Added Dns::Server attribute to DNS REST router.

This is primarily intended to add clarity by reducing clutter. Moreover,
it also has the side effect of improving performance very slightly by
removing the overhead of calling #instance numerous times.

extensions/dns/rest/dns.rb

@@ -12,6 +12,7 @@ module BeEF
 
         # Filters out bad requests before performing any routing
         before do
+          @dns ||= BeEF::Extension::Dns::Server.instance
           config = BeEF::Core::Configuration.instance
 
           # Require a valid API token from a valid IP address
@@ -27,7 +28,7 @@ module BeEF
         # Returns the entire current DNS ruleset
         get '/ruleset' do
           begin
-            ruleset = BeEF::Extension::Dns::Server.instance.get_ruleset
+            ruleset = @dns.get_ruleset
             count = ruleset.length
 
             result = {}
@@ -45,7 +46,7 @@ module BeEF
           begin
             id = params[:id]
 
-            rule = BeEF::Extension::Dns::Server.instance.get_rule(id)
+            rule = @dns.get_rule(id)
             raise InvalidParamError, 'id' if rule.nil?
             halt 404 if rule.empty?
 
@@ -80,7 +81,7 @@ module BeEF
 
               raise InvalidJsonError, 'Wrong "resource" key passed to endpoint /api/dns/rule' unless valid_resources.include?(resource)
 
-              id = BeEF::Extension::Dns::Server.instance.add_rule(
+              id = @dns.add_rule(
                 :pattern => pattern,
                 :resource => eval("Resolv::DNS::Resource::IN::#{resource}"),
                 :response => response
@@ -105,7 +106,7 @@ module BeEF
           begin
             id = params[:id]
 
-            removed = BeEF::Extension::Dns::Server.instance.remove_rule!(id)
+            removed = @dns.remove_rule!(id)
             raise InvalidParamError, 'id' if removed.nil?
 
             result = {}