diff --git a/sys/net/if.c b/sys/net/if.c
--- a/sys/net/if.c
+++ b/sys/net/if.c
@@ -2611,7 +2611,12 @@
 		    (ifp->if_flags & IFF_UP) == 0) {
 			do_ifup = 1;
 		}
-		/* See if permanently promiscuous mode bit is about to flip */
+
+		/*
+		 * See if the promiscuous mode or allmulti bits are about to
+		 * flip.  They require special handling because in-kernel
+		 * consumers increment a counter when toggling them.
+		 */
 		if ((ifp->if_flags ^ new_flags) & IFF_PPROMISC) {
 			if (new_flags & IFF_PPROMISC)
 				ifp->if_flags |= IFF_PROMISC;
@@ -2622,6 +2627,12 @@
                                     ((new_flags & IFF_PPROMISC) ?
                                      "enabled" : "disabled"));
 		}
+		if ((ifp->if_flags ^ new_flags) & IFF_ALLMULTI) {
+			if (new_flags & IFF_ALLMULTI)
+				ifp->if_flags |= IFF_ALLMULTI;
+			else if (ifp->if_amcount == 0)
+				ifp->if_flags &= ~IFF_ALLMULTI;
+		}
 		ifp->if_flags = (ifp->if_flags & IFF_CANTCHANGE) |
 			(new_flags &~ IFF_CANTCHANGE);
 		if (ifp->if_ioctl) {