diff --git a/sys/arm64/broadcom/genet/if_genet.c b/sys/arm64/broadcom/genet/if_genet.c
--- a/sys/arm64/broadcom/genet/if_genet.c
+++ b/sys/arm64/broadcom/genet/if_genet.c
@@ -49,6 +49,7 @@
 #include <sys/mbuf.h>
 #include <sys/socket.h>
 #include <sys/sockio.h>
+#include <sys/sysctl.h>
 #include <sys/module.h>
 #include <sys/taskqueue.h>
 #include <sys/gpio.h>
@@ -97,9 +98,27 @@
 
 #define	TX_MAX_SEGS		20
 
-/* Maximum number of mbufs to send to if_input */
+static SYSCTL_NODE(_hw, OID_AUTO, genet, CTLFLAG_RD | CTLFLAG_MPSAFE, 0,
+    "genet driver parameters");
+
+/* Maximum number of mbufs to pass per call to if_input */
 static int gen_rx_batch = 16 /* RX_BATCH_DEFAULT */;
-TUNABLE_INT("hw.gen.rx_batch", &gen_rx_batch);
+SYSCTL_INT(_hw_genet, OID_AUTO, rx_batch, CTLFLAG_RDTUN,
+    &gen_rx_batch, 0, "max mbufs per call to if_input");
+
+TUNABLE_INT("hw.gen.rx_batch", &gen_rx_batch);	/* old name/interface */
+
+/*
+ * Transmitting packets with only an Ethernet header in the first mbuf
+ * fails.  Examples include reflected ICMPv6 packets, e.g. echo replies;
+ * forwarded IPv6/TCP packets; and forwarded IPv4/TCP packets that use NAT
+ * with IPFW.  Pulling up the sizes of ether_header + ip6_hdr + icmp6_hdr
+ * seems to work for both ICMPv6 and TCP over IPv6, as well as the IPv4/TCP
+ * case.
+ */
+static int gen_tx_hdr_min = 56;		/* ether_header + ip6_hdr + icmp6_hdr */
+SYSCTL_INT(_hw_genet, OID_AUTO, tx_hdr_min, CTLFLAG_RW,
+    &gen_tx_hdr_min, 0, "header to add to packets with ether header only");
 
 static struct ofw_compat_data compat_data[] = {
 	{ "brcm,genet-v1",		1 },
@@ -995,31 +1014,19 @@
 	m = *mp;
 
 	/*
-	 * Reflected ICMPv6 packets, e.g. echo replies, tend to get laid
-	 * out with only the Ethernet header in the first mbuf, and this
-	 * doesn't seem to work.  Forwarded TCP packets over IPv6 also
-	 * fail if laid out with only the Ethernet header in the first mbuf.
-	 * For now, pull up any IPv6 packet with that layout.  Maybe IPv4
-	 * needs it but we haven't run into it.  Pulling up the sizes of
-	 * ether_header + ip6_header + icmp6_hdr seems to work for both
-	 * ICMPv6 and TCP over IPv6.
+	 * Don't attempt to send packets with only an Ethernet header in
+	 * first mbuf; see comment above with gen_tx_hdr_min.
 	 */
-#define IP6_PULLUP_LEN (sizeof(struct ether_header) + \
-		        sizeof(struct ip6_hdr) + 8)
 	if (m->m_len == sizeof(struct ether_header)) {
-		int ether_type = mtod(m, struct ether_header *)->ether_type;
-		if (ntohs(ether_type) == ETHERTYPE_IPV6) {
-			m = m_pullup(m, MIN(m->m_pkthdr.len, IP6_PULLUP_LEN));
-			if (m == NULL) {
-				if (sc->ifp->if_flags & IFF_DEBUG)
-					device_printf(sc->dev,
-					    "IPV6 pullup fail\n");
-				*mp = NULL;
-				return (ENOMEM);
-			}
+		m = m_pullup(m, MIN(m->m_pkthdr.len, gen_tx_hdr_min));
+		if (m == NULL) {
+			if (sc->ifp->if_flags & IFF_DEBUG)
+				device_printf(sc->dev,
+				    "header pullup fail\n");
+			*mp = NULL;
+			return (ENOMEM);
 		}
 	}
-#undef IP6_PULLUP_LEN
 
 	if ((if_getcapenable(sc->ifp) & (IFCAP_TXCSUM | IFCAP_TXCSUM_IPV6)) !=
 	    0) {