better log messages when rejecting cross-origin requests

IPython/html/base/zmqhandlers.py

@@ -42,7 +42,11 b' class ZMQStreamHandler(websocket.WebSocketHandler):'
         host = self.request.headers.get("Host")
 
         # If no header is provided, assume we can't verify origin
-        if(origin is None or host is None):
+        if origin is None:
+            self.log.warn("Missing Origin header, rejecting WebSocket connection.")
+            return False
+        if host is None:
+            self.log.warn("Missing Host header, rejecting WebSocket connection.")
             return False
         
         origin = origin.lower()
@@ -54,13 +58,17 b' class ZMQStreamHandler(websocket.WebSocketHandler):'
         
         # Check CORS headers
         if self.allow_origin:
-            return self.allow_origin == origin
+            allow = self.allow_origin == origin
         elif self.allow_origin_pat:
-            return bool(self.allow_origin_pat.match(origin))
+            allow = bool(self.allow_origin_pat.match(origin))
         else:
             # No CORS headers deny the request
-            self.log.warn("Cross Origin WebSocket Attempt from %s", self.get_origin())
-            return False
+            allow = False
+        if not allow:
+            self.log.warn("Blocking Cross Origin WebSocket Attempt.  Origin: %s, Host: %s",
+                origin, host,
+            )
+        return allow
 
     def clear_cookie(self, *args, **kwargs):
         """meaningless for websockets"""