002_LectureSlides_Jan27.pdf

COMP3170 Web Based Applications

Dr. Curtis GittensLecture II

Learning Objectives

By the end of this lecture you will be able to:Describe the process Apache uses to retrieve files

when a user makes a request

Describe the two main directives involved in file retrieval

Describe how the UserDir directive works to make a users directory Web accessible

Differentiate between the DocumentRoot and DirectoryIndex directives

Differentiate between the different methods for accessing files that are outside the document root

Differentiate absolute from relative paths 2

Quick Overview of How Apache Accesses Web Pages

a brief discussion on how apache serves up web pages, determines web directories and use absolute and relative paths in web documents

3

How Apache Serves Up Web Pages

The default situation is when a user visits a website using a URI that asks for a resource e.g.

Apache uses httpd.confto determine how to respond to all requests

A file request only involves theDocumentRootdirective. A directory request involves the DocumentRoot and DirectoryIndexdirectives.

4

Server passes default file back to user

Server receives request, checks its configuration file httpd.conf for the

document root

Fetches the file

Adds the URL-Path to the end of the DocumentRootindicated in the httpd.conffile

Apache server listens on port 80 for request

User sends a plain IP address URL

Looks at the DirectoryIndexdirective for a default file to display if a directory is requested

2

1

3

4

5

6

7

OR

http://www.fb.com/login.html

The Directives

5

The DocumentRoot Directive

Sets the directory from which Apache serves files for requests

Default value: /usr/local/apache/htdocs

Course server values: /var/www

The path specified in the URL received is appended to the document root

This makes the actual path that is used to retrieve the file

Example6

Document Root:- /var/wwwURL:- http://www.mydomain.com/myfile.htmlDirectory/file referred to: /var/www/myfile.html

The DirectoryIndex Directive

Sets the list of resources (files) to look for when the client requests the directory by specifying a / at the end of the URL

Note: When a trailing / is not specified, Apache rewrites the URL to add the trailing slash and then view it as a directory index request

7

The DirectoryIndex Directive

The resources (files, videos, etc.) are searched for in the order specified

If none found and Indexes is set, it returns a listing of the directory requested

ExampleDirectoryIndex index.html index.php index.txt /cgi-

bin/index.pl

8

Clear as Mud?

Write down two things from the lecture so far that are the most unclear to you. If there is nothing, then say that as well.

Be prepared to share!

In Your Own Words

1. Explain the difference between the DocumentRoot and the DirectoryIndex directive2. Write down what you can remember about how Apache retrieves files based on a

users request

Web Access for Files Outside the Document Root

Options for making files outside the Web file system accessible to the web

11

Accessing Outside Files

Create a symbolic linkSimply make a symbolic link in the document root

of the files you want accessed by the web server Example given /var/www as document root

ln s /dir/out/side/doc/root/foo.html /var/www/foo.html

Using the Alias directive to map any part of the file system into the web space. Example: Alias /mywebspace /dir/out/side/doc/root/

12

Accessing Outside Files

ScriptAliasHas the same behaviour as Alias, but marks the

directory as containing CGI scripts to be used by the CGI script handler

13

User Directories for the Web

14

User Directories for Web Pages

In *NIX systems, user directories can be accessed using ~username

The module userdir extends this to the web

Files in a users directory can be accessed via the URL http://www.somedomain.com/~user/foo.html

Direct access is not given to the users directory

15

User Directories for Web Pages

The UserDir directive is used to specify a subdirectory in the users directory that is web accessibleDefault setting is public_html

If the users home directory is /home/jsmith, then Apache would retrieve the file: /home/jsmith/public_html/foo.html

16

User Directories for Web Pages

17

User Directories for Web Pages

18

User Directories for Web Pages

19

1

2

1. Go to the user, dgittens subdirectory and view its contents where we see the public_html subdirectory

2. View the contents of the public_html directory

User Directories for Web Pages

20

Enter the URL in the browser for ~dgittens/ to view the index.html document over the Web

Accessing the FilesystemUsing Relative and Absolute Paths

21

Relative and Absolute Paths

An absolute path in *NIX systems starts with /Example: /etc/apache2, /var/www/html etc

A relative path starts either with dot ., dot-dot, .. or a directory name without a preceding /

Examples: If the current directory is /home/usr/here, then where are these files located? ./foo.txt, ../dir/other.txt, dir/another.txt

22

Relative and Absolute Paths

Apache views paths based on the document root

These examples assume document root is /var/www looks for var/www/index.html

looks for /var/www/index.html

23

Summary

In this lecture we:Described how Apache retrieve files when a user

makes a request

Described the two main directives involved in file retrieval

Described how the UserDir directive works to make a users directory Web accessible

Discussed how the DocumentRoot and DirectoryIndex directives differ

Examined the different methods for accessing files that are outside the document root

Discuss how absolute and relative paths differ 24