Fallback to IPv6 default routes for network interface detection

doc/scapy/usage.rst

@@ -226,6 +226,9 @@ make\_table()    displays a table according to a lambda function
 Sending packets
 ---------------
 
+.. note::
+   Scapy automatically detects the network interface to be used by default, and stores this result in ``conf.iface``. Packets built by Scapy uses this variable to set relevant fields such as Ethernet source addresses. When sending packets, with functions such as ``send()``, Scapy will use the network interface stored in ``conf.iface``. This behavior can be changed using the ``iface=`` argument.  With IPv6 and link-local addresses, it is mandatory to setup both ``conf.iface`` and ``iface=`` the same value to get the desired result, as Scapy cannot find which interface to use for link-local communications.
+
 .. index::
    single: Sending packets, send
 

scapy/interfaces.py

@@ -372,8 +372,11 @@ def get_if_list():
 def get_working_if():
     # type: () -> NetworkInterface
     """Return an interface that works"""
+
+    # IPv4
     # return the interface associated with the route with smallest
     # mask (route by default if it exists)
+    ipv4_interface = resolve_iface(conf.loopback_name)
     routes = conf.route.routes[:]
     routes.sort(key=lambda x: x[1])
     ifaces = (x[3] for x in routes)
@@ -382,9 +385,28 @@ def get_working_if():
     for ifname in itertools.chain(ifaces, conf.ifaces.values()):
         iface = resolve_iface(ifname)  # type: ignore
         if iface.is_valid():
-            return iface
+            ipv4_interface = iface
+            break
+
+    if ipv4_interface.name != conf.loopback_name:
+        return ipv4_interface
+
+    # IPv6
+    if conf.route6:
+        routes_ipv6 = conf.route6.routes
+        default_routes_ipv6 = [r for r in routes_ipv6 if r[0] == "::"]
+        if default_routes_ipv6:
+            # Sort the default routes using the priority (at index -1)
+            tmp_routes = sorted(default_routes_ipv6, key=lambda r: r[-1])
+
+            # Return the interface (at index 3) of the highest priority default
+            ifname = tmp_routes[-1][3]
+            iface = resolve_iface(ifname)
+            if iface.is_valid():
+                return iface
+
     # There is no hope left
-    return resolve_iface(conf.loopback_name)
+    return ipv4_interface
 
 
 def get_working_ifaces():

scapy/route6.py

@@ -360,3 +360,6 @@ def route(self, dst="", dev=None, verbose=conf.verb):
 
 
 conf.route6 = Route6()
+
+# Reload interfaces
+conf.ifaces.reload()

test/regression.uts

@@ -334,6 +334,20 @@ assert "cuz you know the way to go" + conf.iface  # right +
 
 _test_get_working_if()
 
+# Test IPv6 default route interface selection
+ni4 = NetworkInterface(InterfaceProvider(), {"name": conf.loopback_name, "ips": ["127.0.0.1"], "mac": "aa:aa:aa:aa:aa:aa"})
+ni6 = NetworkInterface(InterfaceProvider(), {"name": "scapy0", "ips": ["::1"], "mac": "aa:aa:aa:aa:aa:aa"})
+
+import mock
+@mock.patch("scapy.interfaces.conf.ifaces.values")
+@mock.patch("scapy.interfaces.conf.route.routes", [(0, 0, '0.0.0.0', 'lo0', '127.0.0.1', 1)])
+@mock.patch("scapy.interfaces.conf.route6.routes", [("::", 0, "", ni6, [""], 0)])
+def _test_get_working_if_v6(ifaces_values):
+    ifaces_values.side_effect = lambda: []
+    assert get_working_if() == ni6
+
+_test_get_working_if_v6()
+
 = Test conf.ifaces
 
 conf.iface