mod_aclr2 is the port of the original mod_aclr module for Apache 1.3 written by Dmitry MikSir.
Original code and documentation are still available here.
Copyright (c) Dmitry MikSir

This module, in contrast to mod_aclr, works only under Apache 2.x.

Original mod_aclr was initially released under the GNU General Public License, but its author granted the permission to release new mod_aclr2 under the BSD license.

Copyright (c) 2011-2012 Andrey Belov
Copyright (c) 2011-2012 Nginx, Inc., http://nginx.com

mod_aclr2 ("aclr" comes from accel redirect) is the module for Apache 2.x that makes it possible to easily deploy NGINX as a local reverse proxy in front of Apache.

As a result, all your static content can now be automatically served by NGINX, while the other legacy and Apache-specific things like RewriteRule, .htaccess, CGI, embedded PHP/Perl will work as they worked before. This setup can be very important and useful for many shared hosting service providers to improve the situation with the concurrency and quality of service.

Efficiency in processing any kind of static content is just one of the NGINX benefits, therefore mod_aclr2 is a good starting point for further improvements to your existing hosting infrastructure - you could do many things better and faster with NGINX.

The module creates a special Apache handler hook. This handler must be processed after all the other registered handlers, and before generating any response to the client (NGINX, in our case).

First, it checks for the X-Accel-Internal request header field presence. If this field is present, and if the result of the request is a regular file, the module sends a special response header field X-Accel-Redirect back to NGINX. This field contains the value of X-Accel-Internal combined with the exact location of the file as it was determined by Apache after processing all previous handlers (mod_rewrite, .htaccess, etc).

All requests with dynamic responses (except SSI) should never reach mod_aclr2 handler. Every SSI request is redirected to the core directly from mod_aclr2 handler if INCLUDES filter was found in the output filters chain.

The fact that the presence of X-Accel-Internal is required allows you to control precisely when to make NGINX serve the static file or go directly to the backend. Please note that in order for NGINX to serve the static content directly, it should run as a local proxy, on the same machine as Apache and having the same access to the files on disk.

syntax: AccelRedirectSet On | Off
context: server config, virtual host, directory
default: Off

    Enables or disables the module.

syntax: AccelRedirectSize size[b|k|m]
context: server config, virtual host, directory
default: -1

    Sets the minimum size of static files for which the request will be handled by NGINX.
    By default, there are no restrictions, and Apache will send redirects to NGINX for all static files.

syntax: AccelRedirectOutsideDocRoot On | Off
context: server config, virtual host, directory
default: Off

    Enables or disables redirects for files outside of DocumentRoot path.

syntax: AccelRedirectDebug 0-4
context: server config
default: 0

    WARNING: module must be built with -DDEBUG to enable this directive.
    Sets the debug level:

• 0 = no debug
• 1 = log successfull redirects only
• 2 = also, log a reason why request has not been redirected
• 3 = also, log a lot of request processing data

1. Compile the module using apxs:

   apxs -c mod_aclr2.c
   

   In order to use AccelRedirectDebug directive, you should use the following command:

   apxs -c -DDEBUG -Wc,-Wall mod_aclr2.c
   

2. Install and activate it.

   apxs -i -a -n aclr mod_aclr2.la
   

3. Configure the module in the Apache configuration file httpd.conf or equivalent. For example:

   AccelRedirectSet On
   AccelRedirectSize 1k
   

4. Reload Apache.

The configuration relies on the fact that mod_aclr2 requires the X-Accel-Internal request header field to be present.

It is very important to transfer a real client IP address to Apache for correct logging, permission checks, etc. You can use the special header field X-Real-IP as shown below along with the module mod_rpaf to make this work.

1. Proxy configuration:

   location / {
       proxy_pass http://127.0.0.1:80;
       proxy_set_header X-Real-IP $remote_addr;
       proxy_set_header X-Accel-Internal /int;
       proxy_set_header Host $host;
   }
   

2. Special internal location for handling redirects:

   location /int/ {
       # path must be equal to Apache's DocumentRoot
       alias /var/www/site.com/;
       internal;
    }
   

According to this configuration, request from NGINX to Apache should look like:

    GET /some/static/file.txt HTTP/1.1
    Host: site.com
    X-Real-IP: 11.22.33.44
    X-Accel-Internal: /int

and response from mod_aclr2 to NGINX should look like:

    HTTP/1.1 200 OK
    Date: Tue, 10 Jan 2012 02:59:02 GMT
    Server: Apache/2.2.21 (FreeBSD)
    X-Accel-Version: 0.01
    X-Accel-Redirect: /int/some/static/file.txt
    Content-Length: 0
    Content-Type: text/plain

and, finally, NGINX sends the file /var/www/site.com/some/static/file.txt back to the client.

Probably exist. Feel free to report. Patches are welcome.