Linux Web Services (HTTP HTML DNS)

DNS domain name resolution

DNS is responsible for translating domain names into IP addresses.

The detailed introduction is well documented in the previous blog.

Here is a review of the DNS resolution process.

Domain Structure

hostname.subdomain.[second-level domain].top-level domain.
                    (root domain)

DNS resolution process (interview questions)

Client -> local caching domain name server -> root domain server -> secondary domain server -> subdomain server -> resolve IP according to host name (iterative, recursive)

The parsing process is divided into iterative mode and recursive mode.

Iterative query is more commonly used, because it is not as easy to occupy the resources of the upper domain server as recursive query

Recursive pattern parsing process

The client sends a request to find the local cache domain server, if there is no

Forward relevant requests to the root domain server for resolution,

The root domain server forwards the request to the corresponding top-level domain server,

The top-level domain server then forwards it to the second-level domain server,

The second-level domain server forwards it to the sub-domain server, and resolves the IP address according to the host name.

The data is returned to the client layer by layer, and the client can use the IP address to access the host of the other party.

At the same time, the corresponding relationship of domain server records is cached locally for the next visit.

Iterative schema parsing process

The client sends a request to find the local cache domain server, if there is no

Send a resolution request to the root domain server and return the address of the top-level domain server

Send a resolution request to the top-level domain server and return the address of the second-level domain server

Send a resolution request to the secondary domain server and return the address of the subdomain server

Send a resolution request to the subdomain server, resolve the IP address and return the result to the local cache domain server

The local cache domain server returns the result to the client, and records the corresponding relationship for the next visit

TCP three-way handshake

TCP three-way handshake and four-way handshake are used to establish a connection before HTTP protocol communication.

The detailed introduction is well documented in the previous blog.

Here is a brief review of the TCP three-way handshake and four-way handshake process.

1. The first handshake: When the connection is established, the client sends a syn packet (seq=x) to the server, and enters the SYN_SENT state, waiting for the server to confirm
2. Second handshake: The server receives the syn packet and must confirm the client’s SYN (ack=x + 1), and at the same time send a SYN packet (seq =y), that is, the SYN + ACK packet, at this time the server enters the SYN_RECV state.
3. The third handshake: the client receives the server’s SYN + ACK package, sends a confirmation package ACK (ack=y + 1) to the server, and this package is sent After completion, the client and server enter the ESTABLISHED (TCP connection successful) state and complete the three-way handshake.

Brief description: pc1 sends a SYN message to pc2, pc2 replies a SYN + ACK message after receiving it, and pc1 replies an ACK message after receiving it, and both ends enter the established state.

TCP waved four times

Brief description: pc1 sends a FIN message to pc2, pc2 replies with an ACK message after receiving it, and then replies with a FIN + ACK message after the data transmission is completed, pc1 sends an ACK message after receiving it, and enters the time wait stage , close the connection after waiting for 2MSL. pc2 also closes the connection after receiving the ACK.

For a detailed description, please refer to the previous blog post

HTML

HTML is called Hypertext Markup Language, which is a specification and a standard, and it uses markup symbols to mark various parts of the webpage to be displayed. A web page file itself is a text file. By adding tags to the text file, you can tell the browser how to display the content.

HTML files can be edited with any text editor that can generate txt files to generate hypertext markup language files, just modify the file name suffix to “.html” or “.htm”.

HTML basic tags

(1) HTML grammar rules
HTML tags adopt the form of double tags, and the front and rear tags correspond to each other, indicating the start and end of the tag respectively, and the content in the middle of the tag is described by the tag. The front tag is represented by ““, and the end tag has one more “/”, which is represented by ““.
(2) HTML file structure
The outermost layer of an HTML file is represented by , indicating that the file is described in HTML language. Inside it are parallel header tags () and content tags (). The most basic HTML file structure is as follows:

<html>
    <head>Description information of the content of the web page</head>
    <body>The content displayed on the webpage</body>
</html>

●Tags commonly used in header tags:

label	description