1. Form Data Encoding GET – URL encoded POST – URL encoded
POST – multipart form

2. <form action="/foo" method="get">
User ID:
<input type="text" name="userid" size="10" maxlength="8">
Password:
<input type="password" name="passwd" size="10" maxlength="8">
Mail message:
<textarea name="mmesg" rows="5" cols="40"></textarea>
File:
<input type="file" name="image_f">

4. GET – URL encoding <form action="/foo" method="GET"> ...
some_form.html
GET /foo?userid=bkoehler&passwd=foo&
mmesg=bow+%26+arrow%0D%0A%3D%0D%0A%3F%3F%3F&
image_f=C%3A%5CTEMP%5Ccgi.txt HTTP/1.1
Host:
...
HTTP request message

5. POST – URL encoding <form action="/foo" method="POST">
some_form.html
POST /foo HTTP/1.1
Host:
Content-Type: application/x-www-form-urlencoded
Content-Length: 150
userid=bkoehler&passwd=foo&
mmesg=bow+%26+arrow%0D%0A%3D%0D%0A%3F%3F%3F&
image_f=C%3A%5CTEMP%5Ccgi.txt
HTTP request message

6. POST – multipart form <form action="/foo" method="POST"
enctype="multipart/form-data">
some_form.html

7. POST /foo HTTP/1.1
Host:
Content-Type: multipart/form-data; boundary=---123
Content-Length: 2421
---123
Content-Disposition: form-data; name="userid"
brian
Content-Disposition: form-data; name="passwd"
foo
Content-Disposition: form-data; name="mmesg"
bow & arrow =
???

8. ---123
Content-Disposition: form-data; name="image_f";
filename="cgi.txt"
Content-Type: text/plain
The contents of the file
would be here.
HTTP request message

9. The Static Web
Doc A
Request
Origin Server
Client
Doc B
Response
Doc C

10. The Dynamic Web ENV Request body Origin Server CGI Program Client
Response
Response

11. Common Gateway Interface (CGI)
convention for interaction between web servers and external applications that process requests
allows external applications to be reasonably portable across different web servers
external programs can be written in any language:
C, C++, COBOL, FORTRAN, Java, Assembly, csh, sh, Perl, Python, etc

12. CGI/1.1 overview
some information about the HTTP request is passed through environment variables
the HTTP request message body (if any) is connected to the external application's standard input stream
the external application must generate a valid HTTP response on its standard output stream

13. CGI Environment Variables
HTTP_*
most of the headers in the request message get passed as environment variables to the CGI program
e.g.: HTTP_ACCEPT, HTTP_ACCEPT_LANGUAGE, HTTP_USER_AGENT

14. REMOTE_* REMOTE_ADDR REMOTE_HOST
the numeric IP address of the client sending the request
this may not be the user agent if there are proxies along the chain
REMOTE_HOST
the fully qualified domain name of the client sending the request

15. SERVER_*
SERVER_SOFTWARE: the name and version of the web server software
SERVER_NAME: the server's hostname or IP address
SERVER_PORT: the port number the request came in on
SERVER_PROTOCOL: the name and version of the protocol the request came in e.g. HTTP/1.1
GATEWAY_INTERFACE: version of CGI, usually CGI/1.1

16. CONTENT_* only generated for POST requests
CONTENT_TYPE: mime type of the message body; usually:
application/x-www-form-urlencoded or
multipart/form-data
CONTENT_LENGTH: length, in characters of the message body

17. Request REQUEST_METHOD: GET, POST, etc.
SCRIPT_NAME: virtual path to script as derived from the URI (no shcheme, host or query component)
e.g. /cgi-bin/foo
QUERY_STRING: all the text past the '?' in the request URI
e.g. arg1=val1&arg2=val2&arg3=val3...

18. Standard Input
when the external CGI program starts running, its standard input is connected to the request message at the beginning of the message body:
POST /foo HTTP/1.1
Host:
Content-Type: application/x-www-form-urlencoded
Content-Length: 150
userid=bkoehler&passwd=foo&
mmesg=bow+%26+arrow%0D%0A%3D%0D%0A%3F%3F%3F&
image_f=C%3A%5CTEMP%5Ccgi.txt

19. POST /foo HTTP/1.1
Host:
Content-Type: multipart/form-data; boundary=---123
Content-Length: 2421
---123
Content-Disposition: form-data; name="userid"
brian
Content-Disposition: form-data; name="passwd"
foo
Content-Disposition: form-data; name="mmesg"
bow & arrow =
???

20. Standard Output
when the external CGI program starts running, anything it writes to standard output will be part of the response to the client
modes:
non-parsed headers (nph): the CGI program must construct a complete HTTP response message which will be delivered unmodified to the client
parsed headers: the web server will fill in any missing required header fields

21. Parsed Headers Content-Type: text/html <html>
...
<== output of CGI program
HTTP/ OK
Date: Mon, 23 Sep :11:21 GMT
Server: Apache/ (Unix) PHP/4.0.6
Content-Length: 699
Content-Type: text/html
<html>
...

22. Apache mod_cgi two methods of invoking CGI programs:
ScriptAlias directive: all files the specified directory are treated as CGI programs
Options +ExecCGI and AddHandler directives: files with specific file extensions are treated as CGI programs