if_tun: Add TUNGIFNAME ioctl to get the interface name

This ioctl helps to get the name of the network interface associated
with the tun control device via its file descriptor.  One generally
obtains such a file descriptor by opening the tun autocloner (i.e.,
/dev/tun).  While the devname(3) or fdevname(3) functions can be used
to determine the name of the actually created tun device, but there is
no easy way to get the name of the associated network interface, which
is though initially the same as the device name but can be renamed.

Bump __DragonFly_version and update tun.4 man page.

See also the relevant Linux patch that added TUNGETIFF to tun:
https://lists.linuxfoundation.org/pipermail/virtualization/2008-August/011546.html

share/man/man4/tun.4

@@ -2,7 +2,7 @@
 .\" $FreeBSD: src/share/man/man4/tun.4,v 1.9.2.4 2001/08/17 13:08:39 ru Exp $
 .\" Based on PR#2411
 .\"
-.Dd July 17, 2018
+.Dd July 31, 2018
 .Dt TUN 4
 .Os
 .Sh NAME
@@ -221,6 +221,14 @@ is declared in
 The argument should be a pointer to an
 .Vt struct tuninfo ,
 where the current MTU, type, and baudrate will be stored.
+.It Dv TUNGIFNAME
+Retrieve the name of the network interface that is associated with the
+control device.
+The argument should be a pointer to a
+.Va struct ifreq .
+The interface name will be returned in the
+.Va ifr_name
+field.
 .It Dv TUNSIFMODE
 The argument should be a pointer to an
 .Vt int ;

sys/net/tun/if_tun.c

@@ -674,6 +674,7 @@ tunioctl(struct dev_ioctl_args *ap)
 	caddr_t data = ap->a_data;
 	struct tun_softc *sc = dev->si_drv1;
 	struct ifnet *ifp = sc->tun_ifp;
+	struct ifreq *ifr;
 	struct tuninfo *tunp;
 	int error = 0;
 
@@ -695,6 +696,11 @@ tunioctl(struct dev_ioctl_args *ap)
 		tunp->baudrate = ifp->if_baudrate;
 		break;
 
+	case TUNGIFNAME:
+		ifr = (struct ifreq *)data;
+		strlcpy(ifr->ifr_name, ifp->if_xname, IFNAMSIZ);
+		break;
+
 	case TUNSDEBUG:
 		tundebug = *(int *)data;
 		break;

sys/net/tun/if_tun.h

@@ -43,8 +43,11 @@ struct tuninfo {
 
 #define	TUNSLMODE	_IOW('t', 93, int)
 #define	TUNSIFMODE	_IOW('t', 94, int)
-#define	TUNSIFPID	_IO('t', 95)
+#define	TUNSIFPID	 _IO('t', 95)
 #define	TUNSIFHEAD	_IOW('t', 96, int)
 #define	TUNGIFHEAD	_IOR('t', 97, int)
 
+/* get the network interface name */
+#define	TUNGIFNAME	_IOR('t', 98, struct ifreq)
+
 #endif /* !_NET_IF_TUN_H_ */

sys/sys/param.h

@@ -185,9 +185,10 @@
  * 500312 - OpenPAM Resedacea upgrade
  * 500313 - remove vmnet support from tap(4) (VMIO_* ioctls)
  * 500314 - add TAPGIFNAME to tap(4)
+ * 500315 - add TUNGIFNAME to tun(4)
  */
 #undef __DragonFly_version
-#define __DragonFly_version 500314	/* propagated to newvers */
+#define __DragonFly_version 500315	/* propagated to newvers */
 
 #include <sys/_null.h>