The modules that provide basic content delivery features are compiled into Stronghold by default. This chapter shows you how to

• use server-side imagemaps
  
  
• perform content negotiation
  
  
• use server-side includes
  
  

Imagemaps are files that define coordinates on an image as links. A single imagemap can act as a link to several different destinations, depending on which part of the image a user clicks. While most browsers now support client-side imagemaps, server-side imagemaps work with all browsers that support images. Stronghold has an imagemap module that processes server-side imagemaps independent of the CGI module.

1. Add or uncomment this line to your httpd.conf file:
   
   

   AddHandler imap-file map
   
   

2. Restart the server:
   
   

   # ServerRoot/bin/reload-server
   
   

   Stronghold now treats all files ending in .map as imagemap files.

With the imagemap handler in place, you can place an image in an HTML files and then create an imagemap file that defines the shapes, coordinates, and destinations of its links. When you link the HTML image tag to the imagemap file, the image becomes an imagemap.

A linked region can take one of four shapes: polygon, circle, rectangle, or point. Your imagemap file must define these parameters:

• the shapes of the linked regions
  
  
• the pixel coordinates (x,y) of the linked regions
  
  
• the (absolute or relative) URLs they are linked to
  
  
• the URL on which relative URLs are based
  
  
• the action to take if a user clicks an unmapped region
  
  

An imagemap file consists of a list of directives that define these parameters. There are six imagemap directives:

Directive	Description
base	The value of base is a URL, and any relative URLs that appear in the imagemap file are interpreted relative to this one. If base is not specified, the default is the root document for this domain. Be sure to include a trailing slash if necessary.
default	This directive specifies the action required if a user clicks an unmapped region of an image. Unless the ImapDefault directive specifies otherwise, the default value is "nocontent," meaning that nothing happens when an unmapped region is clicked.
poly	All shape directives take a value followed by a list of coordinates. A polygon can have from 3 to 100 points in its coordinate list.
circle	Two coordinates are required for a circle: the center point and a point on the circle itself.
rect	Two coordinates are required for a rectangle: the upper left corner point and the lower right corner point.
point	The point directive takes a single coordinate.

For any directive, there are six possible values:

Value	Description
URL	A relative or absolute URL that specifies the destination of a link.
map	Unless the ImapValue directive is set to "none," map causes Stronghold to generate an HTML index of the shapes and links contained in the imagemap file, much like a directory index.
menu	Like map, this value causes Stronghold to create an HTML index of the shapes and links in the imagemap file.
referer	The referer value creates a link to the page indicated in the Referer header of the last client request.
nocontent	Sends the 204 (No Content) status code, which causes the client to take no action and keep displaying the current page.
error	Sends the 500 (Internal Server Error) status code. This value is used only with the default directive.

For example, imagine that you want to use this image as a navigation bar:

Using an image-editing application such as Photoshop, you must first determine the shapes and pixel coordinates of the linked regions on the image:

The imagemap file for this image would look something like this:

base http://www.host.com/
default map
poly previous/ 14,36 66,16 66,60
rect products/ 80,18 196,46
circle help/ 245,40 270,60
rect services/ 293,18 410,46
poly next/ 425,16 425,60 478,38

In this example, the URLs specified in the shape definitions are all relative to http://www.host.com. If a user clicks an unmapped region of the navigation bar, the server returns a linked list of the imagemap's destinations. The linked shapes are defined by x and y pixel coordinates. Polygons require the coordinates of each of their corners, while rectangles only require those of two opposing corners and circles require those of a radius.

With the imagemap file in place, you must link the HTML image reference to it, like this:

<a href=/maps/navigation.map><img src=navbar.gif ISMAP></a>

Test the imagemap by viewing it in your browser and moving the mouse pointer over it. As the pointer passes over a linked region, the destination of the link appears in the browser's status bar. Click an unmapped region to see the linked list defined in the imagemap file. Optionally, you can add quoted description strings to the end of each region definition to make the linked list more descriptive:

rect products/ 80,18 196,46 "BetaCom Products"

The World Wide Web is populated by an enormous variety of users speaking many different languages. To accommodate only the largest segment of users, you can construct a site in English or the language most widely used by your audience, but this alienates other users. To compensate, you can use content negotiation to cater to many varieties of users.

Content negotiation involves an exchange of information between client and server. Under HTTP/1.1, a client's initial request includes headers that specify which file types the client can handle:

GET /home/ HTTP/1.1
Connection: Keep-Alive
User-Agent: Mozilla/3.0 (WinNT; I)
Host: www.host.com
Accept: text/html, text/plain, image/gif, image/jpeg, image/pjpeg ...
Accept-Encoding: compress, gzip, winzip
Accept-Language en; q=1.0, de; q=.8, fr; q=.6,

The request includes a list of acceptable media type in the Accept field, acceptable encoding types in the Accept-Encoding field, and acceptable languages in the Accept-Language field. The numbers in the Accept-Language field are q-values, and they rank the language types according to the user's preference. In this scenario, English is the preferred language, but German and French are also acceptable.

If the root document for the requested directory is only available in a language and format listed in the request, the server simply returns the document. However, imagine that the only root document for /home/ happens to be index.pdf.jp, a Portable Document Format (PDF) file in Japanese. Since the server knows from the request that the client cannot handle this file, it responds like this:

HTTP/1.1 406 Not Acceptable

Most requests can be partially fulfilled. For example, imagine that the only root document for /home/ is index.html.en, and that the file references a QuickTime file. All browsers support HTML, but not all have QuickTime plugins. Even if video/quicktime is not among the values in the Accept header, the server returns the HTML file and the client displays a broken link where the animation would have appeared.

Content negotiation provides a mechanism for preventing 406 status codes and broken links by selecting from among several versions of the same file based on the criteria provided in the client request. Although this requires maintaining and organizing parallel file versions, the result is a clean, universally accessible site.

Server-side includes (SSIs) are simple, HTML-embedded directives that instruct Stronghold to execute a program or include data in the HTML document. As a shorthand alternative to CGI, server-side includes can be handy. However, like CGI, they should be used with caution and an eye for security risks.

Since server-side includes are directives for the server, Stronghold must parse them before it delivers the document that contains them. Scanning every requested file for server-side includes can create considerable processor overhead. Instead, you can instruct Stronghold to look for server-side includes only in certain files by including this line in your httpd.conf file:

AddType text/x-server-parsed-html .shtml

When you start Stronghold with this configuration, it only looks for SSI directives when a client requests a file with the .shtml extension. When it finds them, it parses the directives, carries out their instructions, replaces them with their output, and sends the finished document to the client.

All SSI directives follow this format:

<!--#directive parameter="value"-->

Directives can have more than one parameter, and multiple parameters are separated by white space. The valid SSI directives are as follows:

• Syntax: echo var="environment-variable"
  
• Context: HTML
  

  Echo inserts the value of one or more SSI environment variables.

• Syntax: include file|virtual="path"
  
• Context: HTML
  

  The include directive inserts the text of another document, specified either as a file or a virtual file. Path must be relative to the current document in the case of a file, or it can be a virtual path. For example:

  <!--#include file="sample.html"-->
  <!--#include virtual="/virtual/sample.html"-->
  

• Syntax: fsize file="path"
  
• Context: HTML
  

  The fsize directive inserts the size of the file specified by path, in bytes. For example:

  This file is <!--#fsize file="example.ps"--> bytes.
  

• Syntax: flastmod file="path"
  
• Context: HTML
  

  Similar to fsize, flastmod inserts the modification date for the file specified by path. For example:

  This file was last modified on <!--#flastmod file="example.html"-->.
  

• Syntax: exec cmd|cgi="string"
  
• Context: HTML
  

  The exec directive executes a program, either a command-line string or a CGI program, and inserts the output into the current document. For example:

  Finger C2Net's president:
  <!--#exec cmd="/bin/finger sameer@c2.net"-->
  
  This page has been accessed
  <!--#exec cgi="/cgi-bin/counter.pl"--> times.
  
  

  NOTE:

  The exec directive poses a considerable security risk, since it allows users to execute any program on the server platform. As a precaution, we recommend that you set the Options configuration directive to "IncludesNOEXEC."

  
  

• Syntax: config errmsg|sizefmt|timefmt="string"
  
• Context: HTML
  

  You can use the config directive to modify the default SSI behavior. The parameters are as follows:

  • errmsg: Sets the default error message. For example:
    
    <!--#config errmsg="Error: Try again."-->
    
    
  • sizefmt: Sets the format for file size data returned by the fsize directive. The string for this parameter is either "bytes" for file sizes in bytes, or "abbrev" for file sizes rounded to the nearest kilobyte. For example:
    
    <!--#config sizefmt="abbrev"-->
    
    
  • timefmt: Sets the format for dates and times returned by the flastmod directive and the DATE_LOCAL and DATE_GMT variables. The possible values for this parameter are as follows:

    Format Code	Description
    %a	Abbreviated day of the week, such as "Sun" for Sunday
    %A	Unabbreviated day of the week
    %b	Abbreviated month, such as "Jan" for January
    %B	Unabbreviated month
    %d	Two-digit, numerical day of the month, such as "01"
    %D	Numerical month, day, and year in four-digit format, such as "01/24/1997"
    %e	Numerical day, such as "1"
    %H	24-hour clock hour, such as "17"
    %I	12-hour clock hour, such as "11"
    %j	Numerical day of the year, such as "278"
    %m	Numerical month, such as "11"
    %M	Minutes, such as "08"
    %p	am or pm
    %r	Time, such as "08:23:17 am"
    %S	Seconds, such as "56"
    %T	24-hour time, such as "23:54:56"
    %U	Week of the year, such as "47"
    %w	Numerical day of the week, such as "0" for Sunday and "7" for Saturday
    %y	Year of the current century, such as "97"
    %z	Abbreviated time zone, such as "PST"

    
    
    

• Syntax: printenv
  
• Context: HTML
  

  This directive prints a complete list of all existing SSI variables and their values. It has no attributes. For example:

  <!--#printenv-->
  
  

• Syntax: set var="variable" value="value"
  
• Context: HTML
  

  This directive sets the value of an environment variable. For example:

  <!--#set var="country" value="Liberia"-->
  
  

Stronghold implements Extended SSI, which includes flow control elements much like the ones used in programming languages. With flow control, you can create more sophisticated SSI scripts.

There are four flow control elements:

<!--#if expr="test-condition"-->
<!--#elif expr="test-condition"-->
<!--#else-->
<!--#endif-->

The test-condition is one of the following:

Test Condition	Description
string	True if the string is not empty
string1=string2	True if string1 matches string2
string1!=string2	True if string1 does not match string2
string1>string2	True if string1 is greater than string2
string1>=string2	True if string1 is greater than or equal to string2
string1<string2	True if string1 is less than string2
string1<=string2	True if string1 is less than or equal to string2
(test-condition)	True if test-condition is true
!test-condition	True if test-condition is false
test-condition1 && test-condition2	True if both test-condition1 and test-condition2 are true
test-condition1 || test-condition2	True if either test-condition1 or test-condition2 is true

In a server-side include string, you can use any environment variables available to the server, including the CGI variables listed in "CGI Error Logging" on page 10-1. SSI also has its own environment variables, listed here.

The filename of the current HTML document. For example:

This file is called <!--#echo var="DOCUMENT_NAME"-->.

The URL of the current file. For example:

This document is located at <!--#echo var="DOCUMENT_URL"-->.

An unencoded query string whose metacharacters are escaped with a backslash (\).

The local date and time. For example:

Here in San Francisco, it is now <!--#echo var="DATE_LOCAL"-->.

The current Greenwich Mean Time. For example:

The current time is <!--#echo var="DATE_GMT"--> GMT.

The date and time when the current file was last modified. For example:

I last modified this page on <!--#echo var="LAST_MODIFIED"-->.

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