diff --git a/mercurial/wireproto.py b/mercurial/wireproto.py
--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -1023,7 +1023,7 @@
                                       hint=bundle2requiredhint)
 
                 r = exchange.unbundle(repo, gen, their_heads, 'serve',
-                                      proto._client())
+                                      proto.client())
                 if util.safehasattr(r, 'addpart'):
                     # The return looks streamable, we are in the bundle2 case
                     # and should return a stream.
diff --git a/mercurial/wireprotoserver.py b/mercurial/wireprotoserver.py
--- a/mercurial/wireprotoserver.py
+++ b/mercurial/wireprotoserver.py
@@ -88,6 +88,10 @@
         won't be captured.
         """
 
+    @abc.abstractmethod
+    def client(self):
+        """Returns a string representation of this client (as bytes)."""
+
 def decodevaluefromheaders(req, headerprefix):
     """Decode a long value from multiple HTTP request headers.
 
@@ -164,7 +168,7 @@
             self._ui.fout = oldout
             self._ui.ferr = olderr
 
-    def _client(self):
+    def client(self):
         return 'remote:%s:%s:%s' % (
             self._req.env.get('wsgi.url_scheme') or 'http',
             urlreq.quote(self._req.env.get('REMOTE_HOST', '')),
@@ -399,7 +403,7 @@
     def mayberedirectstdio(self):
         yield None
 
-    def _client(self):
+    def client(self):
         client = encoding.environ.get('SSH_CLIENT', '').split(' ', 1)[0]
         return 'remote:ssh:' + client