diff --git a/hgext/narrow/narrowbundle2.py b/hgext/narrow/narrowbundle2.py
--- a/hgext/narrow/narrowbundle2.py
+++ b/hgext/narrow/narrowbundle2.py
@@ -31,8 +31,9 @@
     stringutil,
 )
 
-_NARROWACL_SECTION = 'narrowhgacl'
+_NARROWACL_SECTION = 'narrowacl'
 _CHANGESPECPART = 'narrow:changespec'
+_RESSPECS = 'narrow:responsespec'
 _SPECPART = 'narrow:spec'
 _SPECPART_INCLUDE = 'include'
 _SPECPART_EXCLUDE = 'exclude'
@@ -142,6 +143,10 @@
 
 @bundle2.parthandler(_SPECPART, (_SPECPART_INCLUDE, _SPECPART_EXCLUDE))
 def _handlechangespec_2(op, inpart):
+    # XXX: This bundle2 handling is buggy and should be removed after hg5.2 is
+    # released. New servers will send a mandatory bundle2 part named
+    # 'Narrowspec' and will send specs as data instead of params.
+    # Refer to issue5952 and 6019
     includepats = set(inpart.params.get(_SPECPART_INCLUDE, '').splitlines())
     excludepats = set(inpart.params.get(_SPECPART_EXCLUDE, '').splitlines())
     narrowspec.validatepatterns(includepats)
@@ -153,6 +158,21 @@
     op.repo.setnarrowpats(includepats, excludepats)
     narrowspec.copytoworkingcopy(op.repo)
 
+@bundle2.parthandler(_RESSPECS)
+def _handlenarrowspecs(op, inpart):
+    data = inpart.read()
+    inc, exc = data.split('\0')
+    includepats = set(inc.splitlines())
+    excludepats = set(exc.splitlines())
+    narrowspec.validatepatterns(includepats)
+    narrowspec.validatepatterns(excludepats)
+
+    if repository.NARROW_REQUIREMENT not in op.repo.requirements:
+        op.repo.requirements.add(repository.NARROW_REQUIREMENT)
+        op.repo._writerequirements()
+    op.repo.setnarrowpats(includepats, excludepats)
+    narrowspec.copytoworkingcopy(op.repo)
+
 @bundle2.parthandler(_CHANGESPECPART)
 def _handlechangespec(op, inpart):
     repo = op.repo
diff --git a/mercurial/exchange.py b/mercurial/exchange.py
--- a/mercurial/exchange.py
+++ b/mercurial/exchange.py
@@ -49,7 +49,7 @@
 urlerr = util.urlerr
 urlreq = util.urlreq
 
-_NARROWACL_SECTION = 'narrowhgacl'
+_NARROWACL_SECTION = 'narrowacl'
 
 # Maps bundle version human names to changegroup versions.
 _bundlespeccgversions = {'v1': '01',
@@ -2213,13 +2213,10 @@
 
     if (kwargs.get(r'narrow', False) and kwargs.get(r'narrow_acl', False)
         and (include or exclude)):
-        narrowspecpart = bundler.newpart('narrow:spec')
-        if include:
-            narrowspecpart.addparam(
-                'include', '\n'.join(include), mandatory=True)
-        if exclude:
-            narrowspecpart.addparam(
-                'exclude', '\n'.join(exclude), mandatory=True)
+        # this is mandatory because otherwise ACL clients won't work
+        narrowspecpart = bundler.newpart('Narrow:responsespec')
+        narrowspecpart.data = '%s\0%s' % ('\n'.join(include),
+                                           '\n'.join(exclude))
 
 @getbundle2partsgenerator('bookmarks')
 def _getbundlebookmarkpart(bundler, repo, source, bundlecaps=None,
diff --git a/tests/test-narrow-acl.t b/tests/test-narrow-acl.t
--- a/tests/test-narrow-acl.t
+++ b/tests/test-narrow-acl.t
@@ -10,7 +10,7 @@
   >   hg commit -m "Add $x"
   > done
   $ cat >> .hg/hgrc << EOF
-  > [narrowhgacl]
+  > [narrowacl]
   > default.includes=f1 f2
   > EOF
   $ hg serve -a localhost -p $HGPORT1 -d --pid-file=hg.pid