diff --git a/.travis.yml b/.travis.yml
index 8154a77..ad9cd96 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -43,8 +43,8 @@ env:
         # -Werror: turn all warnings into fatal errors
         # -Werror=declaration-after-statement: strict ISO C90
         - CFLAGS="-std=c90 -Wno-int-in-bool-context -Werror -Werror=declaration-after-statement"
-        # pass CFLAGS and WITH_GCOV to tox tasks
-        - TOX_TESTENV_PASSENV="CFLAGS WITH_GCOV"
+        # pass CFLAGS, CI (for Travis CI) and WITH_GCOV to tox tasks
+        - TOX_TESTENV_PASSENV="CFLAGS CI WITH_GCOV"
 
 install:
   - pip install "pip>=7.1.0"
diff --git a/Lib/slapdtest.py b/Lib/slapdtest.py
index e0e57c7..ee1fbb2 100644
--- a/Lib/slapdtest.py
+++ b/Lib/slapdtest.py
@@ -18,6 +18,10 @@
 from logging.handlers import SysLogHandler
 import unittest
 
+# Switch off processing .ldaprc or ldap.conf before importing _ldap
+os.environ['LDAPNOINIT'] = '1'
+
+import ldap
 from ldap.compat import quote_plus
 
 # a template string for generating simple slapd.conf file
@@ -52,6 +56,41 @@
 
 LOCALHOST = '127.0.0.1'
 
+
+def identity(test_item):
+    """Identity decorator
+
+    """
+    return test_item
+
+
+def skip_unless_travis(reason):
+    """Skip test unless test case is executed on CI like Travis CI
+    """
+    if os.environ.get('CI', False):
+        return identity
+    else:
+        return unittest.skip(reason)
+
+
+def requires_tls(skip_nss=False):
+    """Decorator for TLS tests
+
+    Tests are not skipped on CI (e.g. Travis CI)
+    
+    :param skip_nss: Skip test when libldap is compiled with NSS as TLS lib
+    """
+    if not ldap.TLS_AVAIL:
+        return skip_unless_travis("test needs ldap.TLS_AVAIL")
+    elif skip_nss and ldap.get_option(ldap.OPT_X_TLS_PACKAGE) == 'MozNSS':
+        return skip_unless_travis(
+            "Test doesn't work correctly with Mozilla NSS, see "
+            "https://bugzilla.redhat.com/show_bug.cgi?id=1519167"
+        )
+    else:
+        return identity
+
+
 def combined_logger(
         log_name,
         log_level=logging.WARN,
diff --git a/Tests/t_cext.py b/Tests/t_cext.py
index 22eb895..be858e9 100644
--- a/Tests/t_cext.py
+++ b/Tests/t_cext.py
@@ -9,7 +9,8 @@
 
 import os
 import unittest
-from slapdtest import SlapdTestCase
+
+from slapdtest import SlapdTestCase, requires_tls
 
 # Switch off processing .ldaprc or ldap.conf before importing _ldap
 os.environ['LDAPNOINIT'] = '1'
@@ -717,12 +718,6 @@ def test_sasl(self):
             return
         # TODO
 
-    def test_tls(self):
-        l = self._open_conn()
-        if not self._require_attr(l, 'start_tls_s'):    # HAVE_TLS
-            return
-        # TODO
-
     def test_cancel(self):
         l = self._open_conn()
         if not self._require_attr(l, 'cancel'):         # FEATURE_CANCEL
@@ -807,7 +802,7 @@ def test_invalid_controls(self):
                 l.sasl_interactive_bind_s, 'who', 'SASLObject', post=(1,))
         self.assertInvalidControls(l.unbind_ext)
 
-    @unittest.skipUnless(_ldap.TLS_AVAIL, "needs tls")
+    @requires_tls(skip_nss=True)
     def test_tls_ext(self):
         l = self._open_conn(bind=False)
         # StartTLS needs LDAPv3
@@ -817,15 +812,17 @@ def test_tls_ext(self):
         l.set_option(_ldap.OPT_X_TLS_NEWCTX, 0)
         l.start_tls_s()
 
-    @unittest.skipUnless(_ldap.TLS_AVAIL, "needs tls")
+    @requires_tls(skip_nss=False)
     def test_tls_ext_noca(self):
         l = self._open_conn(bind=False)
         l.set_option(_ldap.OPT_PROTOCOL_VERSION, _ldap.VERSION3)
-        l.set_option(_ldap.OPT_X_TLS_NEWCTX, 0)
-        with self.assertRaises(_ldap.CONNECT_ERROR):
+        with self.assertRaises(_ldap.CONNECT_ERROR) as e:
             l.start_tls_s()
+        # some platforms return '(unknown error code)' as reason
+        if '(unknown error code)' not in str(e.exception):
+            self.assertIn('not trusted', str(e.exception))
 
-    @unittest.skipUnless(_ldap.TLS_AVAIL, "needs tls")
+    @requires_tls(skip_nss=True)
     def test_tls_ext_clientcert(self):
         l = self._open_conn(bind=False)
         l.set_option(_ldap.OPT_PROTOCOL_VERSION, _ldap.VERSION3)
@@ -836,5 +833,12 @@ def test_tls_ext_clientcert(self):
         l.set_option(_ldap.OPT_X_TLS_NEWCTX, 0)
         l.start_tls_s()
 
+    @requires_tls(skip_nss=False)
+    def test_tls_packages(self):
+        # libldap has tls_g.c, tls_m.c, and tls_o.c with ldap_int_tls_impl
+        package = _ldap.get_option(_ldap.OPT_X_TLS_PACKAGE)
+        self.assertIn(package, {"GnuTLS", "MozNSS", "OpenSSL"})
+
+
 if __name__ == '__main__':
     unittest.main()
diff --git a/Tests/t_ldap_sasl.py b/Tests/t_ldap_sasl.py
index 24c7b20..9acd051 100644
--- a/Tests/t_ldap_sasl.py
+++ b/Tests/t_ldap_sasl.py
@@ -14,7 +14,7 @@
 
 from ldap.ldapobject import SimpleLDAPObject
 import ldap.sasl
-from slapdtest import SlapdTestCase
+from slapdtest import SlapdTestCase, requires_tls
 
 
 LDIF = """
@@ -75,6 +75,7 @@ def test_external_ldapi(self):
             "dn:{}".format(self.server.root_dn.lower())
         )
 
+    @requires_tls(skip_nss=True)
     def test_external_tlscert(self):
         ldap_conn = self.ldap_object_class(self.server.ldap_uri)
         ldap_conn.set_option(ldap.OPT_X_TLS_CACERTFILE, self.server.cafile)
@@ -82,12 +83,7 @@ def test_external_tlscert(self):
         ldap_conn.set_option(ldap.OPT_X_TLS_KEYFILE, self.server.clientkey)
         ldap_conn.set_option(ldap.OPT_X_TLS_REQUIRE_CERT, ldap.OPT_X_TLS_HARD)
         ldap_conn.set_option(ldap.OPT_X_TLS_NEWCTX, 0)
-        try:
-            ldap_conn.start_tls_s()
-        except ldap.CONNECT_ERROR as e:
-            # TODO: On Fedora 27 OpenLDAP server refuses STARTTLS when test
-            # is executed with other tests,
-            raise unittest.SkipTest("buggy start_tls_s: {}".format(e))
+        ldap_conn.start_tls_s()
 
         auth = ldap.sasl.external()
         ldap_conn.sasl_interactive_bind_s("", auth)