Fronting Tomcat with Apache HTTPD to Remove Ports and Context Paths

In this How-To guide, I show a combination of software and configuration to clean up URLs by removing the port numbers of the Java servlet engine (Tomcat) and the context path of the application. The goal is to create "cool URLs" that are are short (removing the unnecessary context path) and follow conventions (using the default port "80" rather than "8080"). OhioLINK also uses a custom access control module -- built for Apache HTTPD -- which makes the fronting of Apache HTTPD for Tomcat even more desirable.

Requirement

We're making use of the latest line of development for the Apache HTTPD series: version 2.2.x. The inclusion of mod_proxy_ajp -- replacing the custom "mod_jk" with a module that extends the httpd proxy engine -- in the latest major release of HTTPD makes our task much easier. This solution also uses HTTPD's mod_rewrite and an add-on module called mod_proxy_html. No additions or changes are needed to the stock Tomcat installation.

The Plan

There are two overall tasks that we're going to ask the HTTPD server to do. First, receive the incoming HTTP request and proxy it to the Tomcat servlet engine using the AJP protocol. Second, rewrite the URL paths of the headers and the X/HTML body from the Tomcat servlet engine to eliminate any instances of the context path. In a visual sense, what we are trying to is rewrite the path so it can be processed by Tomcat (the green box) then remove the extraneous parts of the path in the resulting headers and X/HTML (the red box):

Public Request URLs:	http://e.com		/remaining/path?and=params
URLs sent to Tomcat:	http://e.com	:8080	/context_path	/remaining/path?and=params
URLs as output by Tomcat:	http://e.com	:8080	/context_path	/next/page
URLs as seen by browser:	http://e.com		/next/page

The first half of this problem, modifying a request as they come into the Apache HTTPD server, will be handled by a mod_rewrite rule that rewrites the request to something Tomcat can understand then internally redirects it Tomcat via the AJP proxy. (Note that we are not using simply ProxyPass here because we want to send the request through the AJP interface to the Tomcat server, and RewriteRule allows us to do that with a [P] flag at the end of the RewriteRule line.) The second uses a combination of ProxyPassReverse (a part of Apache-supplied mod_proxy extension that adjusts the URL in the Location, Content-Location and URI headers), ProxyPassReverseCookiePath (also a part of the Apache-supplied mod_proxy extension; it rewrites the path string in Set-Cookie headers), and ProxyHTMLURLMap (from mod_proxy_html, a third-party extension that rewrites URLs inside X/HTML documents).

Preparations

The 'mod_proxy_html' extension is likely new to your Apache HTTPD installation, so we need to download the source, compile it, and move it into the proper directory. Fortunately, this is rather straight forward:

wget 'http://apache.webthing.com/mod_proxy_html/mod_proxy_html-2.5.2.c'
apxs -c -I/usr/include/libxml2 -i mod_proxy_html-2.5.2.c

Note that we are not using the mod_proxy_html author's 3.0 version here. In my set-up, the 3.0 version was causing Apache HTTPD to dump core on _every_request (whether proxied or not), and the prior release works just fine for our purposes. The apxs line will compile, link, and copy the resulting library to the Apache modules directory for us.

The Configuration

This is the contents a 'tomcat-proxy.conf' file that is placed in the 'conf.d' directory of the Apache HTTPD configuration directory (most likely /etc/httpd/conf.d/tomcat-proxy.conf, although your installation may vary).

#
#  Information about 'mod_proxy_html' can be found at 
#   http://apache.webthing.com/mod_proxy_html/
LoadFile    /usr/lib/libxml2.so
LoadModule  proxy_html_module    modules/mod_proxy_html-2.5.2.so

# DON'T TURN ProxyRequests ON!  Bad things will happen
# http://httpd.apache.org/docs/2.2/mod/mod_proxy.html#access
# http://www.akadia.com/services/prevent_abuse_proxy.html
ProxyRequests off

# Necessary to have mod_proxy_html do the rewriting
RequestHeader      unset  Accept-Encoding

# Rewrite the URLs to proxy ("[P]") into the Tomcat server
RewriteEngine     on
RewriteRule ^(/.*)      ajp://localhost:8009/context_path/$1    [P]

# Be prepared to rewrite the HTML/CSS files as they come back
# from Tomcat
SetOutputFilter proxy-html

# Rewrite JavaScript and CSS files in addition to HTML files
ProxyHTMLExtended on

# Output Strict XHTML (add "Legacy" to the end of the line below
# to output Transitional XHTML)
ProxyHTMLDoctype XHTML 

# Rewrite HTTP headers and HTML/CSS links for everything else
ProxyPassReverse /context_path/ /
ProxyPassReverseCookiePath /context_path/ /
ProxyHTMLURLMap /context_path/ /

That's pretty much all there is to it. You should note that mod_proxy_html, like any HTML scraper, requires modestly well-formed X/HTML. If the markup is bad, the output from mod_proxy_html is likely to be unpredictable.