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Apache模块 mod_filter
| 说明 | 根据上下文实际情况对输出过滤器进行动态配置 |
|---|---|
| 状态 | 基本(B) |
| 模块名 | filter_module |
| 源文件 | mod_filter.c |
| 兼容性 | Version 2.1 及以后的版本中可用 |
概述
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).
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
![[This image displays the traditional filter model]](../images/mod_filter_old.gif)
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.
![[This image shows the mod_filter model]](../images/mod_filter_new.gif)
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
FilterDeclaredirective 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
FilterProviderdirective registers a provider with a filter. The filter may have been declared withFilterDeclare; if not, FilterProvider will implicitly declare it with the default type AP_FTYPE_RESOURCE. The provider must have been registered withap_register_output_filterby some module. The remaining arguments toFilterProviderare 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
FilterChaindirective 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_filterin place ofAddOutputFilterByTypeFilterDeclare 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-transformheader 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和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 指令
| 说明 | Configure the filter chain |
|---|---|
| 语法 | FilterChain [+=-@!]filter-name ... |
| 作用域 | server config, virtual host, directory, .htaccess |
| 覆盖项 | Options |
| 状态 | 基本(B) |
| 模块 | 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 指令
| 说明 | Declare a smart filter |
|---|---|
| 语法 | FilterDeclare filter-name [type] |
| 作用域 | server config, virtual host, directory, .htaccess |
| 覆盖项 | Options |
| 状态 | 基本(B) |
| 模块 | 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和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或NETWORK.
FilterProtocol 指令
| 说明 | Deal with correct HTTP protocol handling |
|---|---|
| 语法 | FilterProtocol filter-name [provider-name]
proto-flags |
| 作用域 | server config, virtual host, directory, .htaccess |
| 覆盖项 | Options |
| 状态 | 基本(B) |
| 模块 | 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-transformheader. cache=no- The filter renders the output uncacheable (eg by introducing randomised content changes)
FilterProvider 指令
| 说明 | Register a content filter |
|---|---|
| 语法 | FilterProvider filter-name provider-name
[req|resp|env]=dispatch match |
| 作用域 | server config, virtual host, directory, .htaccess |
| 覆盖项 | Options |
| 状态 | 基本(B) |
| 模块 | 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.
dispatch argument is a string with optional
req=, resp=或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.
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 指令
| 说明 | Get debug/diagnostic information from
mod_filter
|
|---|---|
| 语法 | FilterTrace filter-name level |
| 作用域 | server config, virtual host, directory |
| 状态 | 基本(B) |
| 模块 | 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_filterwill 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.