diff --git a/sys/net/if.c b/sys/net/if.c
--- a/sys/net/if.c
+++ b/sys/net/if.c
@@ -1125,9 +1125,38 @@
 
 	sx_assert(&ifnet_detach_sxlock, SX_XLOCKED);
 
+	/*
+	 * Ideally the interface has been in down state before it been detached,
+	 * but that requires lots of modification of drivers, and also it is a
+	 * little safer to make it down here, as the interface has already been
+	 * removed from the global network interface list.
+	 */
+	if ((ifp->if_flags & IFF_UP) != 0) {
+		if_down(ifp);
+
+		/* Give the driver a chance to teardown */
+		if (ifp->if_ioctl != NULL) {
+			struct ifreq ifr;
+
+			ifr.ifr_flags = ifp->if_flags & 0xffff;
+			ifr.ifr_flagshigh = ifp->if_flags >> 16;
+			(void)(*ifp->if_ioctl)(ifp, SIOCSIFFLAGS, (caddr_t)&ifr);
+		}
+	}
+
 	/*
 	 * At this point we know the interface still was on the ifnet list
 	 * and we removed it so we are in a stable state.
+	 *
+	 * Note that, NET_EPOCH_WAIT() blocks the current thread but does not
+	 * block other threads from entering net epoch. Well it is a nice
+	 * synchronization point, that, threads those enter net epoch will see
+	 * writes before NET_EPOCH_WAIT(), e.g. the ~IFF_UP flag.
+	 *
+	 * Other threads, typically the input / output path, should check the
+	 * interface's up state, aka IFF_UP flag, after (re)entering net epoch.
+	 * Otherwise it is possible to reference an interface in detaching
+	 * progress and see consistency, e.g. NULL if_afdata[AF_INET6].
 	 */
 	NET_EPOCH_WAIT();
 
@@ -1149,8 +1178,6 @@
 	taskqueue_drain(taskqueue_swi, &ifp->if_linktask);
 	taskqueue_drain(taskqueue_swi, &ifp->if_addmultitask);
 
-	if_down(ifp);
-
 #ifdef VIMAGE
 	/*
 	 * On VNET shutdown abort here as the stack teardown will do all