Common Gateway Interface (CGI)

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Common Gateway Interface (CGI)

• Joe Komar

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Common Gateway Interface (CGI)

• HTTP typically requests HTML pages
• Link (URL) can identify a non-HTML page
• WWW server invokes the resource named in the URL
  (a program, batch file, etc.) and lets it do its
  thing
• Results of the CGI program are returned to the
  server in the form of HTML
• Server then passes the HTML to the client

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HyperText Transfer Protocol (HTTP)

• Provides stateless connection
• Client establishes connection
• Client issues a request
• Sever processes the request and returns a
  response
• Server closes the connection
• Webs Remote Procedure Call (RPC) on top of TCP/IP

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HTTP Request

• Request Line -- method (GET), relative path to
  resource (/temp/index.html), protocol version
  (HTTP/1.0)
• Request header fields -- additional parameters in
  the format name, colon (), value
• Entity body -- sometimes used to pass bulk
  information to the server

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HTTP Request Example
GET /path/file.html HTTP/1.0 (header
line) Accept text/html
(request header fields) Accept audio/x
User-agent MacWeb

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HTTP Response

• Response header line -- HTTP version, status of
  the response, explanation of status
• Response header fields -- server attributes and
  document returned metadata
• Entity body -- typically an HTML document

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HTTP Response Example
HTTP/1.0 200 OK (response
header) Server NCSA/1.3 (response
header fields) Mime_Version 1.0
Content_type text/html
Content_length 2000
ltHTMLgt
(entity body) .
lt/HTMLgt

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Common Gateway Interface

• HTML FORMs allow user input
• Input is packaged up in a known format and send
  as an HTTP request (POST or GET) and sent on
  through a CGI protocol
• The program or resource named is given the data
  and left to do with it what it will
• Results are returned in the form of an HTML
  document that the server passes on to the client
• Three-tier client/server model

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Using FORMs and CGI

• FORMs submit button is pushed
• Browser collects data and forms one long string
  of name/value pairs separated by an ampersand ()
  with plus signs () for spaces
• Browser forms a standard HTTP request with the
  long string as the entity body
• Server receives request via a socket and starts a
  CGI interaction

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Using FORMs and CGI continued

• Server sets up environment variables
• shared bulletin board with cgi program
• server_name, request_method, path_info,
  script_name, content_type, content_length, etc.
• Server starts the CGI program
• CGI program reads environment variables
• CGI program receives message body (long string)
  via standard input and parses

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Using FORMs and CGI continued

• CGI program does its thing and formats results in
  HTML
• CGI program returns results via standard output
• Server returns the results to the Browser
• ALL DONE !!
• Since the connection is stateless hidden fields
  are used in sequences of forms

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Example FORM
ltHTMLgt ltBODYgt ltH1gtSending POST Data to a CGI
Programlt/H1gt ltFORM ACTION
"http//140.209.124.107/cgi-bin/CgiPost.bat"
METHOD"POST"gt Enter some data ltINPUT
TYPE"TEXT" NAME"TextField1"gtltBRgt Enter some
more ltINPUT TYPE"TEXT" NAME"TextField2"gtltBRgt
ltINPUT TYPE"SUBMIT" VALUE"Send
It"gt lt/FORMgt lt/BODYgt lt/HTMLgt Link
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Count CGI Version (Client Applet)
import java.awt. import java.applet. import
java.io. import java.net. import
java.util. public class CountCGIClientApplet
extends Applet
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Count CGI Version (Client Applet)
try String sum "0" showStatus("Incrementing
") int count new Integer(textField1.getText()
).intValue() long startTime
System.currentTimeMillis() for (int i 0 i
lt count i ) sum increment(sum)
long stopTime System.currentTimeMillis()
list1.addItem("Avg Ping " ((stopTime -
startTime) / (float)count) " msecs")
showStatus("Sum " sum) catch(Exception
e) showStatus("System Exception" e)
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Count CGI Version (Client Applet)
public String increment(String currentSum)
String script "/cgi-bin/Count.bat" Socket
socket null String rdata "" String line
"" String lastLine ""
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Count CGI Version (Client Applet)
try socket new Socket(getCodeBase().getHost(
), 80) PrintWriter ostream new
PrintWriter(socket.getOutputStream())
BufferedReader istream new BufferedReader(
new InputStreamReader(socket.getInputStream())
) ostream.print("POST " script "
HTTP/1.0\r\n" "Content-type
text/plain\r\n" "Content-length "
("increment " currentSum).length()
"\r\n\r\n") ostream.print("increment "
currentSum "\r\n") ostream.flush()
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Count CGI Version (Client Applet)
try while ((line istream.readLine())
! null) list1.addItem(line)
lastLine line catch (
SocketException e ) istream.close()
ostream.close()
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Count CGI Version (Server)
import java.io. import java.util. class
CountCGIServer public static void main(String
args) int count String line try
// create streams PrintWriter ostream
new PrintWriter(System.out) BufferedReader
istream new BufferedReader(
new InputStreamReader(System.in))
line istream.readLine()
StringTokenizer tokens new StringTokenizer(line)
String myOperation tokens.nextToken()
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Count CGI Version (Server)
if(myOperation.equals("increment"))
String countString tokens.nextToken()
count Integer.parseInt(countString)
count ostream.println(
Integer.toString(count) )
ostream.flush()
istream.close() ostream.close()
catch (Exception e) System.out.println("Erro
r " e)
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Count CGI Html and Batch Files
lthtmlgt ltbodygt lth1gtCount CGI Client
Appletlt/h1gt lthrgt ltcentergt ltapplet
codeCountCGIClientApplet.class width500
height500gt lt/appletgt lt/centergt lt/bodygt lt/htmlgt
d\jdk1.1.7a\bin\java CountCGIServer
Do it!
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Java and CGI Helpful Link
Marty Halls CGI page
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CGI Assignment

• Stay tuned.