René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Can't receive replies to multicast UDP packets","articleBody":"It doesn't appear to be possible to receive replies to multicast UDP messages.  Server-side multicast does work, but only with a bit of extra config.\r\n\r\nGiven the following (working) server code, adapted from the UDP Echo example:\r\n\r\n```\r\nimport asyncio\r\nimport logging\r\nimport socket\r\nimport struct\r\n\r\nBROADCAST_PORT = 1910\r\nBROADCAST_ADDR = \"239.255.255.250\"\r\n\r\n\r\nclass MulticastServerProtocol:\r\n\r\n    def connection_made(self, transport):\r\n        self.transport = transport\r\n\r\n\r\n    def datagram_received(self, data, addr):\r\n        print('Received {!r} from {!r}'.format(data, addr))\r\n        data = \"I received {!r}\".format(data).encode(\"ascii\")\r\n        print('Send {!r} to {!r}'.format(data, addr))\r\n        self.transport.sendto(data, addr)\r\n\r\n\r\n\r\nloop = asyncio.get_event_loop()\r\nloop.set_debug(True)\r\nlogging.basicConfig(level=logging.DEBUG)\r\n\r\n\r\nsock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)\r\nsock.bind(('', BROADCAST_PORT))\r\ngroup = socket.inet_aton(BROADCAST_ADDR)\r\nmreq = struct.pack('4sL', group, socket.INADDR_ANY)\r\nsock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)\r\nsock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)\r\nsock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)\r\n\r\nlisten = loop.create_datagram_endpoint(\r\n    MulticastServerProtocol,\r\n    sock=sock,\r\n)\r\ntransport, protocol = loop.run_until_complete(listen)\r\n\r\nloop.run_forever()\r\nloop.close()\r\n```\r\n\r\n...the following _non-asyncio_ client code sends a broadcast packet and correctly receives the responses:\r\n\r\n```\r\nimport socket\r\nimport struct\r\nimport sys\r\n\r\nBROADCAST_PORT = 1910\r\nBROADCAST_ADDR = \"239.255.255.250\"\r\n\r\n\r\nsock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)\r\nsock.settimeout(3)\r\nttl = struct.pack('b', 1)\r\nsock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, ttl)\r\n\r\ntry:\r\n    sent = sock.sendto(\r\n        sys.argv[1].encode(\"ascii\"),\r\n        (BROADCAST_ADDR, BROADCAST_PORT)\r\n    )\r\n\r\n    while True:\r\n        try:\r\n            data, server = sock.recvfrom(1024)\r\n        except socket.timeout:\r\n            break\r\n        else:\r\n            print(\"Reply from {}: {!r}\".format(server, data))\r\n\r\nfinally:\r\n    sock.close()\r\n```\r\n\r\nHowever, the following code, which I'm pretty sure is the asyncio equivalent, sends out the mutlicast packet correctly but never receives a response:\r\n\r\n```\r\nimport asyncio\r\nimport socket\r\nimport struct\r\nimport sys\r\n\r\nBROADCAST_PORT = 1910\r\nBROADCAST_ADDR = \"239.255.255.250\"\r\n\r\n\r\n\r\nclass DiscoveryClientProtocol:\r\n    def __init__(self, loop):\r\n        self.loop = loop\r\n        self.transport = None\r\n\r\n    def connection_made(self, transport):\r\n        self.transport = transport\r\n        sock = self.transport.get_extra_info('socket')\r\n        sock.settimeout(3)\r\n        ttl = struct.pack('b', 1)\r\n        sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, ttl)\r\n\r\n        self.transport.sendto(sys.argv[1].encode(\"ascii\"))\r\n\r\n    def datagram_received(self, data, addr):\r\n        print(\"Reply from {}: {!r}\".format(addr, data))\r\n        # Don't close the socket as we might get multiple responses.\r\n\r\n    def error_received(self, exc):\r\n        print('Error received:', exc)\r\n\r\n    def connection_lost(self, exc):\r\n        print(\"Socket closed, stop the event loop\")\r\n        self.loop.stop()\r\n\r\n\r\nloop = asyncio.get_event_loop()\r\nconnect = loop.create_datagram_endpoint(\r\n    lambda: DiscoveryClientProtocol(loop),\r\n    remote_addr=(BROADCAST_ADDR, BROADCAST_PORT),\r\n)\r\ntransport, protocol = loop.run_until_complete(connect)\r\nloop.run_forever()\r\ntransport.close()\r\nloop.close()\r\n\r\n```","author":{"url":"https://github.com/gpjt","@type":"Person","name":"gpjt"},"datePublished":"2017-01-04T16:07:09.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/480/asyncio/issues/480"}