Use automatic server address if listen_on value is ''

src/calibre/gui2/actions/device.py

@@ -17,10 +17,10 @@ from calibre.utils.smtp import config as email_config
 
 def local_url_for_content_server():
     from calibre.srv.opts import server_config
-    from calibre.utils.network import is_ipv6_addr
+    from calibre.utils.network import is_ipv6_addr, get_fallback_server_addr
 
     opts = server_config()
-    interface = opts.listen_on or '0.0.0.0'
+    interface = opts.listen_on or get_fallback_server_addr()
 
     addr_map = {'0.0.0.0': '127.0.0.1',
                 '::':      '::1'}

src/calibre/gui2/preferences/server.py

@@ -1299,11 +1299,11 @@ class ConfigWidget(ConfigWidgetBase):
         self.stopping_msg.accept()
 
     def test_server(self):
-        from calibre.utils.network import is_ipv6_addr
+        from calibre.utils.network import is_ipv6_addr, get_fallback_server_addr
 
         prefix = self.advanced_tab.get('url_prefix') or ''
         protocol = 'https' if self.advanced_tab.has_ssl else 'http'
-        lo = self.advanced_tab.get('listen_on') or '0.0.0.0'
+        lo = self.advanced_tab.get('listen_on') or get_fallback_server_addr()
 
         addr_map = {'0.0.0.0': '127.0.0.1',
                     '::':      '::1'}

src/calibre/srv/opts.py

@@ -11,7 +11,6 @@ from collections import OrderedDict, namedtuple
 from functools import partial
 from itertools import zip_longest
 from operator import attrgetter
-from socket import has_dualstack_ipv6
 
 from calibre.constants import config_dir
 from calibre.utils.localization import _
@@ -111,7 +110,7 @@ raw_options = (
     ' there are more than this number of items. Set to zero to disable.'),
 
     _('The interface on which to listen for connections'),
-    'listen_on', '::' if has_dualstack_ipv6() else '0.0.0.0',
+    'listen_on', '',
     _('The default is to listen on all available IPv6 and IPv4 interfaces. You can change this to, for'
     ' example, "127.0.0.1" to only listen for connections from the local machine, or'
     ' to "::" to listen to all incoming IPv6 and IPv4 connections.'),

src/calibre/utils/network.py

@@ -116,3 +116,10 @@ def is_ipv6_addr(addr):
         return True
     except OSError:
         return False
+
+def get_fallback_server_addr():
+    from socket import has_dualstack_ipv6
+    if has_dualstack_ipv6():
+        return '::'
+    else:
+        return '0.0.0.0'