Index: if_sec.c
===================================================================
RCS file: /cvs/src/sys/net/if_sec.c,v
retrieving revision 1.9
diff -u -p -r1.9 if_sec.c
--- if_sec.c	23 Dec 2023 10:52:54 -0000	1.9
+++ if_sec.c	23 Jan 2024 06:42:48 -0000
@@ -315,6 +315,14 @@ sec_output(struct ifnet *ifp, struct mbu
 		}
 	}
 
+	mtag = m_tag_get(PACKET_TAG_GRE, sizeof(ifp->if_index), M_NOWAIT);
+	if (mtag == NULL) {
+		error = ENOBUFS;
+		goto drop;
+	}
+	*(int *)(mtag + 1) = ifp->if_index;
+	m_tag_prepend(m, mtag);
+
 	m->m_pkthdr.ph_family = dst->sa_family;
 
 	error = if_enqueue(ifp, m);