Use IPv6 default route when no IPv4 is available

doc/scapy/usage.rst

@@ -225,6 +225,9 @@ 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

@@ -372,6 +372,8 @@ 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)
     routes = conf.route.routes[:]
@@ -383,6 +385,20 @@ def get_working_if():
         iface = resolve_iface(ifname)  # type: ignore
         if iface.is_valid():
             return iface
+
+    # IPv6
+    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 2) of the highest priority default
+        ifname = tmp_routes[-1][2]
+        iface = resolve_iface(ifname)
+        if iface.is_valid():
+            return iface
+
     # There is no hope left
     return resolve_iface(conf.loopback_name)