CS 335/535 Web Programming - Week 5 - CGI and the Internet

11. CGI and the Internet
    • What is CGI and What Can It Do?
      Common Gateway Interface (CGI) is a standard for interfacing external applications with information servers (i.e. Web servers)

      Web Browser ->HTTP Daemon <--Standard Output-->CGI Program<--TCP/IP Socket-->Database Engine
      

      • Terminology
        • environment variable - named parameter that carries information from the server to the script. Often a variable in the OS's environment
        • Script - software that is invoked by the server via the CGI
        • Server - application program that involes the script in order to service requests
    • What are the Benefits of Using CGI?
      • can retrieve and update real-time data and format dynamic pages (including images & audio)
      • can be used to store and retrieve cookies
      • can write CGI programs in Perl, C/C++, TCL, Visual BASIC, etc.
      • supported by most web servers and platforms
    • What Are the Negatives of Using CGI?
      • security - programs run on host
      • uses resources on host
    • The Protocols communications is via stdout; CGI program needs to decode information, process it, and return the information
      • Environment Variables

        Environment Variable	Purpose/Comment
        GATEWAY_INTERFACE	Revision of CGI Spec to which the server complies CGI/revision
        SERVER_NAME	server's host name, DNS alias, IP address
        SERVER_SOFTWARE	name/version of information server
        SERVER_PROTOCOL	protocol/version of the information protocol this request has
        SERVER_PORT	port # to which the request was sent
        REQUEST_METHOD	method to which the request was made (GET, HEAD, POST, ...
        PATH_INFO	extra path information
        PATH_TRANSLATED	translated version of PATH_INFO (virtual to physical)
        SCRIPT_NAME	virtual path to the script being executed
        QUERY_STRING	query information that follows ?
        REMOTE_HOST	host name making the request
        REMOTE_ADDR	IP address of the remote host
        AUTH_TYPE	protocol specific authentication method
        REMOTE_USER	authenticated user name
        REMOTE_IDENT	remote user name
        CONTENT_TYPE	content type
        CONTENT_LENGTH	content length

        
        
      • Getting Information from the Server
        set QUERY_STRING=mydata+is+here and is passed as string following ? on URL
      • Getting Form Data
        
        1. <FORM ACTION=CGI URL METHOD=GET>
           GET places the data in QUERY_STRING
        2. <FORM ACTION=CGI URL METHOD=POST>
           POST places the data in STDIN with the # of bytes in CONTENT_LENGTH
        Example
      • Returning Information to the Client
        This information must be sent back in proper HTML format wht the first line containing - Content-Type: text/html followed by a blank line and then the HTML code
12. Using CGI in Internet Applications
    • PseudoCode
    • The CGI Program
      • Initialization
      • Determining the Request Method
      • Sending the Form
      • Receiving Information from the User
      • System Processing
      • Sending a Reply to the User

Examples

1. Hello World in C
2. Hello World in Perl
3. Hello World with sh
4. More Sample Programs

Further Information on CGI