Make better error when we have no netmask

Fix #1541

tests/utils/test_interfaces.py

@@ -17,7 +17,7 @@
 
 import sys
 
-from gns3server.utils.interfaces import interfaces, is_interface_up
+from gns3server.utils.interfaces import interfaces, is_interface_up, has_netmask
 
 
 def test_interfaces():
@@ -29,6 +29,16 @@ def test_interfaces():
             assert interface["special"]
 
 
+def test_has_netmask():
+    if sys.platform.startswith("win"):
+        # No loopback
+        pass
+    elif sys.platform.startswith("darwin"):
+        assert has_netmask("lo0") is True
+    else:
+        assert has_netmask("lo") is True
+
+
 def test_is_interface_up():
     if sys.platform.startswith("win"):
         # is_interface_up() always returns True on Windows