From 425690a9e2bef2e5b58fb34d342ee36a3ab8a107 Mon Sep 17 00:00:00 2001
From: Guillaume Valadon <guillaume@valadon.net>
Date: Sat, 16 Mar 2024 11:45:12 +0100
Subject: [PATCH] Use IPv6 default route when no IPv4 is available

---
 doc/scapy/usage.rst |  3 +++
 scapy/interfaces.py | 16 ++++++++++++++++
 test/regression.uts | 13 +++++++++++++
 3 files changed, 32 insertions(+)

diff --git a/doc/scapy/usage.rst b/doc/scapy/usage.rst
index 67b3f02a05c..cabd87a5e29 100644
--- a/doc/scapy/usage.rst
+++ b/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
    
diff --git a/scapy/interfaces.py b/scapy/interfaces.py
index f40f529ea8b..38ca1e0b6b1 100644
--- a/scapy/interfaces.py
+++ b/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 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)
 
diff --git a/test/regression.uts b/test/regression.uts
index 5b5737a570e..0c8a7893edb 100644
--- a/test/regression.uts
+++ b/test/regression.uts
@@ -334,6 +334,19 @@ assert "cuz you know the way to go" + conf.iface  # right +
 
 _test_get_working_if()
 
+# Test IPv6 default route interface selection
+ni = NetworkInterface(InterfaceProvider(), {"name": "scapy0", "ips": ["::1"], "mac": "aa:aa:aa:aa:aa:aa"})
+
+import mock
+@mock.patch("scapy.interfaces.conf.ifaces", {})
+@mock.patch("scapy.interfaces.conf.route.routes", [])
+@mock.patch("scapy.interfaces.conf.route6.routes", [("::", 0, "", ni, [""], 0)])
+def _test_get_working_if_v6():
+
+    assert get_working_if() == ni
+
+_test_get_working_if_v6()
+
 = Test conf.ifaces
 
 conf.iface