Network security (1)

DNS Analytic process
First step , Browser cache
When the user types in the browser www.baidu.com when , The browser will check its cache first , If there is this domain name , It's mapped IP Link to address , Parsing complete

The second step , Local system cache
If the browser cache is not found , Just check the cache of the local operating system , If there is this domain name , It's mapped IP Link to address , Parsing complete

The third step , Local domain name server
If not found in the operating system cache , Then check the cache of the local domain name server , If there is this domain name , It's mapped IP Building links , Parsing complete

Step four , Root domain server
If it is not found in the local domain name server cache , Then ask the root server , The root server will return a primary server in charge of the region IP
The root server is used to manage the home directory of the Internet , Do not store domain names , Instead, it stores the address information of the domain name server responsible for the resolution of each domain , All domain names on the Internet are converted into IP Request for address , In theory, you have to go through the root server

Step five , Recursively request the primary domain name server
The local domain name server returns IP Address , Link to this master domain name server , If the primary domain name server has this domain name , It's mapped IP Building links , Parsing complete
Without this domain name , Then recursively request the next level domain name server , Until you find the corresponding domain name

Step six , Cache and link
The main domain name server will query IP The address is returned to the local domain name server , The local domain name server caches the domain name and its mapping IP , And pass IP Address and web Server links , Show website content , Parsing complete

HTTPS Communication steps
① The client sends a message to the server to start SSL signal communication . The message contains client supported SSL Specify the version 、 Client generated random number 、 Encryption components ( Encryption algorithm and key length ).
② The server sends a response message to the client . Same as client , The message contains SSL edition 、 Server generated random number 、 Encryption components ( Filtered from the received client encryption component ).
③ The server sends the public key certificate to the client , Keep your private key . The server logs its public key into the digital certificate authority , The digital certificate authority signs the public key of the server with its private key and issues the public key certificate .
④ The server sends a message to inform the client of the initial stage SSL End of handshake negotiation .
⑤ The client encrypts the generated random number with the server's public key and sends it to the server . After the client gets the server's public key certificate , Use the public key of the digital certificate authority to verify the digital signature of the public key certificate to the digital certificate authority , To confirm the authenticity of the public key of the server , And then generate a new random number , Encrypt the random number with the public key and send it to the server .
⑥ The server decrypts the private key and obtains the random number sent by the client .
⑦ The client and server are encrypted according to the agreed encryption method , Use the first three random numbers to generate the dialog key , Used to encrypt the next communication process .

Status code

301： Permanent transfer , The browser will automatically jump to a new one after getting the status code returned by the server URL Address , This address can be answered from Location Get... In the first part （ The effect the user sees is the address he enters A In an instant, it becomes another address B）, Old address A Has been permanently removed （ This resource is inaccessible ）, The search engine grabs the new content and exchanges the old URL with the redirected one
302： A temporary transfer , The browser will automatically jump to a new one after getting the status code returned by the server URL Address , This address can be answered from Location Get... In the first part （ The effect the user sees is the address he enters A In an instant, it becomes another address B）, Old address A Our resources are still there （ Still accessible ）, This redirect is only temporarily from the old address A Jump to address B, Search engines will grab new content and save old URLs .
304： Website 304 The error status code of is when the client tries to access the server and prompts each other
307： Temporary redirection , The client sends post Request returns 307 when , The browser asks the user whether to post
308：http Status return code of , Used to indicate that the currently requested resource has been permanently transferred to another uri, Usually by returning header Of location Field display
400： The field name or field type submitted by the front end is different from the entity class in the background , Or the parameters submitted by the front end are inconsistent with the number of parameters required by the background , It makes it impossible to encapsulate .
401： Request for authentication . For pages that need to log in , The server may return this response .
403： The server has understood the request , But refuse to execute it . And 401 The response is different , Authentication doesn't help , And this request should not be submitted repeatedly .
404: The server could not find the resource at the request of the client （ Webpage ）, Called by the front end URL non-existent .
500: Server internal error , Unable to complete request ,500 The error is that the incoming parameter has an exception in business processing
501: The server does not support a function required by the current request . When the server does not recognize the request , And can't support its request for any resources .
502: When a server working as a gateway or proxy attempts to execute a request , Invalid response received from upstream server .
503: Due to temporary server maintenance or overload , The server is currently unable to process the request . This situation is temporary , And will be restored in a period of time