Windows\System32\drivers\etc, which can save the corresponding relationship between domain name and IP, as shown in the following figure:
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Initially, the hosts file is managed through the Internet Information Center. If a new computer wants to connect to the network, or a computer IP changes, it is necessary to apply for changing the hosts file at the Information Center. Other computers also need regular updates to be able to go online. But this is too much trouble. For easier operation, the DNS protocol appeared.
DNS protocol
DNS protocol, the full name of Domain Name System, is an application layer protocol, which is based on UDP or TCP protocol, and uses port 53 by default.
The client communicates through the UDP protocol by default, but because the wide area network is not suitable for transmitting large UDP packets, it is stipulated that when the packet length exceeds 512 bytes, it should be converted to use the TCP protocol for data transmission. Therefore, the DNS protocol is a rare application layer protocol that can use both the UDP protocol and the TCP protocol as the underlying protocol.
The function of this protocol is to convert human-readable domain names (eg, www.qq.com) into machine-readable IP addresses (eg, 119.147.15.13).
Domain name hierarchy:
As shown in the figure above, the middle of the domain name is separated by dots. What does each dot represent?
We need to understand that the domain name is a hierarchical structure, and the domain name server is also a corresponding hierarchical structure, which are the root domain name, the top-level domain name system, the second-level domain name system, and the third-level domain name system, as shown in the following figure:
There are 13 root servers in the world. The Chinese names of these 13 root domain name servers are "A" to "M". Among them, 10 are set in the United States, and the other three are respectively set in the United Kingdom, Sweden, and Japan. One primary root server is placed in the United States, and the remaining 12 are secondary root servers.
For example, the hierarchical structure of the domain name www.baidu.com is as follows:
com: top-level domain name. Indicates that this is a corporate domain name.
baidu: Second-level domain name, refers to the company name.
www: Internet protocol (World Wide Web)
Domain names generally cannot exceed 5 levels, and the levels of domains become higher from left to right, and higher-level domains contain lower-level domains. The domain name is unique in the entire Internet. When the high-level sub-domain is the same, the low-level sub-domain is not allowed to be repeated.
With the domain name structure, there needs to be something to resolve the domain name, and that is the domain name server.
Domain names need to be resolved by domain name servers all over the world. Domain name servers are actually hosts with domain name systems installed.
DNS resolution process
As shown above, let's analyze the process of DNS resolution:
The user wants to access the Baidu server through the client browser - the domain name is www.baidu.com:
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When the user enters the www.baidu.com domain name in the browser, the default priority is to search the browser cache to see if the cache contains the IP address corresponding to the domain name
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When it cannot be found in the browser, it will check whether there is a corresponding IP address in the Hosts file in the system
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If there is no mapping of this domain name in hosts, look for the local DNS server;
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If not, search the root domain name server through the DNS server; the root domain name service returns the IP address of the top-level domain name server available for query;
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The top-level domain name server returns a DNS response message;
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After the host receives the response message, it can access the server normally; and save the result for the next use
The query methods of the DNS protocol are divided into the following two types:
Recursive query : The machine sends a query request to the local domain name server, and waits for the final result. If the local domain name server cannot resolve it, I will query other domain name servers as a DNS client until I get the final IP address and tell this machine
Iterative query : the local domain name server queries the root domain name server, the root domain name server tells it where to query next, and then it checks again, each time it queries each server as a client.
In layman's terms, recursion is to hand over one thing to others. If the thing is not finished, even if it has been done a lot, don't tell me the result. What I want is your final result, not the intermediate result; if you don't After finishing it, please find someone else to finish it.
Iteration is one thing I give you, tell me how much you can do, and then I will do the rest.