Apache Module mod_deflate
Summary
The mod_deflate
module provides the DEFLATE
output filter that allows output from your server to be compressed before being sent to the client over the network.
Sample Configurations
This is a simple sample configuration for the impatient.
Compress only a few types
AddOutputFilterByType DEFLATE text/html text/plain text/xml
The following configuration, while resulting in more compressed content, is also much more complicated. Do not use this unless you fully understand all the configuration details.
Compress everything except images
<Location />
# Insert filter
SetOutputFilter DEFLATE
# Netscape 4.x has some problems...
BrowserMatch ^Mozilla/4 gzip-only-text/html
# Netscape 4.06-4.08 have some more problems
BrowserMatch ^Mozilla/4.0[678] no-gzip
# MSIE masquerades as Netscape, but it is fine
BrowserMatch MSIE !no-gzip !gzip-only-text/html
# Don't compress images
SetEnvIfNoCase Request_URI
.(?:gif|jpe?g|png)$ no-gzip dont-vary
# Make sure proxies don't deliver the wrong content
Header append Vary User-Agent env=!dont-vary
</Location>
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:
Some popular browsers cannot handle compression of all content so you may want to set the gzip-only-text/html
note to 1
to only allow html files to be compressed (see below). If you set this to anything but 1
it will be ignored.
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>
For browsers that have problems even with compression of all file types, use the BrowserMatch
directive to set the no-gzip
note for that particular browser so that no compression will be performed. You may combine no-gzip
with gzip-only-text/html
to get the best results. In that case the former overrides the latter. Take a look at the following excerpt from the configuration example defined in the section above:
BrowserMatch ^Mozilla/4 gzip-only-text/html
BrowserMatch ^Mozilla/4.0[678] no-gzip
BrowserMatch MSIE !no-gzip !gzip-only-text/html
At first we probe for a User-Agent
string that indicates a Netscape Navigator version of 4.x. These versions cannot handle compression of types other than text/html
. The versions 4.06, 4.07 and 4.08 also have problems with decompressing html files. Thus, we completely turn off the deflate filter for them.
The third BrowserMatch
directive fixes the guessed identity of the user agent, because the Microsoft Internet Explorer identifies itself also as "Mozilla/4" but is actually able to handle requested compression. Therefore we match against the additional string "MSIE" (
means "word boundary") in the User-Agent
Header and turn off the restrictions defined before.
Note
The DEFLATE
filter is always inserted after RESOURCE filters like PHP or SSI. It never touches internal subrequests. Note
There is a environment variable
force-gzip
, set via
SetEnv
, which will ignore the accept-encoding setting of your browser and will send compressed output.
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 outputfilter 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.
Note on Content-Length
If you evaluate the request body yourself, don't trust the Content-Length
header! The Content-Length header reflects the length of the incoming data from the client and not the byte count of the decompressed data stream.
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.
Example
Header set Vary *
DeflateBufferSize Directive
The DeflateBufferSize
directive specifies the size in bytes of the fragments that zlib should compress at one time.
DeflateCompressionLevel Directive
The DeflateCompressionLevel
directive specifies what level of compression should be used, the higher the value, the better the compression, but the more CPU time is required to achieve this.
The value must between 1 (less compression) and 9 (more compression).
DeflateFilterNote Directive
The DeflateFilterNote
directive specifies that a note about compression ratios should be attached to the request. The name of the note is the value specified for the directive. You can use that note for statistical purposes by adding the value to your access log.
Example
DeflateFilterNote ratio
LogFormat '"%r" %b (%{ratio}n) "%{User-agent}i"' deflate
CustomLog logs/deflate_log deflate
If you want to extract more accurate values from your logs, you can use the type argument to specify the type of data left as note for logging. type can be one of:
Input
- Store the byte count of the filter's input stream in the note.
Output
- Store the byte count of the filter's output stream in the note.
Ratio
- Store the compression ratio (
output/input * 100
) in the note. This is the default, if the type argument is omitted.
Thus you may log it this way:
Accurate Logging
DeflateFilterNote Input instream
DeflateFilterNote Output outstream
DeflateFilterNote Ratio ratio
LogFormat '"%r" %{outstream}n/%{instream}n (%{ratio}n%%)' deflate
CustomLog logs/deflate_log deflate
See also
DeflateMemLevel Directive
The DeflateMemLevel
directive specifies how much memory should be used by zlib for compression (a value between 1 and 9).
DeflateWindowSize Directive
The DeflateWindowSize
directive specifies the zlib compression window size (a value between 1 and 15). Generally, the higher the window size, the higher can the compression ratio be expected.