TUN/TAP write back to tunnel

Question

My app is using a TUN say tun0. In the design, my app will receive an UDP which includes an full IP layer, then I will take the IP layer out and then use 'file write' to put them into my own tun0 device, supposedly in design, I should can read the packet out again from tun0.

Now the situation is I can see through tcpdump the package is wrote into the tunnel, but I couldn't read them back.

Something wrong with tunnel setting or route setting?

Thanks in advance

Yang

Answer 1

Your second tun0 is not a FIFO queue. You may have a problem in your design of how and why your are using the second tun0 device. Clarify why you are using it and which process should be reading. The proper approach should flow from that clarification.

If you want to read the data you send into you have some options.

• Connect tun0 to an TCP or UDP echo service when you open it. This will then send you back the packets you stuff into it.
• Open a listener for the second tun0 to connect to. Then connect to it and send the packets out that connection. Read your data from the listener side.
• Open a pipe with two file descriptors. Write to one descriptor and read from the other. Pipes are often used for IPC (Inter-Process Communication) when forking children.
• Create a socket and read data from it. Open the other end of the socket for writing. Sockets are often used to allow other processes to communicate with a process. This works well when the calling processes may have a different lifetime than the listening process.
• Create a buffer or queue in memory to store the data.