diff --git a/netwerk/dns/nsDNSService2.cpp b/netwerk/dns/nsDNSService2.cpp
index 3bc5b5e38c9..e87094db13d 100644
--- a/netwerk/dns/nsDNSService2.cpp
+++ b/netwerk/dns/nsDNSService2.cpp
@@ -687,6 +687,15 @@ nsDNSService::Resolve(const nsACString &hostname,
                       uint32_t          flags,
                       nsIDNSRecord    **result)
 {
+    NS_WARNING("Do not use synchronous DNS resolution! This API may be removed soon.");
+
+    // We will not allow this to be called on the main thread. This is transitional
+    // and a bit of a test for removing the synchronous API entirely.
+    if (NS_IsMainThread()) {
+        NS_ERROR("Synchronous DNS resolve failing - not allowed on the main thread!");
+        return NS_ERROR_FAILURE;
+    }
+
     // grab reference to global host resolver and IDN service.  beware
     // simultaneous shutdown!!
     nsRefPtr<nsHostResolver> res;
diff --git a/netwerk/dns/nsIDNSService.idl b/netwerk/dns/nsIDNSService.idl
index 159565de011..c24afb9a075 100644
--- a/netwerk/dns/nsIDNSService.idl
+++ b/netwerk/dns/nsIDNSService.idl
@@ -59,9 +59,8 @@ interface nsIDNSService : nsISupports
                             in nsresult          aReason);
     
     /**
-     * called to synchronously resolve a hostname.  warning this method may
-     * block the calling thread for a long period of time.  it is extremely
-     * unwise to call this function on the UI thread of an application.
+     * Called to synchronously resolve a hostname.  This method will fail
+     * if called from the main thread.
      *
      * @param aHostName
      *        the hostname or IP-address-literal to resolve.