diff --git a/httpd/debian/control b/httpd/debian/control
index 94689c37..bb86b020 100644
--- a/httpd/debian/control
+++ b/httpd/debian/control
@@ -9,7 +9,7 @@ XS-Ruby-Versions: all
 
 Package: symbiosis-httpd
 Architecture: all
-Depends: apache2, libapache2-mod-php7.0, webalizer, libapache2-mod-vhost-bytemark (>= 2015:0101), ${misc:Depends}, symbiosis-common (>= 2017:0328), ruby | ruby-interpreter, ruby-eventmachine
+Depends: apache2, libapache2-mod-php7.0, webalizer, libapache2-mod-vhost-bytemark (>= 2015:0101), ${misc:Depends}, symbiosis-common (>= 2017:0328), ruby | ruby-interpreter, ruby-eventmachine, ruby-diffy
 Recommends: php7.0-mysql | php7.0-mysqli |  php7.0-mysqlnd, php7.0-curl, php7.0-imagick, php7.0-mcrypt, php7.0-mhash, php7.0-xmlrpc, php7.0-gd, geoip-database
 Description: Tools to manage Apache virtual hosting in Symbiosis
  This package contains tools to manage the virtual hosting of websites
diff --git a/httpd/lib/symbiosis/config_files/apache.rb b/httpd/lib/symbiosis/config_files/apache.rb
index 0a723a6b..c1453c84 100644
--- a/httpd/lib/symbiosis/config_files/apache.rb
+++ b/httpd/lib/symbiosis/config_files/apache.rb
@@ -2,6 +2,7 @@
 require 'symbiosis/domain/http'
 require 'symbiosis/host'
 require 'tempfile'
+require 'diffy'
 
 module Symbiosis
   module ConfigFiles
@@ -36,6 +37,19 @@ def ok?
         end
       end
 
+      def diff
+        config = self.generate_config(self.template)
+
+        tempfile = Tempfile.new(File.basename(self.filename))
+        tempfile.puts(config)
+        tempfile.close(false)
+
+        fn = ( File.exists?(self.filename) ? self.filename : '/dev/null' )
+
+        Diffy::Diff.new(fn, tempfile.path, :source => 'files', :include_diff_info => true)
+      ensure
+        tmpfile.unlink
+      end
 
       #
       # This checks a site has its config file linked into the sites-enabled
@@ -275,7 +289,7 @@ def mandatory_ssl?
           false
         end
       end
-    
+
       #
       # This returns the FQDN
       #
diff --git a/httpd/sbin/symbiosis-httpd-configure b/httpd/sbin/symbiosis-httpd-configure
index a9926764..3f94b233 100755
--- a/httpd/sbin/symbiosis-httpd-configure
+++ b/httpd/sbin/symbiosis-httpd-configure
@@ -33,6 +33,9 @@
 #                                     non-SSL sites. Defaults to
 #                                     /etc/symbiosis/apache.d/zz-mass-hosting.template.erb
 #
+# --diff-only, -d   Show the differences in the new configuration, without
+#                   making any changes.
+#
 # --force, -f       Force the re-creation of all sites.
 #
 # --no-reload, -n   Do not reload Apache even if changes have taken place.
@@ -120,12 +123,14 @@ mass_non_ssl_template = nil
 mass_ssl_template     = nil
 apache2_dir      = nil
 no_reload       = false
+diff_only       = false
 
 opts = GetoptLong.new(
          [ '--help',             '-h', GetoptLong::NO_ARGUMENT ],
          [ '--manual',           '-m', GetoptLong::NO_ARGUMENT ],
          [ '--verbose',          '-v', GetoptLong::NO_ARGUMENT ],
          [ '--force',            '-f', GetoptLong::NO_ARGUMENT ],
+         [ '--diff-only',        '-d', GetoptLong::NO_ARGUMENT ],
          [ '--no-reload',        '-n', GetoptLong::NO_ARGUMENT],
          [ '--no-restart',       '-N', GetoptLong::NO_ARGUMENT],
          [ '--ssl-template',     '-s', GetoptLong::REQUIRED_ARGUMENT ],
@@ -156,6 +161,8 @@ begin
       # This is a legacy option.
     when '--no-restart'
       no_reload = true
+    when '--diff-only'
+      diff_only = true
     when '--help'
       help = true
     when '--manual'
@@ -442,6 +449,17 @@ configurations.each do |config|
     # This gets apache2 to check the configuration using a temporary file.
     #
     if recreate
+
+      #
+      # If we're only showing the diff, then check the config, and continue to
+      # the next domain.
+      #
+      if diff_only
+        puts config.diff.to_s(:color)
+        puts "\tW: Apache has rejected this configuration change" unless config.ok?
+        next
+      end
+
       if config.ok?
 
         verbose "\tWriting configuration"