tun-tap-explore

This is an example C++ project to demonstrate tun interface.

Architecture

Machine A has a payload Hellow from client. It writes this payload to a UDP client socket. Destination of this UDP packet is C. Client does this continuously in a different thread.

The client also creates a tun interface (tun1). This is done in the main thread. A routing rule is added in the machine A to send IP packet destined to C towards tun1 interface. Due to this setup, IP packets can not go out of machine A directly (although C is reachable from A, A and C are in the same network in my setup).

The main thread in the client captures (reads continuously) the packets from the tun1 interface and write to a UDP socket again destined to machine B.

Kernel at machine B will receive a UDP packet with a payload which is again a IP packet. Kernel removes the outer IP header and we receive the payload in the application layer. This payload is an IP packet(with destination address C). We want to forward to machine C. But this can not be written to a normal socket destined to machine C, because it will add one more IP header. We dont want any more header.

Therefore server running at machine B also creates one tun interface (tun2). It writes the payload in the tun2 file descriptor.

We need to enable packet forwarding and disbale Reverse path filtering (rp_filter) in the machine B.

      `sysctl -w net.ipv4.ip_forward=1`
      `sysctl -w net.ipv4.conf.tun2.rp_filter=0`
      `sysctl -w net.ipv4.conf.all.rp_filter=0`

Routing table in machine B: (I have not added any more static routes)

      Kernel IP routing table
      Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
      default         _gateway        0.0.0.0         UG    100    0        0 eth0
      10.129.131.0    0.0.0.0         255.255.255.0   U     0      0        0 eth0
      _gateway        0.0.0.0         255.255.255.255 UH    100    0        0 eth0

Instruction to run the project

Three virtual machines, containers or physical machines are required. Assuming IPs are allocated already and they are in the same network.

Machine A:

      g++ client.cpp -pthread
      sudo ./a.out
      
      sudo ifconfig tun1 up
      sudo route add -host 10.129.131.220 tun1

Machine B:

      g++ middle-server.cpp
      sudo ./a.out
      
      sudo ifconfig tun2 up
      sysctl -w net.ipv4.ip_forward=1
      sysctl -w net.ipv4.conf.tun2.rp_filter=0
      sysctl -w net.ipv4.conf.all.rp_filter=0

Machine C:

      g++ sink.cpp
      ./a.out

Authors

• Debashish Deka