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POTUNTAP interface in C (Linux) : Can't capture UDP packets sent on the TUNTAP with sendto()
 

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  1. POTUNTAP interface in C (Linux) : Can't capture UDP packets sent on the TUNTAP with sendto()
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    <p>I am trying to write a tunneling program in C that will take UDP packets from a TUNTAP interface and send them to a serial interface. </p> <p>What I do is allocate the interface from the clone device /dev/net/tun, turn it on and give it an ip address :</p> <pre><code>int tun_setup(char *dev, int flags) { struct sockaddr_in my_addr; struct ifreq ifr; int fd, err; string clonedev = "/dev/net/tun"; // Open clone device file descriptor if( (fd = open(clonedev.c_str() , O_RDWR)) &lt; 0 ) { perror("Opening /dev/net/tun"); return fd; } // Initialise interface parameters structure memset(&amp;ifr, 0, sizeof(ifr)); // Set up flags ifr.ifr_flags = flags; // Set up interface name if (*dev) { strncpy(ifr.ifr_name, dev, IFNAMSIZ); } // Put interface in TUN mode if( (err = ioctl(fd, TUNSETIFF, (void *)&amp;ifr)) &lt; 0 ) { perror("ioctl(TUNSETIFF)"); close(fd); return err; } strcpy(dev, ifr.ifr_name); // Create a socket if ( (s = socket(AF_INET, SOCK_DGRAM, 0)) &lt; 0) { perror("socket"); exit(1); } // Get interface flags if (ioctl(s, SIOCGIFFLAGS, &amp;ifr) &lt; 0) { perror("cannot get interface flags"); exit(1); } // Turn on interface ifr.ifr_flags |= IFF_UP; if (ioctl(s, SIOCSIFFLAGS, &amp;ifr) &lt; 0) { fprintf(stderr, "ifup: failed "); perror(ifr.ifr_name); exit(1); } // Set interface address bzero((char *) &amp;my_addr, sizeof(my_addr)); my_addr.sin_family = AF_INET; my_addr.sin_addr.s_addr = htonl(inet_network("192.168.2.1")); memcpy(&amp;ifr.ifr_addr, &amp;my_addr, sizeof(struct sockaddr)); if (ioctl(s, SIOCSIFADDR, &amp;ifr) &lt; 0) { fprintf(stderr, "Cannot set IP address. "); perror(ifr.ifr_name); exit(1); } // Return interface file descriptor return fd; } </code></pre> <p>Then I create a thread that will poll() on the file descriptor of the created interface and do read() + some other stuff when an event occurs.</p> <pre><code>void* tun_readThreadProc (void* param) { struct pollfd fds[1]; int nread; unsigned char buffer[BUFFERSIZE]; fds[0].fd = tun_fd; fds[0].events = POLLIN; printf("%s : Entered. tun_fd = %d \n",__FUNCTION__,tun_fd); for(;;) { printf("%s : Entered loop\n",__FUNCTION__); if((poll(fds, 1, -1)) == -1) { perror("poll"); exit(1); } printf("%s : Poll sensed something\n",__FUNCTION__); if((nread = read(tun_fd, buffer, BUFFERSIZE)) &lt; 0) { perror("read"); close(tun_fd); exit(1); } printf("%s : Read something : %d bytes\n",__FUNCTION__,nread); } return 0; } </code></pre> <p>On an other part of the program, I bind a UDP socket to the IP address of this TUNTAP interface. </p> <pre><code>void socketInit( void ) { int on = 1; struct sockaddr_in my_addr; unsigned short DefaultPort = 47808; // Create a socket if ( (s1 = socket(AF_INET, SOCK_DGRAM, 0)) &lt; 0) { perror("socket"); exit(1); } // Bind to it bzero((char *) &amp;my_addr, sizeof(my_addr)); my_addr.sin_family = AF_INET; my_addr.sin_addr.s_addr = htonl(inet_network("192.168.2.1")); my_addr.sin_port = htons(DefaultPort); if ( (bind(s, (struct sockaddr *) &amp;my_addr, sizeof(my_addr)) &lt; 0) ) { perror("bind"); } // Allow it to broadcast if (setsockopt(s, SOL_SOCKET, SO_BROADCAST, (char *)&amp;on, sizeof(on)) &lt; 0) { perror("setsockopt"); } } </code></pre> <p>In another function, I use sendto() to send packets with this socket. I am supposed to capture those packets with the poll() + read() thread and then send them on a serial port but poll() never captures events on the TUNTAP interface. </p> <p><strong>I can ping through this interface by using ping -I tun0 [some destination] (tun0 = name of the TUNTAP interface)</strong></p> <p><strong>But if I use ping -I 192.168.2.1 [some destination] (192.168.2.1 = TUNTAP interface address) it goes through the default interface (eth0, the physical NIC).</strong></p> <p><strong>I was able to verify that with Wireshark.</strong></p> <p><strong>This is most probably an ip route configuration problem...</strong></p> <p>I would be really glad if anybody can help me. </p>
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    1. USAlex On The Moon
    1. USAlex On The Moon
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      1. POTUNTAP interface in C (Linux) : Can't capture UDP packets sent on the TUNTAP with sendto()
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    1. COSocket creation code here : http://pastebin.com/raw.php?i=JZLKkuxS
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      1. USAlex On The Moon
    2. COWhat do you mean by UDP packets "on this interface"? Do you mean to destinations routed through that interface?
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      1. POTUNTAP interface in C (Linux) : Can't capture UDP packets sent on the TUNTAP with sendto()
      1. USDavid Schwartz
    3. CO@DavidSchwartz Yes exactly
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      1. USAlex On The Moon
 

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