Index: if_gif.c
===================================================================
RCS file: /cvs/src/sys/net/if_gif.c,v
retrieving revision 1.113
diff -u -p -r1.113 if_gif.c
--- if_gif.c	15 Mar 2018 21:01:18 -0000	1.113
+++ if_gif.c	4 Apr 2018 23:12:02 -0000
@@ -403,6 +403,8 @@ gif_output(struct ifnet *ifp, struct mbu
 		error = ENOBUFS;
 		goto drop;
 	}
+	memcpy((caddr_t)(mtag + 1), &ifp->if_index, sizeof(ifp->if_index));
+	m_tag_prepend(m, mtag);
 
 	m->m_pkthdr.ph_family = dst->sa_family;