[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 <dragorn@kismetwireless.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index effab0b..50b8c67 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -18,6 +18,9 @@
 /*
  *  Changes:
  *
+ *  Mike Kershaw <dragorn@kismetwireless.net> 2005/08/14
+ *    Add TUNSETLINK ioctl to set the link encapsulation
+ *
  *  Mark Smith <markzzzsmith@yahoo.com.au>
  *   Use random_ether_addr() for tap MAC address.
  *
@@ -612,6 +615,18 @@
 		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
 	case TUNSETDEBUG:
 		tun->debug = arg;