[TUNTAP]: Allow setting the linktype of the tap device from userspace

Currently tun/tap only supports the EN10MB ARP type. For use with
wireless and other networking types it should be possible to set the
ARP type via an ioctl.

Patch v2: Included check that the tap interface is down before changing the
link type out from underneath it

Signed-off-by: Mike Kershaw <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/tun.c

···
 /*
  *  Changes:
  *
+ *  Mike Kershaw <[e-mail]> 2005/08/14
+ *    Add TUNSETLINK ioctl to set the link encapsulation
+ *
  *  Mark Smith <[e-mail]>
  *   Use random_ether_addr() for tap MAC address.
  *
···
 		tun->owner = (uid_t) arg;
 
 		DBG(KERN_INFO "%s: owner set to %d\n", tun->dev->name, tun->owner);
+		break;
+
+	case TUNSETLINK:
+		/* Only allow setting the type when the interface is down */
+		if (tun->dev->flags & IFF_UP) {
+			DBG(KERN_INFO "%s: Linktype set failed because interface is up\n",
+				tun->dev->name);
+			return -EBUSY;
+		} else {
+			tun->dev->type = (int) arg;
+			DBG(KERN_INFO "%s: linktype set to %d\n", tun->dev->name, tun->dev->type);
+		}
 		break;
 
 #ifdef TUN_DEBUG

include/linux/if_tun.h

···
 #define TUNSETIFF     _IOW('T', 202, int) 
 #define TUNSETPERSIST _IOW('T', 203, int) 
 #define TUNSETOWNER   _IOW('T', 204, int)
+#define TUNSETLINK    _IOW('T', 205, int)
 
 /* TUNSETIFF ifr flags */
 #define IFF_TUN		0x0001