Overview• A plain HTML document is static• A CGI program is executed in real-time, so

that it can output dynamic information.• CGI (Common Gateway Interface) is the

language or protocol that the browser uses to communicate the data from the form to the web server.

• A standard for interfacing external applications with information servers, such as HTTP or Web servers.

• When the user submits his/her answers on a form, the browser bundles them up and sends them to the web server, which passes them on to your CGI script/program for processing. – The web page itself does not process the dat

a entered on the form. Neither does the web server.

• A CGI script/program is any program which knows how to read that bundle of data.– The script/program must build up and return t

he html source for a web page– Shell and Perl scripts are easier and are mor

e commonly used for CGI scripts.

CGI specification

• Define how these data are passed from the server to the gateway program, and vice versa

Browser

CGIPorgram

Network

CGI

HTTP server

Server Machine

CGI relativescript

stdout

stdincmd lineenv var

Workstation IBM Compatible

Script

Web server

CGI program

Client

Form

Relative Programs and Files

<Form Action=/cgi-bin/php.cgi/~jlwang/php/test.html>. . .</Form>

CGI program: /cgi-bin/php.cgi

Script program: /~jlwang/php/test.html

<Form Action=/cgi-bin/pw.pl>. . .</Form>

CGI program: /cgi-bin/pw.pl

Communication with gateway programs

• Client to Server– URL query string– Extra path information in the URL– Message body (in HTML Forms)

• Server to Gateway program– Command-line argument– Standard input– Environment variables

• Gateway program to Server– Standard output

Client to Server• URL query string

– http://www.mcu.edu.tw/cgi-bin/ex_prog?query_info– Pass the query string query_info to the server– The server launches the gateway program ex_prog and

passes it the query string• Extra path information in the URL

– http://www.mcu.edu.tw/cgi-bin/ex_prog/dir/file?query_info– /dir/file: extra path information– Pass information using environment variables

• Message body– HTTP POST method within HTML FORM

– Pass information using standard I/O

Server to Gateway program• Command-line argument

– GET method within ISINDEX query

• Standard input– POST method

• Environment variables– Everything sent by the client, except POST

data– The query string – The extra path information – The request header

Gateway program to Server

• Standard output– The only way that gateway programs can

pass data to a client– Two parts

• A collection of server directives– Be used to compose the response header

• The data

CGI Environment Variables

• Server CGI

• About the Server

• Request-Specific Variables

• About the user

Server CGI

• The server uses command line arguments as well as environment variables.

• These environment variables are set when the server executes the gateway program.

About the Server• SERVER_SOFTWARE

– The name and version of the information server software answering the request (and running the gateway). Format: name/version

• SERVER_NAME – The server's hostname, DNS alias, or IP

address as it would appear in self-referencing URLs.

• GATEWAY_INTERFACE– The revision of the CGI specification to which

this server complies. Format: CGI/revision

Request-Specific Variables• SERVER_PROTOCOL

– The name and revision of the information protcol this request came in with.

– Format: protocol/revision (HTTP:1.0)• SERVER_PORT

– The port number to which the request was sent. • REQUEST_METHOD

– The method with which the request was made. For HTTP, this is "GET", "HEAD", "POST", etc.

• PATH_INFO – The extra path information, as given by the client.

• PATH_INFO – The extra path information, as given by the client.

• PATH_TRANSLATED – The server provides a translated version of PATH_IN

FO, which takes the path and does any virtual-to-physical mapping to it.

– http://www.mcu.edu.tw/cgi/cgiprogram.cgi/x/y/z– (Assume the document root path:/home1/www/docs)

• PATH_INFO = /a/b/c• PATH_TRANSLATED = /home1/www/docs/x/y/

z• SCRIPT_NAME

– A virtual path to the script being executed, used for self-referencing URLs.

• QUERY_STRING – The information which follows the ? in the URL whi

ch referenced this script. – http://www.mcu.edu.tw/cgi/test.cgi?mcu.edu.tw– QUERY_STRING= mcu.edu.tw

• AUTH_TYPE

– The protocol-specific authentication method used to validate the user.

• REMOTE_USER – If the server supports user authentication, and the

script is protected, this is the username they have authenticated as.

• REMOTE_IDENT – Remote user name retrieved from the server. Usage

of this variable should be limited to logging only. • CONTENT_TYPE

– For queries which have attached information, such as HTTP POST and PUT, this is the content type of the data.

• CONTENT_LENGTH

– The length of the said content as given by the client.

About the user• HTTP_ACCEPT

– The MIME types which the client will accept, as given by HTTP headers.

– Format: type/subtype, type/subtype • HTTP_USER_AGENT

– The browser the client is using to send the request.– Format: software/version library/version.

• REMOTE_HOST– Domain name of the user

• REMOTE_ADDR– IP address of the user

CGI: Output• Standard output

– Document generated by the script– Instructions to the server

• Scripts return – Output a document to their standard output– Output the location of the result document

(either a full URL or a local virtual path)

• The server normally adds the needed HTTP headers to the back-end's results

• Data stream– Header

• One or more lines of text, and is separated from the body by a blank line

– Body• Contains MIME-conforming data whose content

type must be reflected in the header• The server does not interpret or modify the

body in any way

CGI Output Example

print <<EOT;

Content-type: text/html

<HTML> <HEAD>

<TITLE> Search Results</TITLE>

</HEAD>

<BODY> <H1> Search Results </H1>

EOT

Examples

CGI and Database

Browser Database server

GatewayPorgram

DatabaseInterface

Network CGI

HTTP server

Server Machine

Gateway Program

Web BrowserHTML Form

HTTP Server

Common Gateway Interface

Access List

Information Entered by the User

User Authentication Information Entered by the User

GatewayProgram

Database Interface

WebBrowser

HTTP Server

Common Gateway Interface

Error Message

GatewayProgram

ProcessesInformation

Relays ErrorMessage

Database Query

Error Message

DatabaseInterface

ParserDiscoveries

Syntax Error

Interface Error Message

Gateway program transfersthe received data into the database query, which then is passed to database interface.

Database interface parses the received database query

Database Server

Web Browser

Common Gateway Interface

HTTP Server

Formatted Results

GatewayProgram

ProcessesInformation

FormatsResults

Database Query

Query Results

DatabaseInterface

VerifiesQuery

QueryResults

Database Query

Query Results

DatabaseServer

PorcessesQuery

Results From Database

Database interface passes the query to the database server

After handling the query,the database server feed the query result back to the database interface

Local Database Server

PC

Database

Server machineBrowser Network

gateway

user request

output

stringkey tosearch

matcheddata

matcheddata

CGI

Web server

The database server and the Web server are in the same site

Remote Database Server (homogeneous)

Web Browser

Database Server

Database Server

Database Server

GatewayDatabaseInterface

NetworkNetwork

Network

Network

Web server Machine

¹Ï1-6

HTTP server

CGI

Remote Database Server ( heterogeneous )

PC

gatewaydatabaseinterface

DatabasemSQL

Databasesybase

Databaseoracle

Ethe

rnet

user request

output

string

key tosearch key to

search

matcheddata

matcheddata

matcheddata

Browser Network Server machine NetworkDatabasemachine

CGI

Web server

Third party model

PC

gateway

stringkey tosearch

matcheddata

matcheddata

output

user request

matcheddata

key tosearch

Server MachineNetworkBrowser Database MachineNetwork

CGI

Web server 3rd-PartyNetwork Software

Server

DatabaseServer

Database independent