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mod_rewrite for newcomers

What is mod_rewrite?

It's an Apache httpd (web server) module that takes user's requests for pages and diverts them to a resource of a different name (and perhaps type). Why might we want to do this? See previous article for some reasons and alternatives

Here's a simple example of a rewrite rule:

RewriteRule ^train_running.html$ running.html

And that tells the web server to divert requests to train_running.html to running.html instead. We've used rewrite rules as simple as this one to divert links that people have kindly made to our site - but got wrong - to the correct resource:

RewriteRule ^5704.html$ J704.html

But there's much more to mod_rewrite than that as you'll see if you look at the official manual. Actually, there's so much more that I'll give you some further examples - each taken from our web site.

When reading these examples, please bear in mind that you can place them in your httpd.conf main Apache Configuration file if you wish them to propagate through the whole of your site, or in an individual .htaccess file in the directories to which you wish them to apply. Your server administrator has the ability to enable / disable .htaccess, and what may and may not be placed in it

One final introductory note - the incoming URL is specified in the form or a regular expression (a pattern). At its most basic, most characters match one for one to the URL but there are a number of "specials" such as anchors ^ and $ that mean "starting with" and "ending with", character groups such as . which means "any character" and counts such as * which means "0 or more of the preceding item". Round brackets have multiple meanings within regular expressions; you'll see them in my examples below, used to capture parts of the incoming URL to substitute it into the rewritten one. We offer a day's course on Regular Expressions.

Collapsing a whole directory of web pages to a single script

RewriteRule ^(.*)\.html$ /share/index.php?pagename=$1

All requests that end in .html in the area that the .htaccess or httpd.conf file controls are to be passed on to a single PHP script in the /share/ directory called index.php. The name of the page that was passed in to this PHP script is to be placed in a request parameter called pagename.

This is the mechanism we use in our "wiki" ... where (after validation to avoid injection attacks), the following SQL query is run:

mysql_query("select * from sharedata where pagename = '$_REQUEST[pagename]'");

and the fields from the database are used to populate a single template.

Taking all .htm and .html files and passing them on, including GET data filled in to forms

^(.*)\.htm 8.php?pagename=index&sharename=$1&%{QUERY_STRING}

This is an extended example of passing all .htm (and .html) URLs in a directory on to a single script, with various parameters being passed in too, and any data that was submitted via the GET method appended on to the end. Note that by leaving off the $ on the pattern for the incoming URL, you're able to overcome any ".htm or .html" issues. We do the same thing for .php / .php3 / .php4 / .php5 ...

Diverting the home page of a directory

RewriteRule ^$ 8.php?pagename=index&sharename=index&%{QUERY_STRING}

The Regular expression here is "start with and ends right away", so that's a request for a directory rather than anything in a directory.

Referring image requests on to a database via a program

RewriteRule ^(.*)\.jpg /pix/feeder.php?image=$1&%{QUERY_STRING}

All .jpg requests are passed on to a PHP script, with the image name passed in as a parameter. Why do we do this?

• it saves a directory with a large number of pictures getting cluttered
• it allows us to monitor where images are loaded from (the referer) so highlighting any images hotlinked from other web sites
• it allows us to generate dynamic images (for example, this diagram of current train cancellations on First Great Western ;-) )
• it allows us to feed out low res or high res alternatives
• it allows us t maintain image data with the image
• and it allows us to use selected URLs to generate a random image.

Handling a special case - something NOT to rewrite

RewriteRule ^rooms.html rooms.html?%{QUERY_STRING} [L]

Where you have a whole directory / pattern being rewritten and you want to make exceptions, you can do so. In this example, there really is a file called rooms.html that you want to serve! The [L] modifier means "Last" and instructs the httpd web server that it should skip the following rules if this rule has been applied - very useful indeed in preventing some utterly confusing situations!

Rewriting that calls up a page from another server

RewriteRule ^info.php http://www.wellho.info/ [L,P]

This example - with a complete URL in the target position and a [P] modifier, proxies (see mod_proxy) the request on to a different URL on a different computer. Your web server actually becomes a client as it retrieves the page from the other system, and in turn it passes it back to your client. So the following two links will return the same .html source:


Indeed they do return the same source, but you'll need to be very careful indeed of any site-relative links, references to style sheets and images, etc - but mod_proxy is a story for another day!

Other directives in mod_rewrite

As well as RewriteRule directives, you'll also need to make use of a RewriteEngine directive to turn the facility on. The RewriteCond directive allows you to apply a condition to the following RewriteRule, and the RewriteLog allows you to produce a log file of rewrite requests. You can use a RewriteMap directive to call in a separate file of mappings, and that can even include random proxy forwards; what sounds (at first) like a crazy idea actually provides a neat way of spreading your processor load on heavy web based applications around a number of servers - there's a complete example here of forwarding Java requests from Apache httpd to multiple instances of Tomcat.

We cover the configuration of Apache httpd on our Linux Web Server course ... and there's a lot more that you can do other than just mod_rewrite!
(written 2008-12-20, updated 2013-01-02)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
A603 - Web Application Deployment - Further httpd Configuration
  [4307] Identifying and clearing denial of service attacks on your Apache server - (2014-09-27)
  [4001] Helping search engines with appropriate 400 error codes - (2013-02-11)
  [3955] Building up from a small PHP setup to an enterprise one - (2012-12-16)
  [3862] Forwarding a whole domain, except for a few directories - Apache http server - (2012-09-17)
  [3635] Parse error: parse error, unexpected T_STRING on brand new web site - why? - (2012-03-03)
  [3449] Apache Internal Dummy Connection - what is it and what should I do with it? - (2011-09-19)
  [3133] An image from a website that occasionally comes out as hyroglyphics - (2011-01-14)
  [2900] Redirecting a page - silent, temporary or permanent? - (2010-08-03)
  [2478] How did I do THAT? - (2009-10-26)
  [2272] Monitoring and loading tools for testing Apache Tomcat - (2009-07-07)
  [2060] Database connection Pooling, SSL, and command line deployment - httpd and Tomcat - (2009-03-01)
  [1974] Moving a directory on your web site - (2009-01-03)
  [1955] How to avoid duplicating web page maintainance - (2008-12-20)
  [1939] mod_proxy_ajp and mod_proxy_balancer examples - (2008-12-13)
  [1778] Pointing all the web pages in a directory at a database - (2008-08-30)
  [1767] mod_proxy and mod_proxy_ajp - httpd - (2008-08-22)
  [1762] WEB-INF (Tomcat) and .htaccess (httpd) - (2008-08-20)
  [1707] Configuring Apache httpd - (2008-07-12)
  [1636] What to do if the Home Page is missing - (2008-05-08)
  [1619] User and Group settings for Apache httpd web server - (2008-04-22)
  [1566] Strange behaviour of web directory requests without a trailing slash - (2008-03-06)
  [1564] Default file (MiMe types) for Apache httpd and Apache Tomcat - (2008-03-04)
  [1554] Online hotel reservations - Melksham, Wiltshire (near Bath) - (2008-02-24)
  [1551] Which modules are loaded in my Apache httpd - (2008-02-23)
  [1381] Using a MySQL database to control mod_rewrite via PHP - (2007-10-06)
  [1377] Load Balancing with Apache mod_jk (httpd/Tomcat) - (2007-10-02)
  [1355] .php or .html extension? Morally Static Pages - (2007-09-17)
  [1351] Compressing web pages sent out from server. Is it worth it? - (2007-09-14)
  [1207] Simple but effective use of mod_rewrite (Apache httpd) - (2007-05-27)
  [1121] Sharing the load with Apache httpd and perhaps Tomcat - (2007-03-29)
  [1080] httpd.conf or .htaccess? - (2007-02-14)
  [1009] Passing GET parameters through Apache mod_rewrite - (2006-12-27)
  [934] Clustering, load balancing, mod_rewrite and mod_proxy - (2006-11-21)
  [853] To list a directory under httpd on a web server, or not? - (2006-09-02)
  [755] Using different URLs to navigate around a single script - (2006-06-11)
  [662] An unhelpful error message from Apache httpd - (2006-03-30)
  [649] Denial of Service ''attack'' - (2006-03-17)
  [631] Apache httpd to Tomcat - jk v proxy - (2006-03-03)
  [550] 2006 - Making business a pleasure - (2006-01-01)
  [526] Apache httpd - serving web documents from different directories - (2005-12-12)
  [466] Separating 'per instance' data from binaries and web sites - (2005-10-16)
  [345] Spotting a denial of service attack - (2005-06-12)

A607 - Web Application Deployment - Apache httpd mod_rewrite
  [3753] Adding a passcode to a directory - (2012-06-05)
  [3568] Telling which ServerAlias your visitor used - useful during merging domains - (2012-01-04)
  [3339] Simplest ever proxy configuration? - (2011-06-28)
  [3197] Finding and diverting image requests from rogue domains - (2011-03-08)
  [2981] How to set up short and meaningfull alternative URLs - (2010-10-02)
  [2773] Dynamically watching your web site via a PHP wrapper - (2010-05-21)
  [2768] Carrying a long URL around - looking for memorable shorts - (2010-05-17)
  [2728] Redirecting a home page using mod_rewrite - (2010-04-17)
  [2555] Bookkeeping - (2009-12-29)
  [2094] If you have a spelling mistake in your URL / page name - (2009-03-21)
  [1771] More HowTo diagrams - MySQL, Tomcat and Java - (2008-08-24)
  [1731] Apache httpd, MySQL, PHP - installation procedure - (2008-08-01)
  [1376] Choosing between mod_proxy and mod_rewrite - (2007-10-02)
  [1006] Apache httpd and Apache Tomcat together tips - (2006-12-24)

G996 - Well House Consultants - Newsletter Scripts
  [3179] Oops - I typed ci not vi, and have lost my file ... - (2011-02-21)
  [2539] Changing Images - (2009-12-17)
  [2433] Controlling, supressing, enabling PHP error messages - (2009-10-02)
  [2360] Error Handling in Lua with assert and pcall - (2009-08-13)
  [2259] Grouping rows for a summary report - MySQL and PHP - (2009-06-27)
  [2145] Using the internet to remotely check for power failure at home (PHP) - (2009-04-29)
  [2046] Finding variations on a surname - (2009-02-17)
  [1813] Ajax - going Asyncronous and what it means - (2008-09-28)
  [1743] First class functions in Lua lead to powerful OO facilities - (2008-08-07)
  [1665] Factory method example - Perl - (2008-06-04)
  [1601] Replacing the last comma with an and - (2008-04-04)
  [1505] Script to present commonly used images - PHP - (2008-01-13)
  [1487] Efficient PHP applications - framework and example - (2007-12-28)
  [1387] Error logging to file not browser in PHP - (2007-10-11)
  [1321] Resetting session based tests in PHP - (2007-08-26)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [1123] mysqldump and mysqlrestore - (2007-03-30)
  [1066] Final, Finally and Finalize - three special words in Java - (2007-02-05)
  [1001] .pdf files - upload via PHP, store in MySQL, retrieve - (2006-12-19)

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Some other Articles
PHP - Parse error: syntax error, unexpected $end ...
Why are cooks bad tempered?
Images for Christmas
mod_rewrite for newcomers
End of Training, 2008
Small Print
Whisky - Setting and reading cookies from Perl
Copyright of Training Notes and Web Site
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