Apache Module mod_deflate

Supported Encodings

The gzip encoding is the only one supported to ensure complete compatibility with old browser implementations. The deflate encoding is not supported, please check the zlib's documentation for a complete explanation.

Sample Configurations

This is a simple configuration that compresses common text-based content types.

AddOutputFilterByType DEFLATE text/html text/plain text/xml text/css text/javascript application/javascript

Enabling Compression

Output Compression

Compression is implemented by the DEFLATE filter. The following directive will enable compression for documents in the container where it is placed:

SetOutputFilter DEFLATE
SetEnvIfNoCase Request_URI "\.(?:gif|jpe?g|png)$" no-gzip

If you want to restrict the compression to particular MIME types in general, you may use the AddOutputFilterByType directive. Here is an example of enabling compression only for the html files of the Apache documentation:

<Directory "/your-server-root/manual">
    AddOutputFilterByType DEFLATE text/html
</Directory>

Output Decompression

The mod_deflate module also provides a filter for inflating/uncompressing a gzip compressed response body. In order to activate this feature you have to insert the INFLATE filter into the output filter chain using SetOutputFilter or AddOutputFilter, for example:

<Location "/dav-area">
    ProxyPass "http://example.com/"
    SetOutputFilter INFLATE
</Location>

This Example will uncompress gzip'ed output from example.com, so other filters can do further processing with it.

Input Decompression

The mod_deflate module also provides a filter for decompressing a gzip compressed request body . In order to activate this feature you have to insert the DEFLATE filter into the input filter chain using SetInputFilter or AddInputFilter, for example:

<Location "/dav-area">
    SetInputFilter DEFLATE
</Location>

Now if a request contains a Content-Encoding: gzip header, the body will be automatically decompressed. Few browsers have the ability to gzip request bodies. However, some special applications actually do support request compression, for instance some WebDAV clients.

Dealing with proxy servers

The mod_deflate module sends a Vary: Accept-Encoding HTTP response header to alert proxies that a cached response should be sent only to clients that send the appropriate Accept-Encoding request header. This prevents compressed content from being sent to a client that will not understand it.

If you use some special exclusions dependent on, for example, the User-Agent header, you must manually configure an addition to the Vary header to alert proxies of the additional restrictions. For example, in a typical configuration where the addition of the DEFLATE filter depends on the User-Agent, you should add:

Header append Vary User-Agent

If your decision about compression depends on other information than request headers (e.g. HTTP version), you have to set the Vary header to the value *. This prevents compliant proxies from caching entirely.

Header set Vary *

Serving pre-compressed content

Since mod_deflate re-compresses content each time a request is made, some performance benefit can be derived by pre-compressing the content and telling mod_deflate to serve them without re-compressing them. This may be accomplished using a configuration like the following:

<IfModule mod_headers.c>
    # Serve gzip compressed CSS files if they exist 
    # and the client accepts gzip.
    RewriteCond "%{HTTP:Accept-encoding}" "gzip"
    RewriteCond "%{REQUEST_FILENAME}\.gz" -s
    RewriteRule "^(.*)\.css" "$1\.css\.gz" [QSA]
    # Serve gzip compressed JS files if they exist 
    # and the client accepts gzip.
    RewriteCond "%{HTTP:Accept-encoding}" "gzip"
    RewriteCond "%{REQUEST_FILENAME}\.gz" -s
    RewriteRule "^(.*)\.js" "$1\.js\.gz" [QSA]
    # Serve correct content types, and prevent mod_deflate double gzip.
    RewriteRule "\.css\.gz$" "-" [T=text/css,E=no-gzip:1]
    RewriteRule "\.js\.gz$" "-" [T=text/javascript,E=no-gzip:1]
    <FilesMatch "(\.js\.gz|\.css\.gz)$">
      # Serve correct encoding type.
      Header append Content-Encoding gzip
      # Force proxies to cache gzipped & 
      # non-gzipped css/js files separately.
      Header append Vary Accept-Encoding
    </FilesMatch>
</IfModule>