Index: sys/netinet/netdump/netdump_client.c
===================================================================
--- sys/netinet/netdump/netdump_client.c
+++ sys/netinet/netdump/netdump_client.c
@@ -108,6 +108,7 @@
 static int	 netdump_modevent(module_t mod, int type, void *priv);
 static void	 netdump_network_poll(void);
 static void	 netdump_pkt_in(struct ifnet *ifp, struct mbuf *m);
+static void	 netdump_reinit_internal(struct ifnet *ifp);
 static int	 netdump_send(uint32_t type, off_t offset, unsigned char *data,
 		    uint32_t datalen);
 static int	 netdump_send_arp(in_addr_t dst);
@@ -131,8 +132,14 @@
 #define	nd_gateway	nd_conf.kda_gateway.in4
 
 /* General dynamic settings. */
+static struct sx nd_conf_lk;
+SX_SYSINIT(nd_conf, &nd_conf_lk, "netdump configuration lock");
+#define NETDUMP_WLOCK()			sx_xlock(&nd_conf_lk)
+#define NETDUMP_WUNLOCK()		sx_xunlock(&nd_conf_lk)
+#define NETDUMP_ASSERT_WLOCKED()	sx_assert(&nd_conf_lk, SA_XLOCKED)
 static struct ether_addr nd_gw_mac;
 static struct ifnet *nd_ifp;
+static eventhandler_tag nd_detach_cookie;
 static uint16_t nd_server_port = NETDUMP_PORT;
 
 FEATURE(netdump, "Netdump client support");
@@ -1065,52 +1072,86 @@
 
 static struct cdev *netdump_cdev;
 
+static void
+netdump_ifdetach(void *arg __unused, struct ifnet *ifp)
+{
+	struct diocskerneldump_arg kda;
+
+	NETDUMP_WLOCK();
+	if (ifp == nd_ifp) {
+		bzero(&kda, sizeof(kda));
+		kda.kda_index = KDA_REMOVE_DEV;
+		(void)dumper_remove(nd_conf.kda_iface, &kda);
+
+		netdump_mbuf_drain();
+		nd_enabled = 0;
+
+		nd_ifp = NULL;
+		if_rele(ifp);
+	}
+	NETDUMP_WUNLOCK();
+}
+
 static int
 netdump_configure(struct diocskerneldump_arg *conf, struct thread *td)
 {
-	struct epoch_tracker et;
 	struct ifnet *ifp;
 
+	NETDUMP_ASSERT_WLOCKED();
+
 	CURVNET_SET(TD_TO_VNET(td));
 	if (!IS_DEFAULT_VNET(curvnet)) {
 		CURVNET_RESTORE();
 		return (EINVAL);
 	}
-	NET_EPOCH_ENTER(et);
-	CK_STAILQ_FOREACH(ifp, &V_ifnet, if_link) {
-		if (strcmp(ifp->if_xname, conf->kda_iface) == 0)
-			break;
-	}
-	/* XXX ref */
-	NET_EPOCH_EXIT(et);
+	ifp = ifunit_ref(conf->kda_iface);
 	CURVNET_RESTORE();
 
 	if (ifp == NULL)
 		return (ENOENT);
-	if ((if_getflags(ifp) & IFF_UP) == 0)
+	if ((if_getflags(ifp) & IFF_UP) == 0) {
+		if_rele(ifp);
 		return (ENXIO);
-	if (!netdump_supported_nic(ifp) || ifp->if_type != IFT_ETHER)
+	}
+	if (!netdump_supported_nic(ifp) || ifp->if_type != IFT_ETHER) {
+		if_rele(ifp);
 		return (ENODEV);
+	}
 
+	if (nd_ifp != NULL)
+		if_rele(nd_ifp);
 	nd_ifp = ifp;
-	netdump_reinit(ifp);
+
+	netdump_reinit_internal(ifp);
 	memcpy(&nd_conf, conf, sizeof(nd_conf));
 	nd_enabled = 1;
+
 	return (0);
 }
 
 /*
  * Reinitialize the mbuf pool used by drivers while dumping. This is called
- * from the generic ioctl handler for SIOCSIFMTU after the driver has
- * reconfigured itself.
+ * from the generic ioctl handler for SIOCSIFMTU after any NIC driver has
+ * reconfigured itself.  (I.e., it may not be a configured netdump interface.)
  */
 void
 netdump_reinit(struct ifnet *ifp)
+{
+	NETDUMP_WLOCK();
+	if (ifp != nd_ifp) {
+		NETDUMP_WUNLOCK();
+		return;
+	}
+	netdump_reinit_internal(ifp);
+	NETDUMP_WUNLOCK();
+}
+
+static void
+netdump_reinit_internal(struct ifnet *ifp)
 {
 	int clsize, nmbuf, ncl, nrxr;
 
-	if (ifp != nd_ifp)
-		return;
+	NETDUMP_ASSERT_WLOCKED();
 
 	ifp->if_netdump_methods->nd_init(ifp, &nrxr, &ncl, &clsize);
 	KASSERT(nrxr > 0, ("invalid receive ring count %d", nrxr));
@@ -1156,6 +1197,8 @@
 
 	conf = NULL;
 	error = 0;
+	NETDUMP_WLOCK();
+
 	switch (cmd) {
 #ifdef COMPAT_FREEBSD11
 	case DIOCSKERNELDUMP_FREEBSD11:
@@ -1342,6 +1385,7 @@
 	explicit_bzero(&kda_copy, sizeof(kda_copy));
 	if (conf != NULL)
 		explicit_bzero(conf, sizeof(*conf));
+	NETDUMP_WUNLOCK();
 	return (error);
 }
 
@@ -1373,6 +1417,9 @@
 		if (error != 0)
 			return (error);
 
+		nd_detach_cookie = EVENTHANDLER_REGISTER(ifnet_departure_event,
+		    netdump_ifdetach, NULL, EVENTHANDLER_PRI_ANY);
+
 		if ((arg = kern_getenv("net.dump.iface")) != NULL) {
 			strlcpy(conf.kda_iface, arg, sizeof(conf.kda_iface));
 			freeenv(arg);
@@ -1392,10 +1439,13 @@
 			conf.kda_af = AF_INET;
 
 			/* Ignore errors; we print a message to the console. */
+			NETDUMP_WLOCK();
 			(void)netdump_configure(&conf, curthread);
+			NETDUMP_WUNLOCK();
 		}
 		break;
 	case MOD_UNLOAD:
+		NETDUMP_WLOCK();
 		if (nd_enabled) {
 			struct diocskerneldump_arg kda;
 
@@ -1408,7 +1458,10 @@
 			netdump_mbuf_drain();
 			nd_enabled = 0;
 		}
+		NETDUMP_WUNLOCK();
 		destroy_dev(netdump_cdev);
+		EVENTHANDLER_DEREGISTER(ifnet_departure_event,
+		    nd_detach_cookie);
 		break;
 	default:
 		error = EOPNOTSUPP;