How To Save Traffic With Apache2's mod_deflate

Version 1.0
Author: Falko Timme <ft [at] falkotimme [dot] com>
Last edited 05/15/2006

In this tutorial I will describe how to install and configure mod_deflate on an Apache2 web server. mod_deflate allows Apache2 to compress files and deliver them to clients (e.g. browsers) that can handle compressed content which most modern browsers do. With mod_deflate, you can compress HTML, text or XML files to approx. 20 - 30% of their original sizes, thus saving you server traffic and making your modem users happier.

Compressing files causes a slightly higher load on the server, but in my experience this is compensated by the fact that the clients' connection times to your server decrease a lot. For example, a modem user that needed seven seconds to download an uncompressed HTML file might now only need two seconds for the same, but compressed file.

By using mod_deflate you don't have to be afraid that you exclude users with older browsers that cannot handle compressed content. The browser negotiates with the server before any file is transferred, and if the browser does not have the capability to handle compressed content, the server delivers the files uncompressed.

mod_deflate has replaced Apache 1.3's mod_gzip in Apache2. If you want to serve compressed files with Apache 1.3, take a look at this tutorial: mod_gzip - serving compressed content by the Apache webserver

I want to say first that this is not the only way of setting up such a system. There are many ways of achieving this goal but this is the way I take. I do not issue any guarantee that this will work for you!

1 Enable mod_deflate

If you have Apache2 installed, mod_deflate should also already be installed on your system. Now we have to enable it. On Debian, we can do it like this:

a2enmod deflate

Then restart Apache2:

/etc/init.d/apache2 restart

On other distributions you might have to edit Apache2's configuration manually to enable mod_deflate. You might have to add a line like this to the LoadModule section:

LoadModule deflate_module /usr/lib/apache2/modules/mod_deflate.so

Make sure you adjust the path to mod_deflate.so, and restart Apache2 afterwards.

2 Configure mod_deflate

The compression of files can be configured in one of two ways: either explicit exclusion of files by extension or explicit inclusion of files by MIME type. You can enable mod_deflate for your whole Apache2 server, or just for specific virtual sites. Depending on this, either open your Apache2's global server configuration section now or just the vhost configuration section where you want to enable mod_deflate.

2.1 Explicit Inclusion Of Files By MIME Type

If you want to compress HTML, text, and XML files only, add this line to your configuration:

AddOutputFilterByType DEFLATE text/html text/plain text/xml

This is the configuration I'm using because I don't want to compress images or PDF files or already compressed files such as zip files.

2.2 Explicit Exclusion Of Files By Extension

If you want to compress all file types and exclude just a few, you would add something like this to your configuration (instead of the line from section 2.1):

SetOutputFilter DEFLATE
SetEnvIfNoCase Request_URI \.(?:gif|jpe?g|png)$ \
    no-gzip dont-vary
SetEnvIfNoCase Request_URI \
    \.(?:exe|t?gz|zip|bz2|sit|rar)$ \
    no-gzip dont-vary
SetEnvIfNoCase Request_URI \.pdf$ no-gzip dont-vary

This would compress all files except images (gif, jpg, and png), already compressed files (like zip and tar.gz) and PDF files which makes sense because you do not gain much by compressing these file types.

2.3 Further Configuration Directives

Regardless whether you use the configuration from section 2.1 or 2.2, you should add these lines to your configuration:

BrowserMatch ^Mozilla/4 gzip-only-text/html
BrowserMatch ^Mozilla/4\.0[678] no-gzip
BrowserMatch \bMSIE !no-gzip !gzip-only-text/html

These lines are for some older browsers that do not support compression of files other than HTML documents.

The configuration is now finished, and you must now restart Apache2. On Debian, you do it like this:

/etc/init.d/apache2 restart

To learn about further configuration directives, take a look at Apache Module mod_deflate.

3 Testing

To test our compression, we add a few directives to our mod_deflate configuration that log the compression ratio of delivered files. Open your mod_deflate configuration and add the following lines:

DeflateFilterNote Input input_info
DeflateFilterNote Output output_info
DeflateFilterNote Ratio ratio_info
LogFormat '"%r" %{output_info}n/%{input_info}n (%{ratio_info}n%%)' deflate
CustomLog /var/log/apache2/deflate_log deflate

Make sure you replace /var/log/apache2 with your Apache2's log directory. This could be /var/log/httpd, /var/log/httpd2, etc.

Then restart Apache2. On Debian, do it like this:

/etc/init.d/apache2 restart

Now whenever a file is requested this will be logged in /var/log/apache2/deflate_log (or to whatever file you changed it to). A typical log line looks like this:

"GET /info.php HTTP/1.1" 7621/45430 (16%)

You see that the file info.php was requested and delivered. Its original size was 45430 bytes, and it was compressed to 7621 bytes or 16% of its original size! This is a great result, and if your web site mostly consists out of HTML, text, and XML files, mod_deflate will save you a lot of traffic, and for users with a low-bandwidth connection your site will load much faster.

If you don't need the logging after your tests anymore, you can undo the changes from section 3 and restart Apache2.

4 Links

• Apache Module mod_deflate: http://httpd.apache.org/docs/2.0/mod/mod_deflate.html
• mod_gzip - serving compressed content by the Apache webserver: http://www.howtoforge.com/linux_apache_mod_gzip