diff --git a/cfg/ls336_cfg.py b/cfg/ls336_cfg.py
index cb328a4..0b857bb 100644
--- a/cfg/ls336_cfg.py
+++ b/cfg/ls336_cfg.py
@@ -1,10 +1,15 @@
+from os import environ
+
+# either change the uri or set the environment variable 'LS_URI'
+lakeshore_uri = environ.get('LS_URI', 'tcp://<host>:7777')
+
 Node('example_cryo.psi.ch',  # a globally unique identification
      'this is an example cryostat for the Frappy tutorial',  # describes the node
       interface='tcp://10767')  # you might choose any port number > 1024
 Mod('io',  # the name of the module
     'frappy_demo.lakeshore.LakeshoreIO',  # the class used for communication
     'communication to main controller',  # a description
-    uri='tcp://129.129.138.78:7777',  # the serial connection
+    uri=lakeshore_uri,  # the serial connection
     )
 Mod('T',
     'frappy_demo.lakeshore.TemperatureLoop',
diff --git a/frappy/lib/__init__.py b/frappy/lib/__init__.py
index ffe0b70..7a005b3 100644
--- a/frappy/lib/__init__.py
+++ b/frappy/lib/__init__.py
@@ -21,7 +21,7 @@
 # *****************************************************************************
 """Define helpers"""
 
-import os
+import re
 import importlib
 import linecache
 import socket
@@ -296,23 +296,31 @@ def formatException(cut=0, exc_info=None, verbose=False):
     return ''.join(res)
 
 
+HOSTNAMEPAT = re.compile(r'[a-z0-9_.-]+$', re.IGNORECASE)  # roughly checking for a valid hostname or ip address
+
+
 def parseHostPort(host, defaultport):
     """Parse host[:port] string and tuples
 
     Specify 'host[:port]' or a (host, port) tuple for the mandatory argument.
     If the port specification is missing, the value of the defaultport is used.
-    """
 
-    if isinstance(host, (tuple, list)):
-        host, port = host
-    elif ':' in host:
-        host, port = host.rsplit(':', 1)
-        port = int(port)
+    raises TypeError in case host is neither a string nor an iterable
+    raises ValueError in other cases of invalid arguments
+    """
+    if isinstance(host, str):
+        host, sep, port = host.partition(':')
+        if sep:
+            port = int(port)
+        else:
+            port = defaultport
     else:
-        port = defaultport
-    assert 0 < port < 65536
-    assert ':' not in host
-    return host, port
+        host, port = host
+    if not HOSTNAMEPAT.match(host):
+        raise ValueError('illegal host name %r' % host)
+    if 0 < port < 65536:
+        return host, port
+    raise ValueError('illegal port number: %r' % port)
 
 
 def tcpSocket(host, defaultport, timeout=None):
diff --git a/frappy/lib/asynconn.py b/frappy/lib/asynconn.py
index 97cfc37..2aae5da 100644
--- a/frappy/lib/asynconn.py
+++ b/frappy/lib/asynconn.py
@@ -52,6 +52,7 @@ class AsynConn:
     scheme = None
     SCHEME_MAP = {}
     connection = None  # is not None, if connected
+    HOSTNAMEPAT = re.compile(r'[a-z0-9_.-]+$', re.IGNORECASE)  # roughly checking if it is a valid hostname
 
     def __new__(cls, uri, end_of_line=b'\n', default_settings=None):
         scheme = uri.split('://')[0]
@@ -59,17 +60,16 @@ class AsynConn:
         if not iocls:
             # try tcp, if scheme not given
             try:
-                host_port = parseHostPort(uri, None)
-            except (ValueError, TypeError, AssertionError):
+                parseHostPort(uri, 1)  # check hostname only
+            except ValueError:
                 if 'COM' in uri:
                     raise ValueError("the correct uri for a COM port is: "
-                                     "'serial://COM<i>[?<option>=<value>[+<option>=value ...]]'") from None
+                                     "'serial://COM<i>[?<option>=<value>[&<option>=value ...]]'") from None
                 if '/dev' in uri:
                     raise ValueError("the correct uri for a serial port is: "
-                                     "'serial:///dev/<tty>[?<option>=<value>[+<option>=value ...]]'") from None
-                raise ValueError('invalid uri: %s' % uri) from None
+                                     "'serial:///dev/<tty>[?<option>=<value>[&<option>=value ...]]'") from None
+                raise ValueError('invalid hostname %r' % uri) from None
             iocls = cls.SCHEME_MAP['tcp']
-            uri = 'tcp://%s:%d' % host_port
         return object.__new__(iocls)
 
     def __init__(self, uri, end_of_line=b'\n', default_settings=None):
@@ -170,7 +170,6 @@ class AsynTcp(AsynConn):
         super().__init__(uri, *args, **kwargs)
         self.uri = uri
         if uri.startswith('tcp://'):
-            # should be the case always
             uri = uri[6:]
         try:
             self.connection = tcpSocket(uri, self.default_settings.get('port'), self.timeout)
diff --git a/test/test_lib.py b/test/test_lib.py
new file mode 100644
index 0000000..104d5c2
--- /dev/null
+++ b/test/test_lib.py
@@ -0,0 +1,41 @@
+#  -*- coding: utf-8 -*-
+# *****************************************************************************
+#
+# This program is free software; you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free Software
+# Foundation; either version 2 of the License, or (at your option) any later
+# version.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+# FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
+# details.
+#
+# You should have received a copy of the GNU General Public License along with
+# this program; if not, write to the Free Software Foundation, Inc.,
+# 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+#
+# Module authors:
+#   Markus Zolliker <markus.zolliker@psi.ch>
+#
+# *****************************************************************************
+
+import pytest
+from frappy.lib import parseHostPort
+
+
+@pytest.mark.parametrize('hostport, defaultport, result', [
+    (('box.psi.ch', 9999), 1, ('box.psi.ch', 9999)),
+    (('/dev/tty', 9999), 1, None),
+    ('localhost:10767', 1, ('localhost', 10767)),
+    ('www.psi.ch', 80, ('www.psi.ch', 80)),
+    ('/dev/ttyx:2089', 10767, None),
+    ('COM4:', 2089, None),
+    ('underscore_valid.123.hyphen-valid.com', 80, ('underscore_valid.123.hyphen-valid.com', 80)),
+])
+def test_parse_host(hostport, defaultport, result):
+    if result is None:
+        with pytest.raises(ValueError):
+            parseHostPort(hostport, defaultport)
+    else:
+        assert result == parseHostPort(hostport, defaultport)