Internet

1
Internet Communications

• Server-side programming for
• the WWW (lecture 6)
• CGI Programming
• Using Shell Scripts for CGI

2
Server-side programming

• May be useful to send data back to server, and
  run a program there.
• Program might store user details in database or
  file, or do some calculation and return result to
  user.
• A number of technologies available Servlets,
  JSP, CGI, ASP, PHP
• Well look briefly at CGI, Servlets and JSP.

3
Common Gateway Interface (CGI)

• CGI is a very general standard specifying how
  programs can be run on server, from the WWW.
• Any program in any language can be a CGI program
  - it just has to follow the CGI rules..
• These define how programs get data (e.g., HTML
  form data) and how to make sure web server knows
  its a CGI program.

4
CGI

• A CGI program is stored on the server, executed
  on the server, in response to request from
  client.
• By running a CGI program, rather than delivering
  a static HTML page we can
• Put up to date information on web page (e.g.,
  weather forecast product availability).
• Respond appropriately to user input.
• Store user data from form in a file or DB.

5
Call of a CGI Program

• You can call CGI programs in just the same way
  that you get HTML pages.
• A link such as following will run CGI program
  when clicked
• Or you can open that location directly (file
  open)

lta hrefhttp//www.mysite/cgi-bin/myproggt Run
my CGI program lt/agt
6
Security

• If you write a CGI program you are letting anyone
  in the world run a program on your system.
• Malicious users may be able to exploit security
  loopholes, and cause problems on your system.
• Because of this many Web site hosts will not yet
  ordinary users create CGI programs.
• Where use is permitted special wrapper programs
  may be required that do some security checks.

7
How does it know its CGI?

• How does the web server know whether its dealing
  with an HTML page, or a CGI program?
• The Web server will be set up so that it is clear
  which files are to be treated as CGI.
• Common to have to put CGI programs in a cgi-bin
  directory.
• Access may be restricted so ordinary users cant
  access this directory.

8
CGI in MACS

• Users can make their own cgi-bin directories
  (under www directory).
• You can then place CGI programs in this directory
  and try them out.
• But to prevent security problems, a wrapper
  program is used.
• You therefore use the following URL to access
  your CGI programs

Http//www.macs.hw.ac.uk/cgi-bin/cgiwrap/YOURLOGIN
/YOURPROG
9
CGI Examples

• Any programming language can be used for CGI.
• But simplest to illustrate CGI using shell
  scripts - scripts containing Unix commands.
• Every CGI program must write out data to send
  back to web browser.
• The first thing they must write out is MIME type
  of file (e.g., text/plain, text/html)

!/bin/sh echo Content-type text/plain echo ech
o Hello World
Run it
Download it
10
Shell Scripts

• You can try out shell scripts independent of CGI
  programming in Unix environment.
• Create a file containing the following
• Make it executable.
• Run it simply by typing its name at the Unix
  prompt.
• Try out all CGI scripts this way before trying
  them across web.

!/bin/sh echo "Hello"
11
CGI Activity

• Create yourself a cgi-bin directory
• Make sure that it is readable/executable by
  others.
• Create script similar to one on last slide.
• Make it executable by others.
• Run it from a Unix prompt to test.
• Run it from web browser by opening appropriate
  location
• (http//www.macs.hw.ac.uk/cgi-bin/cgiwrap/YOURLOGI
  N/YOURFILE)
• Now make the above URL a link from one of your
  HTML files. Try out the link.

(ie, lta href" http//www.macs.hw.ac.uk/cgi-bin/cg
iwrap/YOURLOGIN/YOURFILE "gt link lt/agt
12
CGI Example 2

• You can add Unix commands to your CGI to get,
  say, a listing of your files, or the date!

!/bin/sh echo Content-type text/plain echo ech
o The date is date echo The files in my
cgi-bin are currently ls
Run it
Download it
13
CGI Example 3

• You can also output HTML (usual practice)

Note content type now text/html
!/bin/sh echo Content-type text/html echo echo
lthtmlgt ltheadgt lt/headgt echo ltbodygt lth1gt My
files lt/h1gt ls echo lt/bodygt lt/htmlgt
Second "echo" line is vital
Download it
Run it
Observe how IE tries to interpret any nonsense as
HTML
14
CGI Input Data

• You can pass data to a CGI program - encoded as
  string looking likevar1val1var2val2var3val3
  ..
• You add this to the URL, after a ?
• Example
• This data is then available through the
  environment variable QUERY_STRING

cgiwrap/alison/test?namealisonaddrEdinburgh
15
CGI Example 4

• Example writing back query string to the browser

!/bin/sh echo Content-type text/plain echo echo
The data passed to this program is echo
QUERY_STRING
Download it
Run it
16
CGI and Forms

• You can use CGI programs to process data from
  forms.
• Specify the CGI program using the action
  attribute of the form
• If methodget then the form data gets put in
  QUERY_STRING as before.

ltform methodget actionhttp//./cgi-bin/cgi
wrap/alison/test.cgigt
17
Example
ltform method"get" action"http//www.macs.hw.
ac.uk/cgi-bin/cgiwrap/alison/inetcomms/example4.cg
i"gt ltpgt Name ltinput type"text" name"username"
/gt lt/pgt ltpgt Age ltinput type"text" name"age" /gt
lt/pgt ltpgt ltinput type"submit" value"Do it" /gt
lt/pgt lt/formgt
Try it

• Exercise Try one of your own forms with the
  above action. Then try writing your own script to
  handle the data.

18
GET vs POST

• Above examples use the "get" method to handle the
  data from the form.
• Using get method
• Data added to URL as ..prog?varval etc.
• This data is put in QUERY_STRING variable
  available to CGI programs
• Alternative is to use post method
• Data is sent separately to URL.
• CGI program reads this data from its standard
  input.

19
Get vs POST

• Get method isnt very secure - all data visible
  in URL.
• Get method suitable for small amounts of data,
  but not for larger amounts.
• But well just use GET..

20
CGI Example 4

• We can use all this to do a very basic version of
  program to store data from form in file
• Stores data from form in "MyFile"

!/bin/sh echo Content-type text/plain echo ech
o QUERY_STRING gtgt MyFile echo Done it
Download
Try it
21
CGI Summary

• CGI is standard for interfacing web client to
  programs run on server.
• Specifies location of files (so server knows to
  execute them!) and how input data is handled.
• Output of CGI must specify MIME type (e.g,
  text/plain, text/html)
• Rest of output displayed on web page
  appropriately.
• Simple examples using shell script, but need more
  serious language for complex ones.