Index: lib/filterClasses.js
===================================================================
--- a/lib/filterClasses.js
+++ b/lib/filterClasses.js
@@ -16,19 +16,21 @@
  */
 
 "use strict";
 
 /**
  * @fileOverview Definition of Filter class and its subclasses.
  */
 
-const {filterNotifier} = require("./filterNotifier");
 const {extend} = require("./coreUtils");
 const {filterToRegExp} = require("./common");
+const {suffixes} = require("./domain");
+const {filterNotifier} = require("./filterNotifier");
+
 const resources = require("../data/resources.json");
 
 /**
  * Map of internal resources for URL rewriting.
  * @type {Map.<string,string>}
  */
 let resourceMap = new Map(
   Object.keys(resources).map(key => [key, resources[key]])
@@ -604,27 +606,23 @@
     if (!docDomain)
       return domains.get("");
 
     if (docDomain[docDomain.length - 1] == ".")
       docDomain = docDomain.replace(/\.+$/, "");
 
     docDomain = docDomain.toLowerCase();
 
-    while (true)
+    for (docDomain of suffixes(docDomain))
     {
       let isDomainIncluded = domains.get(docDomain);
       if (typeof isDomainIncluded != "undefined")
         return isDomainIncluded;
+    }
 
-      let nextDot = docDomain.indexOf(".");
-      if (nextDot < 0)
-        break;
-      docDomain = docDomain.substr(nextDot + 1);
-    }
     return domains.get("");
   },
 
   /**
    * Checks whether this filter is active only on a domain and its subdomains.
    * @param {string} docDomain
    * @return {boolean}
    */