HTTP & Web Servers

1. ncat

   tcp层通话
   ncat -l 9999 //监听端口9999，服务端
   
   ncat localhost 9999//连接至端口9999，客户端

2. python 开本地服务器

   python -m http.server 8000(python3.0)

3. 服务器：接收网络上其他程序发过来的连接

   A server is just a program that accepts connections from other programs on the network.
   
   When you start a server program, it waits for clients to connect to it — like the demo server waiting for your web browser to ask it for a page. 
   Then when a connection comes in, the server runs a piece of code — like calling a function — to handle each incoming connection. 
   A connection in this sense is like a phone call: it's a channel through which the client and server can talk to each other. Web clients send requests over these connections, and servers send responses back.

4. URI : Uniform Resource Identifier, a name for a resource.

   https://en.wikipedia.org/wiki/Fish
    https==>scheme
    en.wikipedia.org==>hostname
    /wiki/Fish==>path

5. scheme

   常见有http/https/file/data/mailto
   file告诉客户端从本地文件系统访问文件
   http/https:从网络*问文件
   data:直接将hardcoded数据放入网页，如图片
   magnet： is used for links to some file-sharing services such as BitTorrent. 

6. hostname:告诉客户端连接哪个服务器

   不是每个URI都有hostname，如mailto:[e-mail]
   
   也可以说明 : 是放在协议后面的,//是放在hostname前面的，mailto没有hostname,因此没有//

7. path : identifie a particular resource on a server, tell the server which resource the client is looking for

   当没写path时，如http://baidu.com，浏览器会自动填充，在后面加个/，http://baidu.com/

8. 其他常见URI组成

   http://baidu.com/search?q=jier
      ?q=jier 叫query，会发送至server
   https://bd.com/wiki/Oxygen#Discovery
      Discovery 叫fragment，不会发送至server，本地浏览器会自动滚动到页面id为Discovery的部分

9. host/nslookup

   host google.com
    google.com has address 111.67.201.52
    google.com has IPv6 address 2404:6800:4012::200e
   nslookup google.com
    Server: 123.207.137.88
    Address: 123.207.137.88#53
   
    Name: google.com
    Address: 111.67.201.52

10. port

    Each packet has the IP addresses of the computer that sent it, and the computer that receives it. 
    And (with the exception of some low-level packets, such as ping) it also has the port number for the sender and recipient. 
    IP addresses distinguish computers; port numbers distinguish programs on those computers.
    
    http 默认80端口
    https 默认443端口

11. HTTP GET REQUESTS

    GET / HTTP/1.1
    GET /1.png HTTP/1.1
    
    GET==>method or a HTTP Verb that clients use when they want a server to send a resource
    /1.png==>path of the resource being requested
    HTTP/1.1==>protocol of the request

12. HTTP responses

    status line:告诉客户端服务器是否理解了请求，是否有请求的资源，下一步的反应及当前protocol版本号
    some headers==>告诉客户端一些response的信息，不会在网页中显示出来
    a response body
    本例中：状态码301表示重定向redirection，告诉客户端如果想获得谷歌主页需要去访问：http://www.google.com.==>浏览器会自动重定向到该网址
    
    
    

1. status code : 状态码 

1xx — Informational. The request is in progress or there's another step to take.
2xx — Success! The request succeeded. The server is sending the data the client asked for.
3xx — Redirection. The server is telling the client a different URI it should redirect to. The headers will usually contain a Location header with the updated URI. Different codes tell the client whether a redirect is permanent or temporary.
4xx — Client error. The server didn't understand the client's request, or can't or won't fill it. Different codes tell the client whether it was a bad URI, a permissions problem, or another sort of error.
5xx — Server error. Something went wrong on the server side

2.head

keyword: value. 
Localtion: www.baidu.com  
Content-type: text/html; charset=UTF-8
Content-type: image/png //图片格式
Content-length: 它告诉客户端响应体的长度（以字节为单位）。如果服务器发送此消息，那么客户端在读取第一响应后可以重用该连接发送另一个请求。浏览器使用这一点，这样就可以在不必重新连接服务器的情况下获取多个数据（例如网页上的图像）。