I have an app that makes a HTTP GET request to a particular URL on the internet. But when the network is down (say, no public wifi - or my ISP is down, or some such thing), I get the following traceback at urllib.urlopen:

70, in get
    u = urllib2.urlopen(req)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 126, in urlopen
    return _opener.open(url, data, timeout)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 391, in open
    response = self._open(req, data)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 409, in _open
    '_open', req)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 369, in _call_chain
    result = func(*args)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 1161, in http_open
    return self.do_open(httplib.HTTPConnection, req)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 1136, in do_open
    raise URLError(err)
URLError: <urlopen error [Errno 8] nodename nor servname provided, or not known>

I want to print a friendly error to the user telling him that his network maybe down instead of this unfriendly "nodename nor servname provided" error message. Sure I can catch URLError, but that would catch every url error, not just the one related to network downtime.

I am not a purist, so even an error message like "The server example.com cannot be reached; either the server is indeed having problems or your network connection is down" would be nice. How do I go about selectively catching such errors? (For a start, if DNS resolution fails at urllib.urlopen, that can be reasonably assumed as network inaccessibility? If so, how do I "catch" it in the except block?)