utils.localinterfaces: Add PUBLIC_IPS to consolidate socket logic

Consolidating IP detection in this module will let us remove
socket.gethostbyname_ex calls from other modules and ensure that
everyone agrees on what our public IPs should be.

IPython/utils/localinterfaces.py

@@ -1,44 +1,51
 """Simple utility for building a list of local IPs using the socket module.
 This module defines two constants:
 
 LOCALHOST : The loopback interface, or the first interface that points to this
             machine.  It will *almost* always be '127.0.0.1'
 
 LOCAL_IPS : A list of IP addresses, loopback first, that point to this machine.
+
+PUBLIC_IPS : A list of public IP addresses that point to this machine.
+             Use these to tell remote clients where to find you.
 """
 #-----------------------------------------------------------------------------
 #  Copyright (C) 2010-2011  The IPython Development Team
 #
 #  Distributed under the terms of the BSD License.  The full license is in
 #  the file COPYING, distributed as part of this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 
 import socket
 
 from .data import uniq_stable
 
 #-----------------------------------------------------------------------------
 # Code
 #-----------------------------------------------------------------------------
 
 LOCAL_IPS = []
 try:
     LOCAL_IPS = socket.gethostbyname_ex('localhost')[2]
 except socket.gaierror:
     pass
 
+PUBLIC_IPS = []
 try:
-    LOCAL_IPS.extend(socket.gethostbyname_ex(socket.gethostname())[2])
+    PUBLIC_IPS = socket.gethostbyname_ex(socket.gethostname())[2]
 except socket.gaierror:
     pass
+else:
+    PUBLIC_IPS = uniq_stable(PUBLIC_IPS)
+    LOCAL_IPS.extend(PUBLIC_IPS)
 
 # include all-interface aliases: 0.0.0.0 and ''
 LOCAL_IPS.extend(['0.0.0.0', ''])
 
 LOCAL_IPS = uniq_stable(LOCAL_IPS)
 
 LOCALHOST = LOCAL_IPS[0]