Apache Module mod_ext_filter

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	Pass the response body through an external program before delivery to the client
	Extension
	ext_filter_module
	mod_ext_filter.c

Summary

presents a simple and familiar programming model for . With this module, a program which reads from stdin and writes to stdout (i.e., a Unix-style filter command) can be a filter for Apache. This filtering mechanism is much slower than using a filter which is specially written for the Apache API and runs inside of the Apache server process, but it does have the following benefits:

• the programming model is much simpler
• any programming/scripting language can be used, provided that it allows the program to read from standard input and write to standard output
• existing programs can be used unmodified as Apache filters

Even when the performance characteristics are not suitable for production use, can be used as a prototype environment for filters.

Directives

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Topics

• 

See also

Generating HTML from some other type of response

# mod_ext_filter directive to define a filter
# to HTML-ize text/c files using the external
# program /usr/bin/enscript, with the type of
# the result set to text/html
ExtFilterDefine c-to-html mode=output \
intype=text/c outtype=text/html \
cmd="/usr/bin/enscript --color -W html -Ec -o - -"


# core directive to cause the new filter to
# be run on output
SetOutputFilter c-to-html

# mod_mime directive to set the type of .c
# files to text/c
AddType text/c .c

# mod_ext_filter directive to set the debug
# level just high enough to see a log message
# per request showing the configuration in force
ExtFilterOptions DebugLevel=1


Implementing a content encoding filter

Note: this gzip example is just for the purposes of illustration. Please refer to for a practical implementation.

# mod_ext_filter directive to define the external filter
ExtFilterDefine gzip mode=output cmd=/bin/gzip


# core directive to cause the gzip filter to be
# run on output
SetOutputFilter gzip

# mod_header directive to add
# "Content-Encoding: gzip" header field
Header set Content-Encoding gzip


Slowing down the server

# mod_ext_filter directive to define a filter
# which runs everything through cat; cat doesn't
# modify anything; it just introduces extra pathlength
# and consumes more resources
ExtFilterDefine slowdown mode=output cmd=/bin/cat \
preservescontentlength


# core directive to cause the slowdown filter to
# be run several times on output
#
SetOutputFilter slowdown;slowdown;slowdown


Using sed to replace text in the response

# mod_ext_filter directive to define a filter which
# replaces text in the response
#
ExtFilterDefine fixtext mode=output intype=text/html \
cmd="/bin/sed s/verdana/arial/g"


# core directive to cause the fixtext filter to
# be run on output
SetOutputFilter fixtext


Tracing another filter

# Trace the data read and written by mod_deflate
# for a particular client (IP 192.168.1.31)
# experiencing compression problems.
# This filter will trace what goes into mod_deflate.
ExtFilterDefine tracebefore \
cmd="/bin/tracefilter.pl /tmp/tracebefore" \
EnableEnv=trace_this_client

# This filter will trace what goes after mod_deflate.
# Note that without the ftype parameter, the default
# filter type of AP_FTYPE_RESOURCE would cause the
# filter to be placed *before* mod_deflate in the filter
# chain. Giving it a numeric value slightly higher than
# AP_FTYPE_CONTENT_SET will ensure that it is placed
# after mod_deflate.
ExtFilterDefine traceafter \
cmd="/bin/tracefilter.pl /tmp/traceafter" \
EnableEnv=trace_this_client ftype=21


SetEnvIf Remote_Addr 192.168.1.31 trace_this_client
SetOutputFilter tracebefore;deflate;traceafter


Here is the filter which traces the data:

#!/usr/local/bin/perl -w
use strict;

open(SAVE, ">$ARGV[0]")
or die "can't open $ARGV[0]: $?";

while () {
print SAVE $_;
print $_;
}

close(SAVE);

	Define an external filter
	ExtFilterDefine filtername parameters
	server config
	Extension
	mod_ext_filter

The ExtFilterDefine directive defines the characteristics of an external filter, including the program to run and its arguments.

filtername specifies the name of the filter being defined. This name can then be used in SetOutputFilter directives. It must be unique among all registered filters. At the present time, no error is reported by the register-filter API, so a problem with duplicate names isn't reported to the user.

Subsequent parameters can appear in any order and define the external command to run and certain other characteristics. The only required parameter is cmd=. These parameters are:

cmd=cmdline

The cmd= keyword allows you to specify the external command to run. If there are arguments after the program name, the command line should be surrounded in quotation marks (e.g., cmd="/bin/mypgm arg1 arg2". Normal shell quoting is not necessary since the program is run directly, bypassing the shell. Program arguments are blank-delimited. A backslash can be used to escape blanks which should be part of a program argument. Any backslashes which are part of the argument must be escaped with backslash themselves. In addition to the standard CGI environment variables, DOCUMENT_URI, DOCUMENT_PATH_INFO, and QUERY_STRING_UNESCAPED will also be set for the program.

mode=mode

mode should be output for now (the default). In the future, mode=input will be used to specify a filter for request bodies.

intype=imt

This parameter specifies the internet media type (i.e., MIME type) of documents which should be filtered. By default, all documents are filtered. If intype= is specified, the filter will be disabled for documents of other types.

outtype=imt

This parameter specifies the internet media type (i.e., MIME type) of filtered documents. It is useful when the filter changes the internet media type as part of the filtering operation. By default, the internet media type is unchanged.

PreservesContentLength

The PreservesContentLength keyword specifies that the filter preserves the content length. This is not the default, as most filters change the content length. In the event that the filter doesn't modify the length, this keyword should be specified.

ftype=filtertype

This parameter specifies the numeric value for filter type that the filter should be registered as. The default value, AP_FTYPE_RESOURCE, is sufficient in most cases. If the filter needs to operate at a different point in the filter chain than resource filters, then this parameter will be necessary. See the AP_FTYPE_foo definitions in util_filter.h for appropriate values.

disableenv=env

This parameter specifies the name of an environment variable which, if set, will disable the filter.

enableenv=env

This parameter specifies the name of an environment variable which must be set, or the filter will be disabled.

	Configure options
	ExtFilterOptions option [option] ...
	ExtFilterOptions DebugLevel=0 NoLogStderr
	directory
	Extension
	mod_ext_filter

The ExtFilterOptions directive specifies special processing options for . Option can be one of

DebugLevel=n

The DebugLevel keyword allows you to specify the level of debug messages generated by . By default, no debug messages are generated. This is equivalent to DebugLevel=0. With higher numbers, more debug messages are generated, and server performance will be degraded. The actual meanings of the numeric values are described with the definitions of the DBGLVL_ constants near the beginning of mod_ext_filter.c.

Note: The core directive should be used to cause debug messages to be stored in the Apache error log.

LogStderr | NoLogStderr

The LogStderr keyword specifies that messages written to standard error by the external filter program will be saved in the Apache error log. NoLogStderr disables this feature.

Example

ExtFilterOptions LogStderr DebugLevel=0

Messages written to the filter's standard error will be stored in the Apache error log. No debug messages will be generated by .

 

用apache 2的mod_ext_filter给html文件加广告。

用apache 2的mod_ext_filter给html文件加广告。 

想在静态页面上自动加个广告条，而不必手动修改每个静态页面。

按照这个需求在google上找了一段时候，不得要领。

后来想起来tomcat里有filter的概念的。

就到apache的文档里找。

瞎猫碰到了这个死耗子。

由于自己学习不认真，并且编程能力不强，笨拙的按照文档解决了这个问题。

个人感觉就是unix管道的理念。

 

服务器环境是centos 4.2 /apache 2.0.52

 

首先加载mod_ext_filter

在/etc/httpd/conf/httpd.conf

的Dynamic Shared Object (DSO) Support节

加入

Code:

 

LoadModule ext_filter_module modules/mod_ext_filter.so

 

 

然后定义filter的名字（advtext）和配置filter要调用程序的名字（gingeradv）。

Code:

 

ExtFilterDefine advtext mode=output intype=text/html cmd="/usr/bin/gingeradv"

 

 

在Directory标签里加入如下行。

Code:

 

SetOutputFilter advtext

 

 

加入后看起来如下

Code:

 

   Options FollowSymLinks

   AllowOverride None

   SetOutputFilter fixtext

 

 

用于加入广告的程序gingeradv是用perl写的，自己笨写的很垃圾。

Code:

 

#!/usr/bin/perl

my @lines = ;

open ADVLINE, "/var/www/html/abc.txt" or die "cant't find the adv files";

my @advlines = ;

print @lines,@advlines;

 

 

记得要给这个文件执行的权限。

Code:

 

chmod a+x gingeradv

 

 

要显示的广告保存在/var/www/html/abc.txt里。

我用于测试的这个是一个google的Google AdSense

abc.txt如下

Code:

 

 

 

然后重新启动apache服务器

Code:

 

apachectl graceful

 

 

在浏览器里访问效果如下。

 

 

 

但是这样虽然解决了问题，但perl程序好像写的很笨拙，估计没有效率。

还有这种工作方式每个请求都会调用这个perl脚本，估计访问量大的时候会不怎么好。

自己猜想写个动态加载的so才是正路，可惜不会，慢慢学习中，但愿我能学习。