diff --git a/config.default.json b/config.default.json
index 7230aa4f047e8a..2a3dfcc0a4a268 100644
--- a/config.default.json
+++ b/config.default.json
@@ -3,4 +3,5 @@
           "https":["auto"],
           "ws":["auto"]},
  "external_domain":"w3c-test.org",
+ "check_subdomains": true,
  "log_level":"debug"}
diff --git a/serve.py b/serve.py
index f037f4f2c27cdc..54194fea5f7a81 100644
--- a/serve.py
+++ b/serve.py
@@ -92,17 +92,14 @@ def kill(self):
         self.proc.terminate()
         self.proc.join()
 
-def probe_subdomains(config):
-    host = config["host"]
+def check_subdomains(config, subdomains):
     port = get_port()
     wrapper = ServerProc()
     wrapper.start(start_http_server, config, port)
 
     rv = {}
 
-    for subdomain in subdomains:
-        #This assumes that the tld is ascii-only or already in punycode
-        punycode = subdomain.encode("idna")
+    for subdomain, (punycode, host) in subdomains.iteritems():
         domain = "%s.%s" % (punycode, host)
         try:
             urllib2.urlopen("http://%s:%d/" % (domain, port))
@@ -121,6 +118,12 @@ def probe_subdomains(config):
 
     return rv
 
+def get_subdomains(config):
+    #This assumes that the tld is ascii-only or already in punycode
+    host = config["host"]
+    return {subdomain: (subdomain.encode("idna"), host)
+            for subdomain in subdomains}
+
 def start_servers(config, ports):
     servers = defaultdict(list)
 
@@ -235,7 +238,9 @@ def normalise_config(config, domains, ports):
 
 def start(config):
     ports = get_ports(config)
-    domains = probe_subdomains(config)
+    domains = get_subdomains(config)
+    if config["check_subdomains"]:
+        domains = check_subdomains(config, domains)
 
     config_ = normalise_config(config, domains, ports)