This document provides an overview of how to build a basic web server in 20 minutes. It explains that a web server follows the representational state transfer (REST) architectural style and discusses requests, responses, and parsing request data. It also describes adding dynamic functionality through common gateway interface (CGI) scripts and potential security issues around user inputs. The goal is to help readers understand the core components of a web server through a simple example implementation.

1. Learn in 20 Min
Build a Useless Web Server

2. Before Course
● You need to know
○ Coding skill
○ Socket
○ Google

3. REST (Representational State Transfer)
● Web Server is based on REST
○ Who: Assign the particular web page
○ Want: Do the particular method
○ What: Ask the particular resource
● www.example.com/image.html?img="正妹"
○ Who: image.html
○ What: GET method
○ What: 正妹

4. Request
● Request from browser or something else
○ Method: GET/POST/ ...
○ Target: /favicon.ico or what you want
○ Options:
■ config, cookie, session, ...

5. Response
● Also, response follows REST
○ Return the status code
○ Specified the content type: plain, html, or else
○ An empty blank line
○ Content for the web page

6. Basically
● Create a socket and listen request, parse
○ Find out the user's target resource.
○ Find out the method user's want.
○ Capture the content if need.
● Based on above information, return
○ Static file: CSS, js or image
○ CGI result: .cgi .php or some else I unknow
○ Else: 403, 404, 500, ...

7. Example
● Sample code...

8. Parser
● Simple plain parser (really?)
○ Split by rn, and find out the blank line
○ Line[0] contains the method, resource and protocol
○ Line[1]~line[m] contains options information
○ Line[m+1]~ contains content
● For example: Get file content

9. So, Implement It...
● Sample code...

10. But Something Bad...
● In this case
○ We return anything from request...
● But we have a smart and kind client agent
○ Request 127.0.0.1/../../../index.html will transfer to
127.0.0.1/index.html
○ We must believe the people as good as VXXXX.

11. Web 2.0
● Only get the static web page is so boring
○ We must interactive with other
○ Calling HCG CGI is the simple way
● CGI is Common Gateway Interface
○ In the easiest way to explain: executable binary
○ Execute and return the web page
○ Usually store in cgi-bin/ folder
○ Call as 127.0.0.1/cgi-bin/index.cgi

12. So, Implement It...

13. Web 2.0
● Only get the static web page is so boring as
cmj
○ We must interactive with other
○ Calling HCG CGI is the simple way
● CGI is Common Gateway Interface
○ In the easiest way to explain: executable binary
○ Usually store in cgi-bin/ folder
○ Call as 127.0.0.1/cgi-bin/index.cgi
● But Someone say
○ We are as good as ?F
○ We will never link to 127.0.0.1/cgi-bin/../../../../bin/ls
to list the files and folders

14. Extra Functionality
● Rewrite engine
● Symlink
● more and more
○ As many as possibles you can implement and twist
its original concept

15. Reference
○ http://www.ics.uci.
edu/~fielding/pubs/dissertation/top.htm
○ http://en.wikipedia.
org/wiki/Common_Gateway_Interface
○ http://en.wikipedia.org/wiki/Rewrite_engine