Apache Module mod_ext_filter

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 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_headers directive to add

"Content-Encoding: gzip" header field Header set Content-Encoding gzip

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

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

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 19

presents a simple and familiar programming model for modextfilterfilters. 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:

Even when the performance characteristics are not suitable for production use,

can be used as a prototype environment for filters.modextfilter

#!/usr/local/bin/perl -w use strict; open(SAVE, ">$ARGV[0]") or die "can't open $ARGV[0]: $?"; while () { print SAVE $; print $; } close(SAVE);

filtername specifies the name of the filter being defined. This name can then be used in

directives. It must be unique among all registered filters. SetOutputFilterAt 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=

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 (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, DOCUMENTURI, DOCUMENTPATHINFO, and QUERYSTRING_UNESCAPED will also be set for the program.mode=mode

for filters which process the request. mode=input

is specified, the filter will be disabled for documents of other types.outtype=imt

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

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.Onfail=[abort|remove]

, the filter is removed and the request continues without it.ExtFilterOptions LogStderr

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Apache HTTP Server Version 2.4