Sam Pointer

Introduction

The mod_proxy_html module from Webthing is to the body of an HTTP request what mod_proxy is to the headers. It is especially useful for frigging with the output of a site or application that is being proxied and has made the interesting decision to use absolute rather than relative URLs in its output and links. Of course, if you are proxying, say, an internal application to the outside world, your internal DNS namespace is not going to be resolvable to visitors, and the links will not work.

For the uninitiated the mod_proxy_html homepage doesn’t give much clue about how to build and install the module. Here is a quick, platform independent, guide.

Assumptions

• You have a working build environment (`which gcc`, `which make`, etc.)
• You have libxml2 and libxml2-devel installed, and the includes are present in /usr/include/libxml2
• You are using Apache 2.x and its base directory is /usr/local/apache
• You have unpacked the source into /usr/local/src/mod_proxy_html, and the two mod_xml2enc source files into /usr/local/src/mod_xml2enc [1][2].

Satisfying Dependencies

The module mod_xml2enc is required to build mod_proxy_html. You don’t have to use this module if you only want to parse ASCII, but it’ll log an anoying message on every restart if you don’t. Given both of these things you may as well build it and use it.

Building mod_xml2enc

Within /usr/local/apache/bin execute the command:

./apxs -aic -I/usr/include/libxml2 /usr/local/src/mod_xml2enc/mod_xml2enc.c

The ‘c’ option actually does the compiling, whilst ‘a’ activates the module by placing the configuration directive into your httpd.conf file, and ‘i’ installs the compiled DSO into your Apache’s ‘modules’ subdirectory. As we’ll see in a minute, just loading the modules with ‘LoadModule’ (as added by the ‘a’ option) isn’t enough to get going.

Providing you see no error messages you’re good to go.

Building mod_proxy_html

Within /usr/local/apache/bin execute the command:

./apxs -aic -I/usr/include/libxml2 -I/usr/local/src/mod_xml2enc /usr/local/src/mod_proxy_html/mod_proxy_html.c

Again, all being well you’ll have the module compiled installed and activated. Notice how we need to include the mod_xml2enc directory too, it’s not a typo. However do not restart your Apache instance just yet.

Loading libxml2 into Apache

Before you restart you Apache instance, as is good practice, test the syntax with the httpd -t command:

# /usr/local/apache/bin/httpd -t
httpd: Syntax error on line 56 of /usr/local/www1/conf/httpd.conf: Cannot load /usr/local/www1/modules/mod_proxy_html.so into server: ld.so.1: httpd: fatal: relocation error: file /usr/local/www1/modules/mod_proxy_html.so: symbol htmlFreeParserCtxt: referenced symbol not found

This is because you need to also load the libxml2.so file into Apache. To do this, open up /usr/local/apache/conf/httpd.conf and locate the two LoadModule lines added by apxs:

LoadModule proxy_html_module modules/mod_proxy_html.so
LoadModule xml2enc_module modules/mod_xml2enc.so

Immediately before the first of these two lines, add:

LoadFile /usr/lib/libxml2.so

Test as before:

/usr/local/apache/bin/httpd -t
Syntax OK

You can now restart your Apache and follow the guides on the site to using the module.

Nothing works! - "No links configured: nothing for proxy-html filter to do"

If nothing seems to be rewritten on your site, try alerting the LogLevel statement in your httpd.conf to be ‘info’. If you then start seeing the above message in your logs you’d do well to take a look at this site, which will explain exactly why. Suffice to say, documentation in general is lacking for this module.