Apache HTTP Server Version 2.0
Apache Module mod_deflate
| Description: | Compress content before it is delivered to the client |
|---|---|
| Status: | Extension |
| Module Identifier: | deflate_module |
| Source File: | mod_deflate.c |
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.
See also
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 \bMSIE !no-gzip !gzip-only-text/html
# NOTE: Due to a bug in mod_setenvif up to Apache 2.0.48
# the above regex won't work. You can use the following
# workaround to get the desired effect:
BrowserMatch \bMSI[E] !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:
SetOutputFilter DEFLATE
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 \bMSIE !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" (\b means
"word boundary") in the User-Agent Header and turn off
the restrictions defined before.
Note
TheDEFLATE filter is always inserted after RESOURCE
filters like PHP or SSI. It never touches internal subrequests.
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
| Description: | Fragment size to be compressed at one time by zlib |
|---|---|
| Syntax: | DeflateBufferSize value |
| Default: | DeflateBufferSize 8096 |
| Context: | server config, virtual host |
| Status: | Extension |
| Module: | mod_deflate |
The DeflateBufferSize directive specifies
the size in bytes of the fragments that zlib should compress at one
time.
DeflateCompressionLevel Directive
| Description: | How much compression do we apply to the output |
|---|---|
| Syntax: | DeflateCompressionLevel value |
| Default: | Zlib's default |
| Context: | server config, virtual host |
| Status: | Extension |
| Module: | mod_deflate |
| Compatibility: | This directive is available since Apache 2.0.45 |
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
| Description: | Places the compression ratio in a note for logging |
|---|---|
| Syntax: | DeflateFilterNote [type] notename |
| Context: | server config, virtual host |
| Status: | Extension |
| Module: | mod_deflate |
| Compatibility: | type is available since Apache 2.0.45 |
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
| Description: | How much memory should be used by zlib for compression |
|---|---|
| Syntax: | DeflateMemLevel value |
| Default: | DeflateMemLevel 9 |
| Context: | server config, virtual host |
| Status: | Extension |
| Module: | mod_deflate |
The DeflateMemLevel directive specifies
how much memory should be used by zlib for compression
(a value between 1 and 9).
DeflateWindowSize Directive
| Description: | Zlib compression window size |
|---|---|
| Syntax: | DeflateWindowSize value |
| Default: | DeflateWindowSize 15 |
| Context: | server config, virtual host |
| Status: | Extension |
| Module: | mod_deflate |
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.