diff --git a/README.md b/README.md
index e12e03d..0026767 100644
--- a/README.md
+++ b/README.md
@@ -41,11 +41,20 @@ use Rack::LiveReload, :min_delay => 500, ...
 
 ## How it works
 
-The necessary `script` tag to bring in a vendored copy of [livereload.js](https://github.com/livereload/livereload-js) is
+The necessary `script` tag to bring in a copy of [livereload.js](https://github.com/livereload/livereload-js) is
 injected right before the closing `head` tag in any `text/html` pages that come through. The `script` tag is built in
 such a way that the `HTTP_HOST` is used as the LiveReload host, so you can connect from external machines (say, to
 `mycomputer:3000` instead of `localhost:3000`) and as long as the LiveReload port is accessible from the external machine,
 you'll connect and be LiveReloading away!
 
+### Which LiveReload script does it use?
+
+* If you've got a LiveReload watcher running on the same machine as the app that responds
+  to `http://localhost:35729/livereload.js`, that gets used, with the hostname being changed when
+  injected into the HTML page.
+* If you don't, the copy vendored with rack-livereload is used.
+* You can force the use of either one (and save on the cost of checking to see if that file
+  is available) with the middleware option `:source => :vendored` or `:source => :livereload`.
+
 As usual, super-alpha!
 
diff --git a/lib/rack/livereload.rb b/lib/rack/livereload.rb
index 09b399e..1bfefdd 100644
--- a/lib/rack/livereload.rb
+++ b/lib/rack/livereload.rb
@@ -1,6 +1,7 @@
 module Rack
   class LiveReload
     LIVERELOAD_JS_PATH = '/__rack/livereload.js'
+    LIVERELOAD_LOCAL_URI = 'http://localhost:35729/livereload.js'
 
     attr_reader :app
 
@@ -9,6 +10,31 @@ module Rack
       @options = options
     end
 
+    def use_vendored?
+      return @use_vendored if @use_vendored
+
+      if @options[:source]
+        @use_vendored = (@options[:source] == :vendored)
+      else
+        require 'net/http'
+        require 'uri'
+
+        uri = URI.parse(LIVERELOAD_LOCAL_URI)
+
+        http = Net::HTTP.new(uri.host, uri.port)
+        http.read_timeout = 1
+
+        begin
+          http.send_request('GET', uri.path)
+          @use_vendored = false
+        rescue Timeout::Error, Errno::ECONNREFUSED
+          @use_vendored = true
+        end
+      end
+
+      @use_vendored
+    end
+
     def call(env)
       if env['PATH_INFO'] == LIVERELOAD_JS_PATH
         deliver_file(::File.expand_path('../../../js/livereload.js', __FILE__))
@@ -21,12 +47,14 @@ module Rack
 
           body.each do |line|
             if !headers['X-Rack-LiveReload'] && line['</head>']
-              src = LIVERELOAD_JS_PATH.dup
-              if @options[:host]
-                src << "?host=#{@options[:host]}"
+              host_to_use = @options[:host] || env['HTTP_HOST'].gsub(%r{:.*}, '')
+
+              if use_vendored?
+                src = LIVERELOAD_JS_PATH.dup + "?host=#{host_to_use}"
               else
-                src << "?host=#{env['HTTP_HOST'].gsub(%r{:.*}, '')}" if env['HTTP_HOST']
+                src = LIVERELOAD_LOCAL_URI.dup.gsub('localhost', host_to_use) + '?'
               end
+
               src << "&mindelay=#{@options[:min_delay]}" if @options[:min_delay]
               src << "&maxdelay=#{@options[:max_delay]}" if @options[:max_delay]
               src << "&port=#{@options[:port]}" if @options[:port]
diff --git a/rack-livereload.gemspec b/rack-livereload.gemspec
index abc8bfd..8c10272 100644
--- a/rack-livereload.gemspec
+++ b/rack-livereload.gemspec
@@ -28,6 +28,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "mocha"
   s.add_development_dependency "guard"
   s.add_development_dependency "guard-rspec"
+  s.add_development_dependency "webmock"
 
   s.add_runtime_dependency "rack"
 end
diff --git a/spec/rack/livereload_spec.rb b/spec/rack/livereload_spec.rb
index 3b1e9de..09421a7 100644
--- a/spec/rack/livereload_spec.rb
+++ b/spec/rack/livereload_spec.rb
@@ -1,7 +1,7 @@
 require 'spec_helper'
 
 describe Rack::LiveReload do
-  let(:middleware) { described_class.new(app) }
+  let(:middleware) { described_class.new(app, options) }
   let(:app) { stub }
 
   subject { middleware }
@@ -9,6 +9,37 @@ describe Rack::LiveReload do
   its(:app) { should == app }
 
   let(:env) { {} }
+  let(:options) { {} }
+
+  describe described_class::LIVERELOAD_LOCAL_URI do
+    context 'does not exist' do
+      before do
+        stub_request(:any, 'localhost:35729/livereload.js').to_timeout
+      end
+
+      it { should use_vendored }
+    end
+
+    context 'exists' do
+      before do
+        stub_request(:any, 'localhost:35729/livereload.js')
+      end
+
+      it { should_not use_vendored }
+    end
+
+    context 'specify vendored' do
+      let(:options) { { :source => :vendored } }
+
+      it { should use_vendored }
+    end
+
+    context 'specify LR' do
+      let(:options) { { :source => :livereload } }
+
+      it { should_not use_vendored }
+    end
+  end
 
   context 'not text/html' do
     let(:ret) { [ 200, { 'Content-Type' => 'image/png' }, [ '<head></head>' ] ] }
@@ -25,6 +56,7 @@ describe Rack::LiveReload do
   context 'text/html' do
     before do
       app.stubs(:call).with(env).returns([ 200, { 'Content-Type' => 'text/html', 'Content-Length' => 0 }, [ '<head></head>' ] ])
+      middleware.stubs(:use_vendored?).returns(true)
     end
 
     let(:host) { 'host' }
@@ -34,13 +66,26 @@ describe Rack::LiveReload do
     let(:body) { ret.last.join }
     let(:length) { ret[1]['Content-Length'] }
 
-    it 'should add the livereload js script tag' do
-      body.should include("script")
-      body.should include(described_class::LIVERELOAD_JS_PATH)
+    context 'vendored' do
+      it 'should add the vendored livereload js script tag' do
+        body.should include("script")
+        body.should include(described_class::LIVERELOAD_JS_PATH)
 
-      length.should == body.length.to_s
+        length.should == body.length.to_s
 
-      described_class::LIVERELOAD_JS_PATH.should_not include(host)
+        described_class::LIVERELOAD_JS_PATH.should_not include(host)
+      end
+    end
+
+    context 'not vendored' do
+      before do
+        middleware.stubs(:use_vendored?).returns(false)
+      end
+
+      it 'should add the LR livereload js script tag' do
+        body.should include("script")
+        body.should include(described_class::LIVERELOAD_LOCAL_URI.gsub('localhost', 'host'))
+      end
     end
 
     context 'set options' do
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 39811a9..96ab0b9 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,7 +1,16 @@
 require 'mocha'
+require 'webmock/rspec'
+
 require 'rack-livereload'
 
 RSpec.configure do |c|
   c.mock_with :mocha
 end
 
+module RSpec::Matchers
+  define :use_vendored do
+    match do |subject|
+      subject.use_vendored?
+    end
+  end
+end