Apache HTTP Server Version 2.1

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# Apache Module mod_filter

Available Languages: en

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

### 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).

### Directives

### Topics

- Smart Filtering
- Filter Declarations, Providers and Chains
- Configuring the Chain
- Examples
- Protocol Handling

## 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 any Request Header, Response Header or Environment Variable. This generalises the limited flexibility offered by `AddOutputFilterByType`, and fixes it to work correctly with dynamic content, regardless of the content generator. The ability to dispatch based on Environment Variables offers the full flexibility of configuration with `mod_rewrite` to anyone who needs it.

## 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 remaining arguments to `FilterProvider` are a dispatch criterion and a match string. The former may be an HTTP request or response header, an environment variable, or the Handler used by this request. The latter is matched to it for each request, to determine whether this provider will be used to implement the filter for this request.
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.

## Examples

Server side Includes (SSI)
  A simple case of using `mod_filter` in place of `AddOutputFilterByType`

  `FilterDeclare SSI
  FilterProvider SSI INCLUDES resp=Content-Type $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 $image
  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 up, 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.

## 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.

proto-flags is one or more of

`change=yes`
  The filter changes the content, including possibly the content length
`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 [req|resp|env]=dispatch match`
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 match declared here matches the value of the header or environment variable declared as dispatch.

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

The dispatch argument is a string with optional `req=`, `resp=` or `env=` prefix causing it to dispatch on (respectively) the request header, response header, or environment variable named. In the absence of a prefix, it defaults to a response header. A special case is the word `handler`, which causes `mod_filter` to dispatch on the content handler.

The match argument specifies a match that will be applied to the filter's dispatch criterion. The match may be a string match (exact match or substring), a regex, an integer (greater, lessthan or equals), or unconditional. The first characters of the match argument determines this:

First, if the first character is an exclamation mark (`!`), this reverses the rule, so the provider will be used if and only if the match fails.

Second, it interprets the first character excluding any leading `!` as follows:

Character | Description
--- | ---
(none) | exact match
`$` | substring match
`/` | regex match (delimited by a second `/`)
`=` | integer equality
`<` | integer less-than
`<=` | integer less-than or equal
`>` | integer greater-than
`>=` | integer greater-than or equal
`*` | Unconditional match

## 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.

Available Languages: en