diff --git a/actionpack/CHANGELOG b/actionpack/CHANGELOG
index 07dc50d398..2aff18340b 100644
--- a/actionpack/CHANGELOG
+++ b/actionpack/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Add ability for relative_url_root to be specified via an environment variable RAILS_RELATIVE_URL_ROOT. [isaac@reuben.com, Nicholas Seckar]
+
 * Fixed link_to "somewhere", :post => true to produce valid XHTML by using the parentnode instead of document.body for the instant form #3007 [Bob Silva]
 
 * Added :function option to PrototypeHelper#observe_field/observe_form that allows you to call a function instead of submitting an ajax call as the trigger #4268 [jonathan@daikini.com]
diff --git a/actionpack/lib/action_controller/request.rb b/actionpack/lib/action_controller/request.rb
index 3bfc8f9385..566ae42ba0 100755
--- a/actionpack/lib/action_controller/request.rb
+++ b/actionpack/lib/action_controller/request.rb
@@ -163,9 +163,18 @@ module ActionController
     end
 
     # Returns the path minus the web server relative installation directory.
-    # This method returns nil unless the web server is apache.
+    # This can be set with the environment variable RAILS_RELATIVE_URL_ROOT.
+    # It can be automatically extracted for Apache setups. If the server is not
+    # Apache, this method returns an empty string.
     def relative_url_root
-      @@relative_url_root ||= server_software == 'apache' ? @env["SCRIPT_NAME"].to_s.sub(/\/dispatch\.(fcgi|rb|cgi)$/, '') : ''
+      @@relative_url_root ||= case
+        when @env["RAILS_RELATIVE_URL_ROOT"]
+          @env["RAILS_RELATIVE_URL_ROOT"]
+        when server_software == 'apache'
+          @env["SCRIPT_NAME"].to_s.sub(/\/dispatch\.(fcgi|rb|cgi)$/, '')
+        else
+          ''
+      end
     end
 
     # Returns the port number of this request as an integer.
diff --git a/actionpack/test/controller/request_test.rb b/actionpack/test/controller/request_test.rb
index a7623f3403..43cd8836fe 100644
--- a/actionpack/test/controller/request_test.rb
+++ b/actionpack/test/controller/request_test.rb
@@ -105,6 +105,19 @@ class RequestTest < Test::Unit::TestCase
     @request.relative_url_root = nil
     @request.env['SCRIPT_NAME'] = "/collaboration/hieraki"
     assert_equal "/collaboration/hieraki", @request.relative_url_root    
+    
+    @request.relative_url_root = nil
+    @request.env['SCRIPT_NAME'] = "/hieraki/dispatch.cgi"
+    @request.env['SERVER_SOFTWARE'] = 'lighttpd/1.2.3'
+    @request.env['RAILS_RELATIVE_URL_ROOT'] = "/hieraki"
+    assert_equal "/hieraki", @request.relative_url_root
+    
+    # @env overrides path guess
+    @request.relative_url_root = nil
+    @request.env['SCRIPT_NAME'] = "/hieraki/dispatch.cgi"
+    @request.env['SERVER_SOFTWARE'] = 'apache/1.2.3 some random text'
+    @request.env['RAILS_RELATIVE_URL_ROOT'] = "/real_url"
+    assert_equal "/real_url", @request.relative_url_root
   end
   
   def test_request_uri