fix: reject non-working socket configuration

agners · agners · commit aefa2985f40b · 2025-04-10T19:21:06.000+02:00
For IPv6, a shared listener/responder socket is not really possible when sending to link-local IPv6 multicast addresses (ff02::/16): The kernel needs to know which interface to use for routing. On IPv4, this is historically a bit different, the kernel just uses what it deems the primary/best route interface based on the routing table. But for IPv6, a message is rejected by Linux with OSError no 99 "Cannot assign requested address" and OSError no 65 "No route to host" on macOS. This change rejects InterfaceChoice.Default IPv6 only or IPv6/IPv4 dual-stack configurations. As a further cleanup, move the socket options for sending multicast packets out of the common socket creation code. For listen only sockets those settings are not needed. Since we allow shared listener/responder sockets, with IPv4 only sockets now, the macOS error address in #392 is not a problem anymore. Actually, we would like to get an exception in case we get into this combination, so remove the explicit exception handling.
diff --git a/src/zeroconf/_core.py b/src/zeroconf/_core.py
@@ -170,7 +170,15 @@ def __init__(
         :param apple_p2p: use AWDL interface (only macOS)
         """
         if ip_version is None:
-            ip_version = autodetect_ip_version(interfaces)
+            if interfaces is InterfaceChoice.Default:
+                ip_version = IPVersion.V4Only
+            else:
+                ip_version = autodetect_ip_version(interfaces)
+
+        # IPv6 requires an explicit interface to work. Use InterfaceChoice.All or pass
+        # a interface list explicitly.
+        if interfaces is InterfaceChoice.Default and ip_version != IPVersion.V4Only:
+            raise RuntimeError("`InterfaceChoice.Default` is only supported with IPv4.")
 
         self.done = False
 
diff --git a/src/zeroconf/_utils/net.py b/src/zeroconf/_utils/net.py
@@ -198,28 +198,33 @@ def set_so_reuseport_if_available(s: socket.socket) -> None:
             raise
 
 
-def set_mdns_port_socket_options_for_ip_version(
+def set_respond_socket_multicast_options(
     s: socket.socket,
-    bind_addr: tuple[str] | tuple[str, int, int],
     ip_version: IPVersion,
 ) -> None:
-    """Set ttl/hops and loop for mdns port."""
-    if ip_version != IPVersion.V6Only:
-        ttl = struct.pack(b"B", 255)
-        loop = struct.pack(b"B", 1)
+    """Set ttl/hops and loop for mDNS respond socket."""
+    if ip_version == IPVersion.V4Only:
         # OpenBSD needs the ttl and loop values for the IP_MULTICAST_TTL and
         # IP_MULTICAST_LOOP socket options as an unsigned char.
-        try:
-            s.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, ttl)
-            s.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_LOOP, loop)
-        except OSError as e:
-            if bind_addr[0] != "" or get_errno(e) != errno.EINVAL:  # Fails to set on MacOS
-                raise
-
-    if ip_version != IPVersion.V4Only:
+        ttl = struct.pack(b"B", 255)
+        loop = struct.pack(b"B", 1)
+        s.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, ttl)
+        s.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_LOOP, loop)
+    elif ip_version == IPVersion.V6Only:
         # However, char doesn't work here (at least on Linux)
         s.setsockopt(_IPPROTO_IPV6, socket.IPV6_MULTICAST_HOPS, 255)
         s.setsockopt(_IPPROTO_IPV6, socket.IPV6_MULTICAST_LOOP, True)
+    else:
+        # A shared sender socket is not really possible, especially with link-local
+        # multicast addresses (ff02::/16), the kernel needs to know which interface
+        # to use for routing.
+        #
+        # It seems that macOS even refuses to take IPv4 socket options if this is an
+        # AF_INET6 socket.
+        #
+        # In theory we could reconfigure the socket on each send, but that is not
+        # really practical for Python Zerconf.
+        raise RuntimeError("Dual-stack responder socket not supported")
 
 
 def new_socket(
@@ -244,9 +249,6 @@ def new_socket(
     s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
     set_so_reuseport_if_available(s)
 
-    if port == _MDNS_PORT:
-        set_mdns_port_socket_options_for_ip_version(s, bind_addr, ip_version)
-
     if apple_p2p:
         # SO_RECV_ANYIF = 0x1104
         # https://opensource.apple.com/source/xnu/xnu-4570.41.2/bsd/sys/socket.h
@@ -402,6 +404,7 @@ def new_respond_socket(
             socket.IP_MULTICAST_IF,
             socket.inet_aton(cast(str, interface)),
         )
+    set_respond_socket_multicast_options(respond_socket, IPVersion.V6Only if is_v6 else IPVersion.V4Only)
     return respond_socket
 
 
@@ -423,6 +426,8 @@ def create_sockets(
     if not unicast and interfaces is InterfaceChoice.Default:
         for interface in normalized_interfaces:
             add_multicast_member(cast(socket.socket, listen_socket), interface)
+        # Sent responder socket options to the dual-use listen socket
+        set_respond_socket_multicast_options(cast(socket.socket, listen_socket), ip_version)
         return listen_socket, [cast(socket.socket, listen_socket)]
 
     respond_sockets = []
diff --git a/tests/test_core.py b/tests/test_core.py
@@ -87,16 +87,16 @@ def test_close_multiple_times(self):
     def test_launch_and_close_v4_v6(self):
         rv = r.Zeroconf(interfaces=r.InterfaceChoice.All, ip_version=r.IPVersion.All)
         rv.close()
-        rv = r.Zeroconf(interfaces=r.InterfaceChoice.Default, ip_version=r.IPVersion.All)
-        rv.close()
+        with pytest.raises(RuntimeError):
+            r.Zeroconf(interfaces=r.InterfaceChoice.Default, ip_version=r.IPVersion.All)
 
     @unittest.skipIf(not has_working_ipv6(), "Requires IPv6")
     @unittest.skipIf(os.environ.get("SKIP_IPV6"), "IPv6 tests disabled")
     def test_launch_and_close_v6_only(self):
         rv = r.Zeroconf(interfaces=r.InterfaceChoice.All, ip_version=r.IPVersion.V6Only)
         rv.close()
-        rv = r.Zeroconf(interfaces=r.InterfaceChoice.Default, ip_version=r.IPVersion.V6Only)
-        rv.close()
+        with pytest.raises(RuntimeError):
+            r.Zeroconf(interfaces=r.InterfaceChoice.Default, ip_version=r.IPVersion.V6Only)
 
     @unittest.skipIf(sys.platform == "darwin", reason="apple_p2p failure path not testable on mac")
     def test_launch_and_close_apple_p2p_not_mac(self):
diff --git a/tests/utils/test_net.py b/tests/utils/test_net.py
@@ -162,23 +162,19 @@ def test_set_so_reuseport_if_available_not_present():
         netutils.set_so_reuseport_if_available(sock)
 
 
-def test_set_mdns_port_socket_options_for_ip_version():
+def test_set_respond_socket_multicast_options():
     """Test OSError with errno with EINVAL and bind address ''.
 
     from setsockopt IP_MULTICAST_TTL does not raise."""
     sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
 
-    # Should raise on EPERM always
-    with pytest.raises(OSError), patch("socket.socket.setsockopt", side_effect=OSError(errno.EPERM, None)):
-        netutils.set_mdns_port_socket_options_for_ip_version(sock, ("",), r.IPVersion.V4Only)
-
-    # Should raise on EINVAL always when bind address is not ''
+    # Should raise on EINVAL always
     with pytest.raises(OSError), patch("socket.socket.setsockopt", side_effect=OSError(errno.EINVAL, None)):
-        netutils.set_mdns_port_socket_options_for_ip_version(sock, ("127.0.0.1",), r.IPVersion.V4Only)
+        netutils.set_respond_socket_multicast_options(sock, r.IPVersion.V4Only)
 
-    # Should not raise on EINVAL when bind address is ''
-    with patch("socket.socket.setsockopt", side_effect=OSError(errno.EINVAL, None)):
-        netutils.set_mdns_port_socket_options_for_ip_version(sock, ("",), r.IPVersion.V4Only)
+    # Should raise on EINVAL always
+    with pytest.raises(RuntimeError):
+        netutils.set_respond_socket_multicast_options(sock, r.IPVersion.All)
 
 
 def test_add_multicast_member(caplog: pytest.LogCaptureFixture) -> None:
