Better algorithm for detecting the "external" network interface

Queries the kernel routing tables

src/calibre/utils/ip_routing.py

@@ -0,0 +1,68 @@
+#!/usr/bin/env python2
+# vim:fileencoding=utf-8
+# License: GPLv3 Copyright: 2016, Kovid Goyal <kovid at kovidgoyal.net>
+
+from __future__ import (unicode_literals, division, absolute_import,
+                        print_function)
+import subprocess, re
+from calibre.constants import iswindows, isosx
+
+def get_address_of_default_gateway(family='AF_INET'):
+    import netifaces
+    ip = netifaces.gateways()['default'][getattr(netifaces, family)][0]
+    if isinstance(ip, bytes):
+        ip = ip.decode('ascii')
+    return ip
+
+def get_addresses_for_interface(name, family='AF_INET'):
+    import netifaces
+    for entry in netifaces.ifaddresses(name)[getattr(netifaces, family)]:
+        if entry.get('broadcast'):  # Not a point-to-point address
+            addr = entry.get('addr')
+            if addr:
+                if isinstance(addr, bytes):
+                    addr = addr.decode('ascii')
+                yield addr
+
+if iswindows:
+
+    def get_default_route_src_address():
+        # Use -6 for IPv6 addresses
+        raw = subprocess.check_output('route -4 print 0.0.0.0'.split()).decode('utf-8', 'replace')
+        in_table = False
+        default_gateway = get_address_of_default_gateway()
+        for line in raw.splitlines():
+            parts = line.strip().split()
+            if in_table:
+                if len(parts) == 6:
+                    network, destination, netmask, gateway, interface, metric = parts
+                elif len(parts) == 5:
+                    destination, netmask, gateway, interface, metric = parts
+                if gateway == default_gateway:
+                    return interface
+            else:
+                if parts == 'Network Destination Netmask Gateway Interface Metric'.split():
+                    in_table = True
+
+
+elif isosx:
+
+    def get_default_route_src_address():
+        # Use -inet6 for IPv6
+        raw = subprocess.check_output('route -n get -inet default'.split()).decode('utf-8')
+        m = re.search(r'^\s*interface:\s*(\S+)\s*$', raw, flags=re.MULTILINE)
+        if m is not None:
+            interface = m.group(1)
+            for addr in get_addresses_for_interface(interface):
+                return addr
+else:
+
+    def get_default_route_src_address(to='1.1.1.1'):
+        # Use -6 for IPv6 addresses
+        raw = subprocess.check_output(('ip -4 route get ' + to).split()).decode('utf-8')
+        m = re.search('^%s\s+via\s+%s\s+.+?src\s+(.+)' % (to, get_address_of_default_gateway()), raw, flags=re.MULTILINE)
+        if m is not None:
+            return m.group(1)
+
+if __name__ == '__main__':
+    print(get_default_route_src_address())