Apache HTTP Server Version 2.2

Apache > HTTP Server > Documentation > Version 2.2 > Rewrite

# Advanced Techniques with mod_rewrite

Available Languages: en

This document supplements the `mod_rewrite` reference documentation. It provides a few advanced techniques and tricks using mod_rewrite.

Note that many of these examples won't work unchanged in your particular server configuration, so it's important that you understand them, rather than merely cutting and pasting the examples into your configuration.

### See also

- Module documentation
- mod_rewrite introduction
- Redirection and remapping
- Controlling access
- Virtual hosts
- Proxying
- Using RewriteMap
- When not to use mod_rewrite

- Comments

## URL-based sharding accross multiple backends

Description:

  A common technique for distributing the burden of server load or storage space is called "sharding". When using this method, a front-end server will use the url to consistently "shard" users or objects to separate backend servers.

Solution:

  A mapping is maintained, from users to target servers, in external map files. They look like:

  `user1 physical_host_of_user1
  user2 physical_host_of_user2
  : :`

  We put this into a `map.users-to-hosts` file. The aim is to map;

  `/u/user1/anypath`

  to

  `http://physical_host_of_user1/u/user/anypath`

  thus every URL path need not be valid on every backend physical host. The following ruleset does this for us with the help of the map files assuming that server0 is a default server which will be used if a user has no entry in the map:

  `RewriteEngine on
  RewriteMap users-to-hosts txt:/path/to/map.users-to-hosts
  RewriteRule ^/u/([^/]+)/?(.*) http://${users-to-hosts:$1|server0}/u/$1/$2`

See the `RewriteMap` documentation for more discussion of the syntax of this directive.

## On-the-fly Content-Regeneration

Description:

  We wish to dynamically generate content, but store it statically once it is generated. This rule will check for the existence of the static file, and if it's not there, generate it. The static files can be removed periodically, if desired (say, via cron) and will be regenerated on demand.

Solution:
  This is done via the following ruleset:

      # This example is valid in per-directory context only
      RewriteCond %{REQUEST_URI}   !-U
      RewriteRule ^(.+)\.html$          /regenerate_page.cgi   [PT,L]

  The `-U` operator determines whether the test string (in this case, `REQUEST_URI`) is a valid URL. It does this via a subrequest. In the event that this subrequest fails - that is, the requested resource doesn't exist - this rule invokes the CGI program `/regenerate_page.cgi`, which generates the requested resource and saves it into the document directory, so that the next time it is requested, a static copy can be served.

  In this way, documents that are infrequently updated can be served in static form. if documents need to be refreshed, they can be deleted from the document directory, and they will then be regenerated the next time they are requested.

## Load Balancing

Description:

  We wish to randomly distribute load across several servers using mod_rewrite.

Solution:

  We'll use `RewriteMap` and a list of servers to accomplish this.

  `RewriteEngine on
  RewriteMap lb rnd:/path/to/serverlist.txt
  RewriteRule ^/(.*) http://${lb:servers}/$1 [P,L]`

  `serverlist.txt` will contain a list of the servers:

  `## serverlist.txt
  servers one.example.com|two.example.com|three.example.com
  `

  If you want one particular server to get more of the load than the others, add it more times to the list.

Discussion

  Apache comes with a load-balancing module - `mod_proxy_balancer` - which is far more flexible and featureful than anything you can cobble together using mod_rewrite.

## Document With Autorefresh

Description:

  Wouldn't it be nice, while creating a complex web page, if the web browser would automatically refresh the page every time we save a new version from within our editor? Impossible?

Solution:

  No! We just combine the MIME multipart feature, the web server NPH feature, and the URL manipulation power of `mod_rewrite`. First, we establish a new URL feature: Adding just `:refresh` to any URL causes the 'page' to be refreshed every time it is updated on the filesystem.

  `RewriteRule ^(/[uge]/[^/]+/?.*):refresh /internal/cgi/apache/nph-refresh?f=$1`

  Now when we reference the URL

  `/u/foo/bar/page.html:refresh`

  this leads to the internal invocation of the URL

  `/internal/cgi/apache/nph-refresh?f=/u/foo/bar/page.html`

  The only missing part is the NPH-CGI script. Although one would usually say "left as an exercise to the reader" ;-) I will provide this, too.

      #!/sw/bin/perl
      ##
      ##  nph-refresh -- NPH/CGI script for auto refreshing pages
      ##  Copyright (c) 1997 Ralf S. Engelschall, All Rights Reserved.
      ##
      $| = 1;

      #   split the QUERY_STRING variable
      @pairs = split( /&/, $ENV{'QUERY_STRING'} );
      foreach $pair (@pairs) {
          ( $name, $value ) = split( /=/, $pair );
          $name =~ tr/A-Z/a-z/;
          $name = 'QS_' . $name;
          $value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
          eval "\$$name = \"$value\"";
      }
      $QS_s = 1    if ( $QS_s eq '' );
      $QS_n = 3600 if ( $QS_n eq '' );
      if ( $QS_f eq '' ) {
          print "HTTP/1.0 200 OK\n";
          print "Content-type: text/html\n\n";
      print "&lt;b&gt;ERROR&lt;/b&gt;: No file given\n";
          exit(0);
      }
      if ( !-f $QS_f ) {
          print "HTTP/1.0 200 OK\n";
          print "Content-type: text/html\n\n";
      print "&lt;b&gt;ERROR&lt;/b&gt;: File $QS_f not found\n";
          exit(0);
      }

      sub print_http_headers_multipart_begin {
          print "HTTP/1.0 200 OK\n";
          $bound = "ThisRandomString12345";
          print "Content-type: multipart/x-mixed-replace;boundary=$bound\n";
          &print_http_headers_multipart_next;
      }

      sub print_http_headers_multipart_next {
          print "\n--$bound\n";
      }

      sub print_http_headers_multipart_end {
          print "\n--$bound--\n";
      }

      sub displayhtml {
          local ($buffer) = @_;
          $len = length($buffer);
          print "Content-type: text/html\n";
          print "Content-length: $len\n\n";
          print $buffer;
      }

      sub readfile {
          local ($file) = @_;
          local ( *FP, $size, $buffer, $bytes );
          ( $x, $x, $x, $x, $x, $x, $x, $size ) = stat($file);
          $size = sprintf( "%d", $size );
      open(FP, "&lt;$file");
          $bytes = sysread( FP, $buffer, $size );
          close(FP);
          return $buffer;
      }

      $buffer = &readfile($QS_f);
      &print_http_headers_multipart_begin;
      &displayhtml($buffer);

      sub mystat {
          local ($file) = $_[0];
          local ($time);

          ( $x, $x, $x, $x, $x, $x, $x, $x, $x, $mtime ) = stat($file);
          return $mtime;
      }

      $mtimeL = &mystat($QS_f);
      $mtime  = $mtime;
      for ( $n = 0 ; $n & lt ; $QS_n ; $n++ ) {
          while (1) {
              $mtime = &mystat($QS_f);
              if ( $mtime ne $mtimeL ) {
                  $mtimeL = $mtime;
                  sleep(2);
                  $buffer = &readfile($QS_f);
                  &print_http_headers_multipart_next;
                  &displayhtml($buffer);
                  sleep(5);
                  $mtimeL = &mystat($QS_f);
                  last;
              }
              sleep($QS_s);
          }
      }

      &print_http_headers_multipart_end;

      exit(0);

      ##EOF##

## Structured Userdirs

Description:

  Some sites with thousands of users use a structured homedir layout, i.e. each homedir is in a subdirectory which begins (for instance) with the first character of the username. So, `/~larry/anypath` is `/home/l/larry/public_html/anypath` while `/~waldo/anypath` is `/home/w/waldo/public_html/anypath`.

Solution:

  We use the following ruleset to expand the tilde URLs into the above layout.

  `RewriteEngine on
  RewriteRule ^/~(([a-z])[a-z0-9]+)(.*) /home/$2/$1/public_html$3`

## Redirecting Anchors

Description:

  By default, redirecting to an HTML anchor doesn't work, because mod_rewrite escapes the `#` character, turning it into `%23`. This, in turn, breaks the redirection.

Solution:

  Use the `[NE]` flag on the `RewriteRule`. NE stands for No Escape.

Discussion:
  This technique will of course also work with other special characters that mod_rewrite, by default, URL-encodes.

## Time-Dependent Rewriting

Description:

  We wish to use mod_rewrite to serve different content based on the time of day.

Solution:

  There are a lot of variables named `TIME_xxx` for rewrite conditions. In conjunction with the special lexicographic comparison patterns `<STRING`, `>STRING` and `=STRING` we can do time-dependent redirects:

  `RewriteEngine on
  RewriteCond %{TIME_HOUR}%{TIME_MIN} >0700
  RewriteCond %{TIME_HOUR}%{TIME_MIN} <1900
  RewriteRule ^foo\.html$ foo.day.html [L]
  RewriteRule ^foo\.html$ foo.night.html`

  This provides the content of `foo.day.html` under the URL `foo.html` from `07:01-18:59` and at the remaining time the contents of `foo.night.html`.

  `mod_cache`, intermediate proxies and browsers may each cache responses and cause the either page to be shown outside of the time-window configured. `mod_expires` may be used to control this effect. You are, of course, much better off simply serving the content dynamically, and customizing it based on the time of day.

## Set Environment Variables Based On URL Parts

Description:

  At time, we want to maintain some kind of status when we perform a rewrite. For example, you want to make a note that you've done that rewrite, so that you can check later to see if a request can via that rewrite. One way to do this is by setting an environment variable.

Solution:

  Use the [E] flag to set an environment variable.

  `RewriteEngine on
  RewriteRule ^/horse/(.*) /pony/$1 [E=rewritten:1]`

  Later in your ruleset you might check for this environment variable using a RewriteCond:

  `RewriteCond %{ENV:rewritten} =1`

Available Languages: en

## Comments

Notice:
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