Question

I’m having a hard time figuring this problem out – I am trying to write a program that will interact with the Linux tunnel driver. At a very basic level, I simply want to create an application that is able to transfer data over a network tunnel. However, I am completely at a loss as to how to properly set up the tunnel driver in order to accomplish this.

I am developing on Ubuntu 9.04, and I have the tunnel driver kernel module loaded.

There exists the device /dev/net/tun, however there are no /dev/tunX devices. I am unable to create these devices using ifconfig – whenever I run /sbin/ifconfig tun0 up, for example, I get the following error:

tun0: ERROR while getting interface flags: No such device.

If I attempt to look at the /dev/net/tun device, the following error is presented:

cat: /dev/net/tun: File descriptor in bad state.

Attempting to open /dev/tunX via a small program, basically, a simple

tun_fd = open( "/dev/tun0", O_RDWR )

returns -1: the application is running as root and still cannot open this tunnel device. It is possible to open /dev/net/tun, however this does not appear to generate a new /dev/tunX device to use instead.

So, in summary – how does one go about writing an application that wishes to use the Linux tunnel driver? Any insights would be greatly appreciated.

Thanks;
~Robert

Answer 1

Read /usr/src/linux/Documentation/networking/tuntap.rst.

You are supposed to open the /dev/net/tun device. A subsequent ioctl on the open fd will create the tun0 (or whatever you wish to name it) network interface. Linux’s network interfaces do not correspond to any /dev/* device.