Simple Python UDP Server: trouble receiving packets from clients other than localhostI am trying to listen datagrams over ipv6. Here are the messages I am interrested in:$ sudo tcpdump | grep 20001[sudo] password for miroslavv: tcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes13:00:59.981338 IP6 fd01:e671:2015:5c01:a00:27ff:fe50:275f.20001 > ff05:e671:2015:5c01:a00:27ff:fe50:275f.20001: UDP, length 8613:00:59.981348 IP6 fd01:e671:2015:5c01:a00:27ff:fe50:275f.20001 > ff05:e671:2015:5c01:a00:27ff:fe50:275f.20001: UDP, length 4713:00:59.981733 IP6 fd01:e671:2015:5c01:a00:27ff:fe50:275f.20001 > ff05:e671:2015:5c01:a00:27ff:fe50:275f.20001: UDP, length 11413:00:59.981744 IP6 fd01:e671:2015:5c01:a00:27ff:fe50:275f.20001 > ff05:e671:2015:5c01:a00:27ff:fe50:275f.20001: UDP, length 6713:00:59.981940 IP6 fd01:e671:2015:5c01:a00:27ff:fe50:275f.20001 > ff05:e671:2015:5c01:a00:27ff:fe50:275f.20001: UDP, length 6013:00:59.982276 IP6 fd01:e671:2015:5c01:a00:27ff:fe50:275f.20001 > ff05:e671:2015:5c01:a00:27ff:fe50:275f.20001: UDP, length 4713:00:59.982492 IP6 fd01:e671:2015:5c01:a00:27ff:fe50:275f.20001 > ff05:e671:2015:5c01:a00:27ff:fe50:275f.20001: UDP, length 4713:00:59.982656 IP6 fd01:e671:2015:5c01:a00:27ff:fe50:275f.20001 > ff05:e671:2015:5c01:a00:27ff:fe50:275f.20001: UDP, length 4713:00:59.982974 IP6 fd01:e671:2015:5c01:a00:27ff:fe50:275f.20001 > ff05:e671:2015:5c01:a00:27ff:fe50:275f.20001: UDP, length 6913:00:59.982985 IP6 fd01:e671:2015:5c01:a00:27ff:fe50:275f.20001 > ff05:e671:2015:5c01:a00:27ff:fe50:275f.20001: UDP, length 11113:00:59.983335 IP6 fd01:e671:2015:5c01:a00:27ff:fe50:275f.20001 > ff05:e671:2015:5c01:a00:27ff:fe50:275f.20001: UDP, length 152My platform is Debian testing. I tried the python examples, which seem to communicate perfectly with each other on the same machine. Yet, when the server is configured to listen for the above listed datagrams, it hangs in recv().Firewall settings:$ sudo iptables -L[sudo] password for miroslavv: Chain INPUT (policy ACCEPT)target     prot opt source               destination         Chain FORWARD (policy ACCEPT)target     prot opt source               destination         Chain OUTPUT (policy ACCEPT)target     prot opt source               destinationI have an ipv6:$ ifconfigeth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500        inet 192.168.42.92  netmask 255.255.248.0  broadcast 192.168.47.255        inet6 fd01:e671:2015:e3ff:219:fff:fe26:a27e  prefixlen 48  scopeid 0x0<global>        inet6 fe80::219:fff:fe26:a27e  prefixlen 64  scopeid 0x20<link>        ether 00:19:0f:26:a2:7e  txqueuelen 1000  (Ethernet)        RX packets 3360785  bytes 943676498 (899.9 MiB)        RX errors 0  dropped 6185  overruns 0  frame 0        TX packets 473236  bytes 77103917 (73.5 MiB)        TX errors 1  dropped 0 overruns 0  carrier 0  collisions 0lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536        inet 127.0.0.1  netmask 255.0.0.0        inet6 ::1  prefixlen 128  scopeid 0x10<host>        loop  txqueuelen 1  (Local Loopback)        RX packets 140  bytes 7912 (7.7 KiB)        RX errors 0  dropped 0  overruns 0  frame 0        TX packets 140  bytes 7912 (7.7 KiB)        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0netcat is also silent:$ netcat -u -l -p20001A peculiar observation is the following. I managed to get some C++ code to read messages if at the same time boost::asio is reading from the same ip and port. Should boost polling be stopped, my code also stops receiving messages.I already tried a bunch of example C programs, but am unable to receive even one UDP packet from anyone: 1, 2, 3, 4.What is going on here? 
UDP multicast client over ipv6
debian;socket;udp