> So, you should create socket, fill interface name field in ifr,
> get flags via ioctl with this socket, and work with derived data.
> That's what ifconfig does.
I've fixed it now. I misunderstood something. The file descriptor I was
calling the ioctl(fd ,SIOCSIFFLAGS, &ifr) on before was the file
descriptor returned by open("/dev/net/tun"). So it was the file
descriptor to the tunX interface.
I thought I should set the if flags and send them to this device. Using
a dgram socket to do the job worked.
Here's the code I have now:
/* Returns a fildes to the network device */
if ( (fd_tun = open("/dev/net/tun", O_RDWR) ) < 0)
return fd_tun;
memset(&ifr, 0, sizeof(ifr));
ifr.ifr_flags = IFF_TUN | IFF_NO_PI; /* Create a TUN device */
if ( *dev )
strncpy(ifr.ifr_name, dev, IFNAMSIZ);
/* Create the device */
if ( (err = ioctl(fd_tun, TUNSETIFF, (void*) &ifr)) < 0 )
return err;
/* Copy the actual name of the device that was created back */
strcpy(dev, ifr.ifr_name);
/* Set no check sum - what is this? */
ioctl(fd_tun, TUNSETNOCSUM, 1);
/* Get the flags that are set */
skfd = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);
if (skfd < 0 )
return skfd;
err = ioctl(skfd, SIOCGIFFLAGS, (void*) &ifr);
if ( err < 0 )
return err;
/* Set the flags that bring the device up */
ifr.ifr_flags |= ( IFF_UP | IFF_RUNNING );
err = ioctl(skfd, SIOCSIFFLAGS, (void*) &ifr);
if ( err < 0 )
return err;
return fd_tun;
It works, but I'm a bit confused. Why do I have to use a new dgram
socket to get/set the device flags? It seems more intuative to use the
fd to the device to set the device flags. Is this new dgram socket
useful for anything else?
Thanks for you help,
Nick Martin
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