Apache HTTP Server Version 2.4

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# Apache Module mod_filter

Description: | Context-sensitive smart filter configuration module
Status: | Base
Module Identifier: | filter_module
Source File: | mod_filter.c
Compatibility: | Version 2.1 and later

### Summary

This module enables smart, context-sensitive configuration of output content filters. For example, apache can be configured to process different content-types through different filters, even when the content-type is not known in advance (e.g. in a proxy).

`mod_filter` works by introducing indirection into the filter chain. Instead of inserting filters in the chain, we insert a filter harness which in turn dispatches conditionally to a filter provider. Any content filter may be used as a provider to `mod_filter`; no change to existing filter modules is required (although it may be possible to simplify them).

### Topics

- Smart Filtering
- Filter Declarations, Providers and Chains
- Configuring the Chain
- Filtering and Response Status
- Upgrading from Apache HTTP Server 2.2 Configuration
- Examples
- Protocol Handling

### Directives

### Bugfix checklist

- httpd changelog
- Known issues
- Report a bug

## Smart Filtering

In the traditional filtering model, filters are inserted unconditionally using `AddOutputFilter` and family. Each filter then needs to determine whether to run, and there is little flexibility available for server admins to allow the chain to be configured dynamically.

`mod_filter` by contrast gives server administrators a great deal of flexibility in configuring the filter chain. In fact, filters can be inserted based on complex boolean expressions This generalises the limited flexibility offered by `AddOutputFilterByType`.

## Filter Declarations, Providers and Chains

Figure 1: The traditional filter model

In the traditional model, output filters are a simple chain from the content generator (handler) to the client. This works well provided the filter chain can be correctly configured, but presents problems when the filters need to be configured dynamically based on the outcome of the handler.

Figure 2: The `mod_filter` model

`mod_filter` works by introducing indirection into the filter chain. Instead of inserting filters in the chain, we insert a filter harness which in turn dispatches conditionally to a filter provider. Any content filter may be used as a provider to `mod_filter`; no change to existing filter modules is required (although it may be possible to simplify them). There can be multiple providers for one filter, but no more than one provider will run for any single request.

A filter chain comprises any number of instances of the filter harness, each of which may have any number of providers. A special case is that of a single provider with unconditional dispatch: this is equivalent to inserting the provider filter directly into the chain.

## Configuring the Chain

There are three stages to configuring a filter chain with `mod_filter`. For details of the directives, see below.

Declare Filters
  The `FilterDeclare` directive declares a filter, assigning it a name and filter type. Required only if the filter is not the default type AP_FTYPE_RESOURCE.
Register Providers
  The `FilterProvider` directive registers a provider with a filter. The filter may have been declared with `FilterDeclare`; if not, FilterProvider will implicitly declare it with the default type AP_FTYPE_RESOURCE. The provider must have been registered with `ap_register_output_filter` by some module. The final argument to `FilterProvider` is an expression: the provider will be selected to run for a request if and only if the expression evaluates to true. The expression may evaluate HTTP request or response headers, environment variables, or the Handler used by this request. Unlike earlier versions, mod_filter now supports complex expressions involving multiple criteria with AND / OR logic (&& / ||) and brackets. The details of the expression syntax are described in the ap_expr documentation.
Configure the Chain
  The above directives build components of a smart filter chain, but do not configure it to run. The `FilterChain` directive builds a filter chain from smart filters declared, offering the flexibility to insert filters at the beginning or end of the chain, remove a filter, or clear the chain.

## Filtering and Response Status

mod_filter normally only runs filters on responses with HTTP status 200 (OK). If you want to filter documents with other response statuses, you can set the filter-errordocs environment variable, and it will work on all responses regardless of status. To refine this further, you can use expression conditions with `FilterProvider`.

## Upgrading from Apache HTTP Server 2.2 Configuration

The `FilterProvider` directive has changed from httpd 2.2: the match and dispatch arguments are replaced with a single but more versatile expression. In general, you can convert a match/dispatch pair to the two sides of an expression, using something like:

`"dispatch = 'match'"`

The Request headers, Response headers and Environment variables are now interpreted from syntax %{req:foo}, %{resp:foo} and %{env:foo} respectively. The variables %{HANDLER} and %{CONTENT_TYPE} are also supported.

Note that the match no longer support substring matches. They can be replaced by regular expression matches.

## Examples

Server side Includes (SSI)
  A simple case of replacing `AddOutputFilterByType`

      FilterDeclare SSI
      FilterProvider SSI INCLUDES "%{CONTENT_TYPE} =~ m|^text/html|"
      FilterChain SSI

Server side Includes (SSI)
  The same as the above but dispatching on handler (classic SSI behaviour; .shtml files get processed).

      FilterProvider SSI INCLUDES "%{HANDLER} = 'server-parsed'"
      FilterChain SSI

Emulating mod_gzip with mod_deflate
  Insert INFLATE filter only if "gzip" is NOT in the Accept-Encoding header. This filter runs with ftype CONTENT_SET.

      FilterDeclare gzip CONTENT_SET
      FilterProvider gzip inflate "%{req:Accept-Encoding} !~ /gzip/"
      FilterChain gzip

Image Downsampling
  Suppose we want to downsample all web images, and have filters for GIF, JPEG and PNG.

      FilterProvider unpack jpeg_unpack "%{CONTENT_TYPE} = 'image/jpeg'"
      FilterProvider unpack gif_unpack  "%{CONTENT_TYPE} = 'image/gif'"
      FilterProvider unpack png_unpack  "%{CONTENT_TYPE} = 'image/png'"

      FilterProvider downsample downsample_filter "%{CONTENT_TYPE} = m|^image/(jpeg|gif|png)|"
      FilterProtocol downsample "change=yes"

      FilterProvider repack jpeg_pack "%{CONTENT_TYPE} = 'image/jpeg'"
      FilterProvider repack gif_pack  "%{CONTENT_TYPE} = 'image/gif'"
      FilterProvider repack png_pack  "%{CONTENT_TYPE} = 'image/png'"
      <Location "/image-filter">
          FilterChain unpack downsample repack
      </Location>

## Protocol Handling

Historically, each filter is responsible for ensuring that whatever changes it makes are correctly represented in the HTTP response headers, and that it does not run when it would make an illegal change. This imposes a burden on filter authors to re-implement some common functionality in every filter:

- Many filters will change the content, invalidating existing content tags, checksums, hashes, and lengths.
- Filters that require an entire, unbroken response in input need to ensure they don't get byteranges from a backend.
- Filters that transform output in a filter need to ensure they don't violate a `Cache-Control: no-transform` header from the backend.
- Filters may make responses uncacheable.

`mod_filter` aims to offer generic handling of these details of filter implementation, reducing the complexity required of content filter modules. This is work-in-progress; the `FilterProtocol` implements some of this functionality for back-compatibility with Apache 2.0 modules. For httpd 2.1 and later, the `ap_register_output_filter_protocol` and `ap_filter_protocol` API enables filter modules to declare their own behaviour.

At the same time, `mod_filter` should not interfere with a filter that wants to handle all aspects of the protocol. By default (i.e. in the absence of any `FilterProtocol` directives), `mod_filter` will leave the headers untouched.

At the time of writing, this feature is largely untested, as modules in common use are designed to work with 2.0. Modules using it should test it carefully.

## AddOutputFilterByType Directive

Description: | assigns an output filter to a particular media-type
Syntax: | `AddOutputFilterByType filter[;filter...] media-type [media-type] ...`
Context: | server config, virtual host, directory, .htaccess
Override: | FileInfo
Status: | Base
Module: | mod_filter
Compatibility: | Had severe limitations before being moved to `mod_filter` in version 2.3.7

This directive activates a particular output filter for a request depending on the response media-type.

The following example uses the `DEFLATE` filter, which is provided by `mod_deflate`. It will compress all output (either static or dynamic) which is labeled as `text/html` or `text/plain` before it is sent to the client.

    AddOutputFilterByType DEFLATE text/html text/plain

If you want the content to be processed by more than one filter, their names have to be separated by semicolons. It's also possible to use one `AddOutputFilterByType` directive for each of these filters.

The configuration below causes all script output labeled as `text/html` to be processed at first by the `INCLUDES` filter and then by the `DEFLATE` filter.

    <Location "/cgi-bin/">
        Options Includes
        AddOutputFilterByType INCLUDES;DEFLATE text/html
    </Location>

### See also

- `AddOutputFilter`
- `SetOutputFilter`
- filters

## FilterChain Directive

Description: | Configure the filter chain
Syntax: | `FilterChain [+=-@!]filter-name ...`
Context: | server config, virtual host, directory, .htaccess
Override: | Options
Status: | Base
Module: | mod_filter

This configures an actual filter chain, from declared filters. `FilterChain` takes any number of arguments, each optionally preceded with a single-character control that determines what to do:

`+filter-name`
  Add filter-name to the end of the filter chain
`@filter-name`
  Insert filter-name at the start of the filter chain
`-filter-name`
  Remove filter-name from the filter chain
`=filter-name`
  Empty the filter chain and insert filter-name
`!`
  Empty the filter chain
`filter-name`
  Equivalent to `+filter-name`

## FilterDeclare Directive

Description: | Declare a smart filter
Syntax: | `FilterDeclare filter-name [type]`
Context: | server config, virtual host, directory, .htaccess
Override: | Options
Status: | Base
Module: | mod_filter

This directive declares an output filter together with a header or environment variable that will determine runtime configuration. The first argument is a filter-name for use in `FilterProvider`, `FilterChain` and `FilterProtocol` directives.

The final (optional) argument is the type of filter, and takes values of `ap_filter_type` - namely `RESOURCE` (the default), `CONTENT_SET`, `PROTOCOL`, `TRANSCODE`, `CONNECTION` or `NETWORK`.

## FilterProtocol Directive

Description: | Deal with correct HTTP protocol handling
Syntax: | `FilterProtocol filter-name [provider-name] proto-flags`
Context: | server config, virtual host, directory, .htaccess
Override: | Options
Status: | Base
Module: | mod_filter

This directs `mod_filter` to deal with ensuring the filter doesn't run when it shouldn't, and that the HTTP response headers are correctly set taking into account the effects of the filter.

There are two forms of this directive. With three arguments, it applies specifically to a filter-name and a provider-name for that filter. With two arguments it applies to a filter-name whenever the filter runs any provider.

Flags specified with this directive are merged with the flags that underlying providers may have registerd with `mod_filter`. For example, a filter may internally specify the equivalent of `change=yes`, but a particular configuration of the module can override with `change=no`.

proto-flags is one or more of

`change=yes|no`
  Specifies whether the filter changes the content, including possibly the content length. The "no" argument is supported in 2.4.7 and later.
`change=1:1`
  The filter changes the content, but will not change the content length
`byteranges=no`
  The filter cannot work on byteranges and requires complete input
`proxy=no`
  The filter should not run in a proxy context
`proxy=transform`
  The filter transforms the response in a manner incompatible with the HTTP `Cache-Control: no-transform` header.
`cache=no`
  The filter renders the output uncacheable (eg by introducing randomised content changes)

## FilterProvider Directive

Description: | Register a content filter
Syntax: | `FilterProvider filter-name provider-name expression`
Context: | server config, virtual host, directory, .htaccess
Override: | Options
Status: | Base
Module: | mod_filter

This directive registers a provider for the smart filter. The provider will be called if and only if the expression declared evaluates to true when the harness is first called.

provider-name must have been registered by loading a module that registers the name with `ap_register_output_filter`.

expression is an ap_expr.

### See also

- Expressions in Apache HTTP Server, for a complete reference and examples.
- `mod_include`

## FilterTrace Directive

Description: | Get debug/diagnostic information from `mod_filter`
Syntax: | `FilterTrace filter-name level`
Context: | server config, virtual host, directory
Status: | Base
Module: | mod_filter

This directive generates debug information from `mod_filter`. It is designed to help test and debug providers (filter modules), although it may also help with `mod_filter` itself.

The debug output depends on the level set:

`0` (default)
  No debug information is generated.
`1`
  `mod_filter` will record buckets and brigades passing through the filter to the error log, before the provider has processed them. This is similar to the information generated by mod_diagnostics.
`2` (not yet implemented)
  Will dump the full data passing through to a tempfile before the provider. For single-user debug only; this will not support concurrent hits.