Apache HTTP Server Version 2.4

Apache > HTTP Server > Documentation > Version 2.4 > Modules

# 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.

### Topics

- Supported Encodings
- Sample Configurations
- Enabling Compression
- Dealing with proxy servers
- Serving pre-compressed content

### Directives

### Bugfix checklist

- httpd changelog
- Known issues
- Report a bug

### See also

- Filters

## 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

### Compression and TLS

Some web applications are vulnerable to an information disclosure attack when a TLS connection carries deflate compressed data. For more information, review the details of the "BREACH" family of attacks.

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

### Compress only a few types

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

## Enabling Compression

### Compression and TLS

Some web applications are vulnerable to an information disclosure attack when a TLS connection carries deflate compressed data. For more information, review the details of the "BREACH" family of attacks.

### 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>

### Note

The `DEFLATE` filter is always inserted after RESOURCE filters like PHP or SSI. It never touches internal subrequests.

### Note

There is an 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 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.

### 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 *

## 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 and JS files if they exist
        # and the client accepts gzip.
        RewriteCond "%{HTTP:Accept-encoding}" "gzip"
        RewriteCond "%{REQUEST_FILENAME}\.gz" -s
        RewriteRule "^(.*)\.(css|js)"         "$1\.$2\.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>

## DeflateAlterETag Directive

Description: | How the outgoing ETag header should be modified during compression
Syntax: | `DeflateAlterETag AddSuffix|NoChange|Remove`
Default: | `DeflateAlterETag AddSuffix`
Context: | server config, virtual host
Status: | Extension
Module: | mod_deflate
Compatibility: | Available in Apache 2.4.58 and later

The `DeflateAlterETag` directive specifies how the ETag header should be altered when a response is compressed.

AddSuffix

  Append the compression method onto the end of the ETag, causing compressed and uncompressed representations to have unique ETags. This has been the default since 2.4.0, but prevents serving "HTTP Not Modified" (304) responses to conditional requests for compressed content.

NoChange

  Don't change the ETag on a compressed response. This was the default prior to 2.4.0, but does not satisfy the HTTP/1.1 property that all representations of the same resource have unique ETags.

Remove

  Remove the ETag header from compressed responses. This prevents some conditional requests from being possible, but avoids the shortcomings of the preceding options.

## 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. If the compressed response size is bigger than the one specified by this directive then httpd will switch to chunked encoding (HTTP header `Transfer-Encoding` set to `Chunked`), with the side effect of not setting any `Content-Length` HTTP header. This is particularly important when httpd works behind reverse caching proxies or when httpd is configured with `mod_cache` and `mod_cache_disk` because HTTP responses without any `Content-Length` header might not be cached.

## 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

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

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 a 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

- `mod_log_config`

## DeflateInflateLimitRequestBody Directive

Description: | Maximum size of inflated request bodies
Syntax: | `DeflateInflateLimitRequestBody value`
Default: | `None, but LimitRequestBody applies after deflation`
Context: | server config, virtual host, directory, .htaccess
Status: | Extension
Module: | mod_deflate
Compatibility: | 2.4.10 and later

The `DeflateInflateLimitRequestBody` directive specifies the maximum size of an inflated request body. If it is unset, `LimitRequestBody` is applied to the inflated body. A value of `0` means no limit is applied to the inflated body size (but other limits such as `LimitRequestBody` may still apply).

## DeflateInflateRatioBurst Directive

Description: | Maximum number of times the inflation ratio for request bodies can be crossed
Syntax: | `DeflateInflateRatioBurst value`
Default: | `DeflateInflateRatioBurst 3`
Context: | server config, virtual host, directory, .htaccess
Status: | Extension
Module: | mod_deflate
Compatibility: | 2.4.10 and later

The `DeflateInflateRatioBurst` directive specifies the maximum number of times the `DeflateInflateRatioLimit` can be crossed before terminating the request.

## DeflateInflateRatioLimit Directive

Description: | Maximum inflation ratio for request bodies
Syntax: | `DeflateInflateRatioLimit value`
Default: | `DeflateInflateRatioLimit 200`
Context: | server config, virtual host, directory, .htaccess
Status: | Extension
Module: | mod_deflate
Compatibility: | 2.4.10 and later

The `DeflateInflateRatioLimit` directive specifies the maximum ratio of deflated to inflated size of an inflated request body. This ratio is checked as the body is streamed in, and if crossed more than `DeflateInflateRatioBurst` times, the request will be terminated.

## 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.