fix: bind to a specific local IPv6 address during pair verify

This will prevent future traffic from picking the "best" address to
communicate with an accessory. Current products tie sessions to the
original IPv6 address and return a CoAP error code 4.04 when a new
address attempts to continue the session.

It is likely that this could explode in fun, new ways: the DHCPv6 server
returning a new address, the host selecting a new random SLAAC address,
etc.

aiohomekit/controller/coap/connection.py

@@ -19,6 +19,7 @@
 import asyncio
 import logging
 import random
+import socket
 import struct
 from typing import Any
 import uuid
@@ -244,8 +245,10 @@ def __init__(self, owner, host, port):
         self.address = f"[{host}]:{port}"
         self.connection_lock = asyncio.Lock()
         self.enc_ctx = None
+        self.host = host
         self.owner = owner
         self.pair_setup_client = None
+        self.port = port
 
     async def reconnect_soon(self):
         if not self.enc_ctx:
@@ -322,14 +325,23 @@ async def do_pair_setup_finish(self, pin, salt, srpB):
 
         return pairing
 
+    def _get_local_ipv6_addr(self, remote):
+        s = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
+        s.connect(remote)
+        addr = s.getsockname()[0]
+        s.close()
+        return addr
+
     async def do_pair_verify(self, pairing_data):
         if self.is_connected:
             logger.warning("Connecting to connected device?")
             await self.enc_ctx.coap_ctx.shutdown()
             self.enc_ctx = None
 
+        local_address = self._get_local_ipv6_addr((self.host, self.port))
+
         root = resource.Site()
-        coap_client = await Context.create_server_context(root, bind=("::", 0))
+        coap_client = await Context.create_server_context(root, bind=(local_address, 0))
         uri = "coap://%s/2" % (self.address)
         logger.debug(f"Pair verify uri={uri}")
 

aiohomekit/controller/coap/pairing.py

@@ -53,6 +53,8 @@ def _async_endpoint_changed(self) -> None:
         self.connection.address = (
             f"[{self.description.address}]:{self.description.port}"
         )
+        self.connection.host = self.description.address
+        self.connection.port = self.description.port
         async_create_task(self.connection.reconnect_soon())
 
     @property