Stupid Server Tricks - Zeekat Softwareontwikkeling

An introduction to webservers from a client-side programming perspective.

Author & version

 Title:  Stupid Server Tricks
 Url:    http://zeekat.nl/joost/server-tricks/
 Id:     $Id: server-tricks.pod,v 1.3 2005/04/23 22:16:23 joost Exp $
 Author: Joost Diepenmaat - joost@zeekat.nl
 Source: http://zeekat.nl/joost/server-tricks/server-tricks.pod

COPYING

 Copyright (c) 2001 - 2005 Joost Diepenmaat.
 Permission is granted to copy, distribute and/or modify this
 document under the terms of the GNU Free Documentation License,
 Version 1.1 or any later version published by the Free Software
 Foundation; with no Invariant Sections, with no Front-Cover Texts,
 and with no Back-Cover Texts. A copy of the license is included 
 in the section entitled "GNU Free Documentation License".

This document is intended as an introduction for client-side WWW programmers who want to learn about the server-side of the WWW and how you can use your web-server to simplify designing and maintaining websites.

As this document is still unfinished, some of the information is incomplete and some of it may be wrong. If you find any bugs in this file, let me know (please check the online version first to see if you have the most recent version).

Intended audience

This document assumes that you have some experience in building websites with HTML. Some parts assume some (programming) experience in JavaScript, so being able to read JavaScript code might help. Also, to test the examples you need a working web-server. Mostly I will be working with the Apache HTTP Server, the free and very popular webserver by the apache group, but other webservers might work for you too. Apache-specific sections will be recognizable as such (like Apache configuration).

Overview

Almost all traffic on the WWW is transported using the HTTP protocol. It is based on a simple client-server model and uses plain (ASCII) text.

 HTTP Client (Browser)                        HTTP Server
 +------------------------+                   +---------------------+
 | Wait for user action   |                   |   Wait for request  |
 | Request document     >---- HTTP Request  ----> Recieve request   |
 | Wait for response      |                   | | Interpret request |
 | Display document     <---- HTTP Response ----< Return Document   |
 | Back to top            |                   |   Back to top       |
 +------------------------+                   +---------------------+

Abstract view of the HTTP protocol.

In the HTTP protocol a transmission is driven by the client (browser) which opens a connection with the server and sends a request over the connection. When the server recieves a request, it sends a response and closes the connection.* This also means that the server cannot send commands or documents to the client unless it is asked to do so: you cannot, for instance re-arrange the HTML layout without the browser reloading the document.

* in the HTTP/1.1 model, a connection is kept open by default, until the browser or the server sends a command to close the connection. This makes downloading several documents in succession quicker, because opening the connection takes a relatively long time. This does not affect the rest of the protocol very much though.

Detailed view of an HTTP Request

A simple HTTP/1.1 request looks like this:

 01     GET /index.html HTTP/1.1                                
 02     Host: zeekat.nl
 03     Accept: text/*;q=1.0, image/png;q=1.0, image/jpeg;q=1.0, 
                image/gif;q=1.0, image/*;q=0.8, */*;q=0.5  
 04     Accept-Charset: iso-8859-1;q=1.0, *;q=0.9, utf-8;q=0.8
 05     Accept-Encoding: x-gzip; q=1.0, gzip; q=1.0, identity
 06     Accept-Language: nl, en_US, en  
 07     Connection: Keep-Alive
 08     User-Agent: Mozilla/5.0 (compatible; Konqueror/2.1; X11)
 09

An HTTP/1.1 request (line 03 wrapped for better viewability)

As you can see, the request starts with a line specifying what the request is about; an method: GET, for retrieving a document, the location of the document on the server: /index.html, and the version of the HTTP protocol used: HTTP/1.1. This line may be followed by a number of headers, each on their own line, ending with an empty line. The order of the headers is not important.

Methods: GET and POST

The 2 most familiar methods - if you have ever worked with HTML Forms - are GET and POST. The methods are similar; both request some resource (data) from the server, and both can send user data to the server. However, there are some differences: a GET method can only send a limited amount of data to the server (limited by the length of the URL - all user supplied data is appended to the actual location of the requested resource) and it is not supposed to have any side-effects. (Side-effects are things like sending submitted data via email to someone or purchasing something from an online shop.) Because GET requests are not supposed to have any side-effects, the browser will not warn you when you send data to the sever via a GET method. A POST method may send an almost unlimited amount of data to the server, and may be used for instance to upload files. POST requests are assumed to have side-effects and that is the reason that most browsers will warn you before sending a form-data with a POST method.

There are other methods, but you will probably not want (or be able) to use them with standard browsers. If you really want to know, check out RFC 2616: HTTP 1.1.

Headers

An HTTP header consists of a directive and a value, seperated by a colon. Headers are case-insensitive and none is required except for the Host: header, which is required in HTTP/1.1.

For our purposes, the more interesting directives in this request are:

Accept: This specifies the document types the browser will support, in this case with quality information (the ;q=x.x information) specifying what documents are preferred.

Accept-Charset: Specifies the character set the browser supports. iso-8859-1 is sort of ASCII-like. utf-8 is unicode.

Accept-Encoding: The supported encodings. x-gzip and gzip mean that the browser support gzip-compressed files. indentity means 'as-is': no encoding.; See also Compression.

Accept-Language: The accepted languages, in order of preference: Dutch, US English and English. This can be used by the server to decide which document to serve, if it is available in different languages.; See also Multi-lingual sites.

User-Agent: The browser & OS type; in this case Konqueror 2.1 on X11 (Actually; Linux). This line is usually build up like: Mozilla/X.X (compatible; BrowserName/Y.Y; OSName) *, mostly because of historical reasons.

* You can read that as (loosly) meaning: Browser BrowserName version Y.Y on OS OSName, compatible with Mozilla (a.k.a. Netscape) version X.X (except of course that Netscape 6 is actually Netscape 5 and still identifies itself as Mozilla 5.0, that the recent Mozilla betas identify themselves as gecko, and the first full release of Mozilla will probably identify as Mozilla 1.0 - I'm getting a headache).

Detailed view of an HTTP Response

The HTTP/1.1 response looks like this:

 01     HTTP/1.1 200 OK
 02     Date: Thu, 26 Jul 2001 16:08:44 GMT
 03     Server: Apache/1.3.14 (Unix)  (Red-Hat/Linux) mod_perl/1.23
 04     Last-Modified: Fri, 27 Jul 2001 11:21:48 GMT
 05     ETag: "898-2258-3b614ecc"
 06     Connection: close
 07     Content-Type: text/html
 08     
 09     <html>
 10     <head><title>Some HTML file</title></head>
 11     <body>This is an HTML file</body>
 12     </html>

An HTTP/1.1 response

An HTTP response starts with a status line, specifying the HTTP version of the response: HTTP/1.1 and a status code: 200 OK - see HTTP status codes. After the status line we find more headers, followed by an empty line signifying the start of the content. While none of the headers are strictly required, the content-type is almost always sent.

The most interesting header in this response is:

Content-Type: The content-type specifies the MIME-type of the returned document. Browser use this field to determine how they should render the document. (By using the built-in html viewer, a plugin or an external program, or some other way of presenting.) Without a content-type header, a browser cannot determine what kind of document it is recieving and may try to show it as HTML, as plain text, or just try to save the document and not render it at all.

* Microsoft Internet Explorer (and maybe some other browsers?) also try to determine the content-type based on the extention of the filename, which this works most of the time, but can also cause strange results because the mapping of file-extention to mime-type is based on registered extentions on the client side, while the server might have a completely different extention-scheme - for instance to determine content-language.

Sending HTTP headers with META tags

The Apache HTTP server is a very flexible program, and I will not try to include a complete configuration manual here - the manual included with the software is very good, and books are available to teach you more; see Where to find out more. This section is merely intended to give some hints to get you started.

After you have succesfully installed the Apache server (or if you use an already installed server) you will probably need to adjust the configuration in order to make use of things like server-side includes and content-negotiation. You can adjust the configuration in either the httpd.conf file or other global configuration files, or via .htaccess files in the document directories. I will only cover .htaccess files here. For more information, see the Apache manual located on your system, and on http://httpd.apache.org/docs/

.htaccess files

You can override the apache configuration on a per-directory basis by inserting a file called .htaccess (notice the leading dot). Any configuration directive specified in this file will be valid for that directory and its subdirectories *.

* The main apache configuration file httpd.conf can specify which directives may be overridden, but I will assume a setting of AllowOverride All in this document. If you find your server has disabled cetain options you need, contact your server-administrator, or edit the httpd.conf file yourself.

Multi-language sites

Lots of sites today are multi-lingual. There are many ways of dealing with this; you can just build a site, copy all the content and then translate, you can go for a complete content-management package or you might think of something in between.

One of the first you will come up against with a multi-lingual site is this: what language should you present a user with the first time they show up at your homepage? You could start in English, but what if the user can't read English?

There is a solution to this: language-based content-negotiation. Browsers can send to the server which language their user prefers, and the server can then automatically send the right document back. All you need to do, is to keep the pages you want to be negotiated to a naming scheme the server can recognize.

Say you have a directory introduction filled like this:

 /introduction/index.html.en (english)
 /introduction/index.html.nl (dutch)
 /introduction/index.html.de (german)

Directory with multi-language index.html files

Now a browser sends a request for the introduction/index.html file, also sending the Accept-Language header, in this case Accept-Language: nl, en_US, en. With the proper configuration, the server sees that there is no literal index.html page in the directory and will then try to match with the index.html.* files instead. The best match for this user is /introduction/index.html.nl. The user is instantly greeted with an introduction in his or her preferred language!

 Options +MultiViews
 AddLanguage en .en
 AddLanguage nl .nl
 AddLanguage de .de
 DefaultLanguage en
 DirectoryIndex index

Sample Apache configuration directives for language-base content-negotiation

If you combine this technique with Server side includes, you can get some pretty impressive results without even using a special content-management system or templating tools.

Compression

Some browsers also provide an Accept-Encoding header, this allows for compressed files to be server to the client, saving transmision time.

Standard compression types include gzip and compress:

 Options +MultiViews
 AddEncoding x-gzip .gz
 AddEncoding x-compress .Z

Now you can add gzip'ed and compress'ed versions of files to the directory, like this:

 file.html
 file.html.gz
 file.html.Z

And the correct file will be selected when a browser requests file or file.html.

Parsed HTML

You can configure most webservers to parse the HTML files before they are send to the user. This means that you can do things like: include menus in 'static' html files, print out the age of the file and even create different output based on the link followed to the page, without relying on any browser-specific techniques like JavaScript. All this is done using HTML comments () with a special syntax:

SSI (server-side include) tags look like this:

 <!--#command [argument(s)] -->

There are several commands, and I will show the most usefull here:

include virtual

 <!--#include virtual=path/file -->

Includes a file or the output of a script/dynamic page (CGI, PHP etc.) from a path relative to the including page.* You cannot specify a hostname, so the included page MUST be on the same server as the including page. You can specify 'absolute' URLs by using /rootrelative/file.html syntax.

include file

 <!--#include file=path/file -->

Includes a file without any parsing. The path is taken to be relative to the directory containing the current document being parsed. It cannot contain ../, nor can it be a root-relative path (on apache that is *). Included files are supposed to be static (No CGI-scripts, PHP or other dynamic content).

* Microsoft IIS users should watch out, because the default SSI parser for IIS expects the the PATH to be relative to the the document-root in case of an include virtual statement, but relative in case of an include file statement. IIS also accepts paths with ../ in include file.

echo

 <!--#echo var=variable -->

Echo the value of a variable on the page. You can use this for instance to make dynamic framesets. This is the code I used on one site:

  <frameset  rows="200,*" border"0">
    <frame name="" src="navigation.html" marginwidth="0" 
      marginheight="0" scrolling="no" frameborder="0" border="0">
    <frame name="content" src="<!--#echo var="QUERY_STRING"-->" 
      marginwidth="0" marginheight="0" scrolling="no" frameborder="0" 
      border="0">
  </frameset>

Sourcecode for frameset.shtml

Now you can create links to pages that automatically load that page in a frameset like this: http://some.site.com/frameset.shtml?somepage.html.

set

 <!--#set var="varname" value="Stringvalue" -->

Sets variable varname to value Stringvalue, for instance:

 <!--#set var="hellovar" value="world" -->

 <h1>Hello <!--#echo var="hellovar" --></h1>

Outputs

 <h1>Hello world</h1>

if else endif

 <!--#if expr="$somevar" -->
   Somevar is not empty
 <!--#else -->
   Somevar is empty
 <!--#endif -->

Will output Somevar is not empty if $somevar is true (any expression that does not results in an empty string is considerd to be true). Otherwise it will output Somevar is empty

exec

 <!--#exec cmd="command" -->

Executes a system command named 'command' and returns the output in the page. This is certainly NOT secure and most system-administrators will have this command disabled.

 <!--#exec cgi="script.cgi" -->

Executes a cgi-script and returns the output in the page. This will run ANY file that is available to the webserver as a cgi-script and so this isn't very secure either. Again, most system-administrators will have this command disabled.

Apache specific section

If you have a directory containing files that need to be indexed, but you do not want to create a full html file with links to all the files, you can use the built in FancyIndexing options to create a nice looking page with an HTML header and footer.

Example .htaccess file:

        Options +Indexes
        IndexOptions +ScanHTMLTitles +FancyIndexing
        IndexOptions +DescriptionWidth=* +SuppressHTMLPreamble
        HeaderName HEADER.html
        ReadmeName README.html
        IndexIgnore *.css ..

Short explanation of the settings:

Options +Indexes

Turn directory indexing on. You won't see anything without this.
IndexOptions ...
- FancyIndexing
  
  Turn on special options.
- ScanHTMLTitles
  
  Read the titles from the HTML files in this directory and show them in the Description field.
- DescriptionWidth=*
  
  Show the full titles in the description
- SuppressHTMLPreamble
  
  Do not create the standard HTML up to the <BODY> tag, read it from the Header instead.
HeaderName HEADER.html

Insert a header from the HEADER.html file.
ReadmeName README.html

Insert a footer from the README.html file.

Note: on some versions of apache you must specify the HeaderName and ReadmeName WITH .html extention, on others you should not - YMMV.
IndexIgnore *.css ..

Do not show .css files or the parent directory in the list.

For more information take a look at The Apache group.

The W3C website: http://www.w3.org/ The site for all official specifications for HTTP, HTML, XHTML, CSS and more.

RFC 2616: HTTP 1.1: ftp://ftp.isi.edu/in-notes/rfc2616.txt The RFC describing the HTTP/1.1 protocol

The Apache group: http://apache.org/ The source for the free Apache HTTP Server and its documentation.

Articles: http://www.zdnet.com/devhead/stories/articles/0,4413,1600807,00.html introduction to SSI and comparison between different webservers.

I want to thank my former employer, Netlinq Framfab ( http://www.framfab.nl/ ) for letting me distribute this document, which was mostly written during working hours, on the web. I would also like to thank my colleage Peter Paul Koch for inspiration and criticism - visit his JavaScript pages at http://www.quirksmode.org/ .

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 Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.
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 Free Documentation License".

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