I have an ubuntu machine with SixXS set up and running. I have a windows 7 machine with an auto configured teredo interface. Both machines are behind a NAT router. I have wireshark running on both machines. I want to send a UDP packet from one to the other. (I have also tried TCP connections, but they behave the same.) I have two problems. 1. It only seems to work one way at a time. i.e. Server (SixXS, Ubuntu) to client works for a bit, then it stops. On the way that isn't working, I see the packet leave the machine but not get to the other machine. I think this might be because the teredo router is confused because they both come from the same IP address and it thinks it is a routing loop - not sure. Any comment on this would be welcome. 2. More importantly, I see perfectly good UDP packets coming into both machines at times, but they are not picked up by the app with the socket. I made a java app at both ends. I also used netcat to do this. I tried UDP and TCP. The sockets are listening properly (I can see them in netstat and I get prot not reachable errors when they are not listening), but the packets are never delivered. the java apps work if I use IPv4, or link local IPv6 addresses, so I guess this is a security setting somewhere. Any pointers would be appreciated. Same problem on windows and ubuntu.