Application Layer Protocol Overview

Application Layer Protocol Overview

Application Layer: provides network services for applications.

The main agreement: DNS, HTTP, SMTP, POP3, IMAP, Telnet, FTP, TFTP ...

A, DNS (Domain Name System) : establish a mapping between domain names and ip addresses.

· In simple terms, DNS is similar to a phone book, the name is the domain name, ip address is the phone number you want to call Joe Smith, it is necessary to ask the phone book, Joe Smith telephone number, phonebook respond, then by ip address to dial Joe Smith.

· Public DNS server: Google's 8.8.8.8 and 114.114.114.114 114dns of.

· Commonly used commands: nslookup DNS debugging tools

ipconfig / displaydns view the DNS cache

ipconfig / flushdns to clear the DNS cache

· Recursive queries, and iterative queries difference

Two, HTTP: Hypertext Transfer Protocol

Helping clients to access the World Wide Web (world wide web).

· HTTP is not encrypted, data is transmitted in clear text, unsafe; compared to the HTTPS HTTP, added to the SSL layer to encrypt data transmission, it can not easily be cracked.

· GET and post difference:

1.get directly in the browser input, post need tools to send requests

2.get url or cookie with parameter passing, post the data in the body

3.get have the URL length limit, post data can be very large

4.post see than get data security, because the URL

5.get used to obtain the data, post used to transmit data

· Request header meaning:

Accept: refers to the browser or other client can take the love of MIME file format. Analyzing it can Servlet and return the appropriate file format.

User-Agent: is the name of the client browser

Host: the corresponding Web URL name and port number in the URL.

Accept-Langeuage: pointed out that the browser can accept the kind of language, such as en or en-us, refers to English.

connection: used to tell whether the server can maintain a constant HTTP connection. http is connectionless, HTTP / 1.1 Keep-Alive use default values, so that when (such as an HTML file and the associated graphics files) browser needs multiple files, do not need to establish a connection every time

Cookie: browser with this property is sent to the server Cookie. Cookie is registered in the browser small data volume, the user may specify that the information relative to the server, a session can be used to implement the function.

Referer: show generated web page URL request. As the ratio from click /icconcept/index.jsp a link to a page / icwork / search, send a GET request to the server / icwork / search in the middle, Referer is http: // hostname: 8080 / icconcept / index. jsp. This property can be used to track Web request is from what site come.

Content-Type: used to type the name of the table of contents of request. Can () method of obtaining a getContentType HttpServletRequest.

Accept-Charset: pointed out that the browser can accept the character encoding. English browser default value is ISO-8859-1.

Accept-Encoding: pointed encoding the browser can accept. Unlike encoding file format, which is to archive file delivery speed and acceleration. Browser early decoding after receiving the response from the Web, and then check the file format.

Third, mail service

· SMTP Simple Mail Transfer Protocol used to send e-mail

· POP3 Post Office Protocol version 3 for incoming mail

·IMAP 互联网邮件访问协议类似POP3,功能更多

·在通过foxmail客户端管理邮件时，需开启以上协议（一般强制要求IMAP），才能正常收发邮件。

四、Telnet（Terminal Network，终端网络）：远程管理的主要协议。

·常用终端工具：SecureCRT、Putty、xshell

·windows自带telnet工具，但出于安全因素（telnet不加密）考虑，一般默认关闭，如需使用，可在控制面板中开启telnet客户端服务。

·命令:telnet IP地址或域名，例如：telnet 192.168.168.168

五、FTP和TFTP

·FTP:文件传输协议提供可靠的文件传输服务，具有认证、权限服务

Application Layer Protocol Overview

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