diff --git a/mitogen/core.py b/mitogen/core.py
index c228079d..a14286f9 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -1937,18 +1937,28 @@ class MitogenProtocol(Protocol):
     :class:`Protocol` implementing mitogen's :ref:`stream protocol
     <stream-protocol>`.
     """
-    #: If not :data:`None`, :class:`Router` stamps this into
-    #: :attr:`Message.auth_id` of every message received on this stream.
-    auth_id = None
-
     #: If not :data:`False`, indicates the stream has :attr:`auth_id` set and
     #: its value is the same as :data:`mitogen.context_id` or appears in
     #: :data:`mitogen.parent_ids`.
     is_privileged = False
 
-    def __init__(self, router, remote_id):
+    def __init__(self, router, remote_id, auth_id=None,
+                 local_id=None, parent_ids=None):
         self._router = router
         self.remote_id = remote_id
+        #: If not :data:`None`, :class:`Router` stamps this into
+        #: :attr:`Message.auth_id` of every message received on this stream.
+        self.auth_id = auth_id
+
+        if parent_ids is None:
+            parent_ids = mitogen.parent_ids
+        if local_id is None:
+            local_id = mitogen.context_id
+
+        self.is_privileged = (
+            (remote_id in parent_ids) or
+            auth_id in ([local_id] + parent_ids)
+        )
         self.sent_modules = set(['mitogen', 'mitogen.core'])
         self._input_buf = collections.deque()
         self._input_buf_len = 0
@@ -2800,8 +2810,8 @@ class Router(object):
     broker_exit_msg = 'Broker has exitted'
     no_route_msg = 'no route to %r, my ID is %r'
     unidirectional_msg = (
-        'routing mode prevents forward of message from context %d via '
-        'context %d'
+        'routing mode prevents forward of message from context %d to '
+        'context %d via context %d'
     )
 
     def __init__(self, broker):
@@ -3152,7 +3162,9 @@ class Router(object):
                 (in_stream.protocol.is_privileged or
                  out_stream.protocol.is_privileged):
             self._maybe_send_dead(msg, self.unidirectional_msg,
-                in_stream.protocol.remote_id, out_stream.protocol.remote_id)
+                in_stream.protocol.remote_id,
+                out_stream.protocol.remote_id,
+                mitogen.context_id)
             return
 
         out_stream.protocol._send(msg)
@@ -3641,7 +3653,12 @@ class ExternalContext(object):
         os.close(in_fd)
 
         out_fp = os.fdopen(os.dup(self.config.get('out_fd', 1)), 'wb', 0)
-        self.stream = MitogenProtocol.build_stream(self.router, parent_id)
+        self.stream = MitogenProtocol.build_stream(
+            self.router,
+            parent_id,
+            local_id=self.config['context_id'],
+            parent_ids=self.config['parent_ids']
+        )
         self.stream.accept(in_fp, out_fp)
         self.stream.name = 'parent'
         self.stream.receive_side.keep_alive = False
diff --git a/mitogen/unix.py b/mitogen/unix.py
index 645b061d..1af1c0ec 100644
--- a/mitogen/unix.py
+++ b/mitogen/unix.py
@@ -162,10 +162,9 @@ class Listener(mitogen.core.Protocol):
         stream = mitogen.core.MitogenProtocol.build_stream(
             router=self._router,
             remote_id=context_id,
+            auth_id=mitogen.context_id,
         )
         stream.name = u'unix_client.%d' % (pid,)
-        stream.protocol.auth_id = mitogen.context_id
-        stream.protocol.is_privileged = True
         stream.accept(sock, sock)
         LOG.debug('listener: accepted connection from PID %d: %s',
                   pid, stream.name)