Apache

As we said last day this is the most common web server.
It has orginally had three configuration files, but in modern usage only the httpd.conf file is used.
This file consists of a bunch of lines in a pseudo-tagged based language. # is used for comments
Here is a brief list of common directives:
- ServerName --- names the server system
- ServerRoot --- path to the server root
- DocumentRoot --- path to the document root
- LoadModule --- loads a module (like PHP to be used with the server)
- AddModule --- makes the added module available for use
- AddType --- associate a file name extension with a MIME type.
- Alias -- alias a URL path to a physical directory path
- <Directory> </Directory> --- control how the contents of a particular directory can be served. .htaccess files
Even if you don't have access to the httpd.conf file, if the webserver admin allows it, you might be able to use .htaccess files in the directories you control.
An .htaccess file allows you to do configurations similar to what can be done in <Directory></Directory> tags.

Example

AuthUserFile  "/Volumes/Library/WebDocuments/library/.htpasswd"
AuthGroupFile /dev/null
AuthName library
AuthType Basic

<Limit GET POST>
require valid-user
</Limit>

Quiz

Which of the following statements is true?

A web browser will make a connection to a web server to find out the IP address corresponding to a fully qualified domain name.
The document root directory of a web server always corresponds to the root directory of the server's filesystem.
The HTTP protocol was originally developed by a team that included Tim Berners-Lee.

MIME

MIME stands for Multipurpose Internet Mail Extensions.
It was originally developed as a way to send attachments containing non-text documents over the internet.
Nowadays, it provides a way for a browser to determine the format of the documents it receives from the server.
A MIME specification has the format:
```
type/subtype.
```
The most common types are text, image, video.
Some example complete specifications are: text/plain, text/html, image/gif, image/jpeg, video/mpeg.
In addition to the above, you sometimes see types such as application/xhtml+xml, application/xml, or experimental types which begin with an x-. For example, video/x-msvideo

HTTP

The details of HTTP (hypertext transfer protocol) can be found in the RFC 2616 available at http://www.w3.org
HTTP consists of two phases, the request and the response.

A request has the format:

HTTP method  Domain part of URL  HTTP version
Header fields
Blank line
Message Body

For example, at a Unix prompt trying typing:

For example, at a Unix prompt trying typing:
telnet www.cs.sjsu.edu 80  
GET /index.shtml HTTP/1.0
<blank line>

More HTTP

The most commonly used HTTP requests are:

GET --- request contents of specified document
HEAD --- request header contents of specified document
POST --- Executes the specified document, using the enclosed data
PUT --- replaces the specified document with the  enclosed data
DELETE ---deletes the specified document.

Of these, GET and POST are the most common.

Header Fields

After the first line of the HTTP request one can have any number of header fields.
There are four kinds of headers: general, those used in the request, those used in the response, and entity headers which are used in both.
Two common headers are:
- Accept: says what MIME types the browser can handle
- Host: (required in HTTP/1.1) Says the name of the host that you are trying to contact. Host is useful because www.widgets.com, www.sprockets.com might be hosted on the same machine (virtual hosting). This says which one you are trying to contact.

As an example:

GET /respond.html HTTP/1.1
Host: blanca.uccs.edu
Accept: text/*
Accept: image/gif

Response Phase

An HTTP response has the following format:

Status line
Response header fields
Blank line
Response body

An example status line might be:
```
HTTP/1.1   200   OK
```
In general, one will have a 3-digit status code and a message. The codes come in one of five categories: 1 - information, 2 success, 3 - redirection, 4 - client error, 5 - server error.

An example response header

HTTP/1.1 200 OK
Date: Mon, 28 Aug 2006 19:09:45 GMT
Server: Apache/2.2.2 (Fedora)
Accept-Ranges: bytes
Connection: close
Content-Type: text/html
In the above, Date:, Server:, Accept-Ranges:, etc are all response fields.

XHTML

HTML was original specified in the Standard Generalized Markup Language (SGML) which is an ISO standard for specifying text-formatting languages.
HTML was designed to specify content rather than how it was formatted (presentation).
The reason was to allow it to be displayed in as many different kinds of computers and browsers as possible.
As a specification language for simple tag based languages, SGML is severe overkill, so a stripped-down specification language called XML was developed in the late 90's.
XHTML is HTML specified using XML.

HTML Variants

The most common variant of HTML today are:

ISO HTML (HTML 4.01) which is specified using SGML a
XHTML 1.0 Transitional
XHTML 1.0 STRICT
XHTML 1.1

XHTML 2.0 has been under development for several years, but looks like it is being abandoned.
HTML 5 is also currently under development and looks like it might be used.

Basic Syntax

The fundamental structural unit of HTML is a pair of tags.
For example, a paragraph might be specified with: <p>The quick brown fox..</p>
Here <p> is called the open tag and </p> is called the close tag. The quick brown fox is the content of the tag.
Unlike earlier variants of HTML, all tags names in XHTML are lower case.
To be a legal document, every open tag must be closed. Further, the nesting must be legal.

For example,

<img src="my_picture.png" ></img> 
   <!--this is okay. BTW, this is an example HTML comment -->
<img src="my_picture.png" /> 
   <!--this is also okay and is an abbreviation for the line above -->
<img src="my_picture.png" >  <!--not okay by itself -->
<P>old style paragraph</P> <!--not okay -->
<p><i>Hello</i></p> <!--okay -->
<p><i>Hello</p></i> <!--not okay -->

Open tags may have attributes. For example, src in the image tag above. The value of an attribute must be given in double quotes.

HTTP and XHTML

Outline