Apply by doing:
	cd /usr/src
	patch -p0 < 005_ifrtlabel.patch

And then build, install and boot a new kernel:
	cd /usr/src/sys/arch/`arch -s`/conf
	config GENERIC
	cd ../compile/GENERIC
	make depend && make && sudo make install

If you are using the multiprocessor kernel, replace GENERIC by
GENERIC.MP above.

Index: sys/net/if.c
===================================================================
RCS file: /cvs/src/sys/net/if.c,v
retrieving revision 1.165
retrieving revision 1.165.2.1
diff -u -p -r1.165 -r1.165.2.1
--- sys/net/if.c	6 Jul 2007 14:00:59 -0000	1.165
+++ sys/net/if.c	11 Jan 2008 12:41:26 -0000	1.165.2.1
@@ -1,4 +1,4 @@
-/*	$OpenBSD: if.c,v 1.165 2007/07/06 14:00:59 naddy Exp $	*/
+/*	$OpenBSD: if.c,v 1.165.2.1 2008/01/11 12:41:26 henning Exp $	*/
 /*	$NetBSD: if.c,v 1.35 1996/05/07 05:26:04 thorpej Exp $	*/
 
 /*
@@ -1293,10 +1293,13 @@ ifioctl(struct socket *so, u_long cmd, c
 		break;
 
 	case SIOCGIFRTLABEL:
-		label = rtlabel_id2name(ifp->if_rtlabelid);
-		strlcpy(ifrtlabelbuf, label, RTLABEL_LEN);
-		error = copyoutstr(ifrtlabelbuf, ifr->ifr_data, RTLABEL_LEN,
-		    &bytesdone);
+		if (ifp->if_rtlabelid &&
+		    (label = rtlabel_id2name(ifp->if_rtlabelid)) != NULL) {
+			strlcpy(ifrtlabelbuf, label, RTLABEL_LEN);
+			error = copyoutstr(ifrtlabelbuf, ifr->ifr_data,
+			    RTLABEL_LEN, &bytesdone);
+		} else
+			error = ENOENT;
 		break;
 
 	case SIOCSIFRTLABEL:
Index: sys/net/route.c
===================================================================
RCS file: /cvs/src/sys/net/route.c,v
retrieving revision 1.84
retrieving revision 1.84.2.1
diff -u -p -r1.84 -r1.84.2.1
--- sys/net/route.c	14 Jun 2007 18:31:49 -0000	1.84
+++ sys/net/route.c	11 Jan 2008 12:41:26 -0000	1.84.2.1
@@ -1,4 +1,4 @@
-/*	$OpenBSD: route.c,v 1.84 2007/06/14 18:31:49 reyk Exp $	*/
+/*	$OpenBSD: route.c,v 1.84.2.1 2008/01/11 12:41:26 henning Exp $	*/
 /*	$NetBSD: route.c,v 1.14 1996/02/13 22:00:46 christos Exp $	*/
 
 /*
@@ -1031,8 +1031,8 @@ rtinit(struct ifaddr *ifa, int cmd, int 
 	info.rti_flags = flags | ifa->ifa_flags;
 	info.rti_info[RTAX_DST] = dst;
 	info.rti_info[RTAX_GATEWAY] = ifa->ifa_addr;
-	if (ifa->ifa_ifp->if_rtlabelid) {
-		label = rtlabel_id2name(ifa->ifa_ifp->if_rtlabelid);
+	if (ifa->ifa_ifp->if_rtlabelid &&
+	    (label = rtlabel_id2name(ifa->ifa_ifp->if_rtlabelid)) != NULL) {
 		bzero(&sa_rl, sizeof(sa_rl));
 		sa_rl.sr_len = sizeof(sa_rl);
 		sa_rl.sr_family = AF_UNSPEC;