diff --git a/lib/sinatra/base.rb b/lib/sinatra/base.rb
index 30aef2e7..981189a7 100644
--- a/lib/sinatra/base.rb
+++ b/lib/sinatra/base.rb
@@ -29,6 +29,10 @@ module Sinatra
       alias secure? ssl?
     end
 
+    def forwarded?
+      @env.include? "HTTP_X_FORWARDED_HOST"
+    end
+
     def route
       @route ||= begin
         path = Rack::Utils.unescape(path_info)
@@ -104,7 +108,16 @@ module Sinatra
     def uri(addr = nil, absolute = true, add_script_name = true)
       return addr if addr =~ /^https?:\/\//
       uri = [host = ""]
-      uri << request.host_with_port if absolute
+      if absolute
+        host << 'http'
+        host << 's' if request.secure?
+        host << "://"
+        if request.forwarded? or request.port != (request.secure? ? 443 : 80)
+          host << request.host_with_port
+        else
+          host << request.host
+        end
+      end
       uri << request.script_name.to_s if add_script_name
       uri << (addr ? addr : request.path_info).to_s
       File.join uri
diff --git a/test/helpers_test.rb b/test/helpers_test.rb
index 935cc2ab..bee2d005 100644
--- a/test/helpers_test.rb
+++ b/test/helpers_test.rb
@@ -111,6 +111,18 @@ class HelpersTest < Test::Unit::TestCase
       response = request.get('/', 'SERVER_PORT' => '444')
       assert_equal 'http://example.org:444/foo', response['Location']
     end
+
+    it 'works behind a reverse proxy' do
+      mock_app do
+        get '/' do
+          redirect '/foo'
+        end
+      end
+
+      request = Rack::MockRequest.new(@app)
+      response = request.get('/', 'HTTP_X_FORWARDED_HOST' => 'example.com', 'SERVER_PORT' => '8080')
+      assert_equal 'http://example.com/foo', response['Location']
+    end
   end
 
   describe 'error' do