tweak the vnetid so it can be optional and therefore cleared/deleted.

the abstract vnetid is promoted to a uin32_t, and adds a SIOCDVNETID
ioctl so it can be cleared.

this is all because i set an assignment on implementing a virtual
network interface and the students got confused when vnetid 0 didnt
show up in ifconfig output.

the vnetid in the vxlan(4) protocol is optional, but the current
code confuses 0 with no vnetid being set. this makes it clear.

ok reyk@ who also simplified my diff

2 files changed, 18 insertions, 5 deletions

diff --git a/sbin/ifconfig/ifconfig.8 b/sbin/ifconfig/ifconfig.8
index 35137b537bb..3412f74f09a 100644
--- a/sbin/ifconfig/ifconfig.8
+++ b/sbin/ifconfig/ifconfig.8
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: ifconfig.8,v 1.257 2015/10/06 17:23:21 benno Exp $
+.\"	$OpenBSD: ifconfig.8,v 1.258 2015/10/23 01:19:04 dlg Exp $
 .\"	$NetBSD: ifconfig.8,v 1.11 1996/01/04 21:27:29 pk Exp $
 .\"     $FreeBSD: ifconfig.8,v 1.16 1998/02/01 07:03:29 steve Exp $
 .\"
@@ -31,7 +31,7 @@
 .\"
 .\"     @(#)ifconfig.8	8.4 (Berkeley) 6/1/94
 .\"
-.Dd $Mdocdate: October 6 2015 $
+.Dd $Mdocdate: October 23 2015 $
 .Dt IFCONFIG 8
 .Os
 .Sh NAME
@@ -1504,6 +1504,7 @@ for a complete list of the available protocols,
 .Op Cm tunnel Ar src_address dest_address
 .Op Cm tunneldomain Ar tableid
 .Op Cm vnetid Ar network-id
+.Op Cm -vnetid
 .Ek
 .nr nS 0
 .Pp
@@ -1559,6 +1560,8 @@ to identify packets with a virtual network.
 The accepted size of the number depends on the individual tunnel protocol;
 it is a 24-bit number for
 .Xr vxlan 4 .
+.It Cm -vnetid
+Clear the virtual network identifier.
 .El
 .\" VLAN
 .Sh VLAN
diff --git a/sbin/ifconfig/ifconfig.c b/sbin/ifconfig/ifconfig.c
index 5c015f67930..6c681e3fa5e 100644
--- a/sbin/ifconfig/ifconfig.c
+++ b/sbin/ifconfig/ifconfig.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: ifconfig.c,v 1.302 2015/10/03 10:44:23 florian Exp $	*/
+/*	$OpenBSD: ifconfig.c,v 1.303 2015/10/23 01:19:04 dlg Exp $	*/
 /*	$NetBSD: ifconfig.c,v 1.40 1997/10/01 02:19:43 enami Exp $	*/
 
 /*
@@ -181,6 +181,7 @@ void	deletetunnel(const char *, int);
 void	settunnelinst(const char *, int);
 void	settunnelttl(const char *, int);
 void	setvnetid(const char *, int);
+void	delvnetid(const char *, int);
 #ifdef INET6
 void	setia6flags(const char *, int);
 void	setia6pltime(const char *, int);
@@ -413,6 +414,7 @@ const struct	cmd {
 	{ "tunneldomain", NEXTARG,	0,		settunnelinst } ,
 	{ "tunnelttl",	NEXTARG,	0,		settunnelttl } ,
 	{ "vnetid",	NEXTARG,	0,		setvnetid },
+	{ "-vnetid",	0,		0,		delvnetid },
 	{ "pppoedev",	NEXTARG,	0,		setpppoe_dev },
 	{ "pppoesvc",	NEXTARG,	0,		setpppoe_svc },
 	{ "-pppoesvc",	1,		0,		setpppoe_svc },
@@ -2868,7 +2870,7 @@ phys_status(int force)
 
 	if (dstport)
 		printf(":%u", ntohs(dstport));
-	if (ioctl(s, SIOCGVNETID, (caddr_t)&ifr) == 0 && ifr.ifr_vnetid > 0)
+	if (ioctl(s, SIOCGVNETID, (caddr_t)&ifr) == 0)
 		printf(" vnetid %d", ifr.ifr_vnetid);
 	if (ioctl(s, SIOCGLIFPHYTTL, (caddr_t)&ifr) == 0 && ifr.ifr_ttl > 0)
 		printf(" ttl %d", ifr.ifr_ttl);
@@ -3391,7 +3393,7 @@ void
 setvnetid(const char *id, int param)
 {
 	const char *errmsg = NULL;
-	int vnetid;
+	uint32_t vnetid;
 
 	vnetid = strtonum(id, 0, UINT_MAX, &errmsg);
 	if (errmsg)
@@ -3403,6 +3405,14 @@ setvnetid(const char *id, int param)
 		warn("SIOCSVNETID");
 }
 
+/* ARGSUSED */
+void
+delvnetid(const char *ignored, int alsoignored)
+{
+	if (ioctl(s, SIOCDVNETID, &ifr) < 0)
+		warn("SIOCDVNETID");
+}
+
 void
 mpe_status(void)
 {