Comp2513 Forms and CGI Server Applications

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Comp2513Forms and CGI Server Applications

• Daniel L. Silver, Ph.D.

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Objectives

• To discuss HTML Forms and CGI Scripts
• To introduce the concept of server applications
  and discuss their use as a part of an E-Commerce
  infrastructure
• References Ch. 2 Sharma (p.38-41), DDEA
  p.115-124

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Outline

• HTML Forms
• HTTP GET and POST Methods
• CGI ServerApplications
• Drawbacks of CGI
• Forms and Javascript
• Cookies

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HTML Forms

• Forms are part of an HTML document
• ltFORM ACTION someApplicationgt
• .. input elements like text fields, radio
  buttons, etc ..
• .. one or more submit buttons ..
• lt/FORMgt
• Simple form example greet_shell2.html
• User enters data, selects options
• User sends request by clicking on a submit button
• Data is processed by Javascript or sent back to
  client for processing using a CGI script
• The results returned to the browser as HTML

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CGI Common Gateway Interface

• CGI is a standard for HTTP client to server
  application communications that defines
• How a client can request to run an application on
  a server and use specified input data
• How the data is passed to the server application
• How the server application can pass the response
  back to the client
• CGI is NOT a programming langauge

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Forms and CGI Examples

• A barebones CGI request for execution of a sever
  application Hello_time.html
• Passing parameters to a program on a server via
  the CGI protocol greet_shell.html
• Combining forms and CGI greet_shell2.html

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How is User Data Passed to the Server?

• Either GET or POST HTTP method is used
• See the forms tutorial
• The default and the one used in the previous
  example is GET
• Recall the HTTP Request Header
• GET /demo/Hello.html HTTP 1.0
• Accept text/plain
• Accept text/html
• User-Agent Mozilla/2.0
• ltCR/LFgt

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How is User Data Passed to the Server?

• With the GET method, the browser appends a ?
  to the URL followed by the user entered FORM
  data. So you see
• http//eagle.acadiau.ca/demo/cgi-bin/greet_shell.c
  gi?nameDanny
• The server reads the data following the ? and
  makes it available in the form of environment
  variable, QUERY_STRING
• The CGI application on the server must read and
  parse this environment variable

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How is User Data Passed to the Server?

• With the POST method, the browser creates a
  message containing the user entered FORM data.
• The message is sent to the server and forwards it
  on to the requested application in the form of an
  input stream
• The CGI application on the server must read and
  parse the input stream
• An example RequestParamExample.html,
  RequestParamExample.java

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POST versus Get Methods

• Advisable to use POST
• GET is limited to 1024 characters (restricted by
  the environment variable size limits)
• POST provides a first order level of security
• Why?

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Other Data Available at Server

• The server application that reads the FORM data
  can also access other information provided by the
  CGI standard
• REMOTE_ADDR the IP address of the client
• REMOTE_HOST fully qualified URL of host
• CONTENT_LENGTH length of FORM data
• Checkout Request Info and Request Headers
• http//eagle.acadiau.ca8080/examples/servlets/

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CGI Server Applications

• A CGI Script can be any program that can execute
  on the server
• Shell script, Perl script, C, C
• Perl Example perl_greeting.html
• Perl code perl_greeting.cgi
• NOTE to see Perl code open in source view

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Drawbacks of CGI

• Each time a CGI application is requested by an
  HTML page the server is requested to start a
  separate process
• This is true even if it is a Java program
• doThis.cgi !/bin/sh
• java doThis.class
• A new JVM is started each time
• Takes time to set up and take down
• Uses memory resources on the server

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Forms and Javascript

• Javascript was introduced by NetScape
• A client-side language
• Provides program logic embedded in HTML for
  generation of dynamic webpages and minor
  computation
• Manipulation of objects in HTML page including
  the creation and movement of browser windows
• Most importantly allows validation of entered
  FORM data calculator, greet_javascript

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Cookies

• Recall the problem of web sessions being
  connectionless
• TCP/IP is connection oriented but each HTTP
  request/response uses one such connection and
  then terminates
• State is not maintained from page to page
• Each item you order is a separate request
• So how does a E-Comm site know how to accumulate
  orders for you?

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Whats a Cookie

• A Cookie is a small piece of data placed on a
  client system that is used by the server to
  identify the client
• Client, about to make a request to a server,
  checks to see if it has an associated cookie
• If cookie, then send it with the request
• Server checks for cookie in request
• If cookie, then pass it to any applications
  called
• Server may create a new cookie and return it with
  the response to the client
• Client receives response and checks for new
  cookie
• If cookie, then it saves it for this server URL

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Cookies are not programs

• Contain 4K of text or less
• There limits stored by a browser (default 20 per
  site, 300 in total, oldest are deleted)
• Only the originating domain can ever use the
  contents of their cookies
• Written with or without an expiry date
• Turn on your browsers cookie warnings to observe
  how frequent they are used

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Break down of a Cookie

• C\Program Files\Netscape\ Users\defaultuser\cooki
  es.txt
• www.goto.com FALSE / FALSE 1293231196 UserID
  7481BA1DC3F68F71
• First Boolean value (FALSE) indicates whether the
  cookie is available throughout the domain, the
  second denotes whether the cookie data should be
  transmitted only over secure channels
• 1293231196 is the expiry date milliseconds
  since 1970
• UserID is the cookie name
• 7481BA1DC3F68F71 is the cookie data

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Cookies are Useful

• Saving user preferences and profile
• Remembering pages visited and when
• Greeting people by name
• Notifying visitor of changes since last visit
• Retaining data from one page (or frame) to
  another
• Using server side code cookie data can be used
  track user visits and movement patterns

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Cookie Examples

• Javascript (client controlled) example
  Samplecookie1.htm
• Java servlet (server controled) example
  Servercookies.html

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Web References

• http//www.jmarshall.com/easy/cgi/
• http//www.library.uq.edu.au/quik-it/pub_adv.html
  forms
• http//www.nlc-bnc.ca/pubs/netnotes/notes19.htm
• http//hoohoo.ncsa.uiuc.edu/cgi/
• http//www.cgidir.com/
• http//cgi.resourceindex.com/

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THE ENDdanny.silver_at_acadiau.ca