diff --git a/sys/net/if_tuntap.c b/sys/net/if_tuntap.c
--- a/sys/net/if_tuntap.c
+++ b/sys/net/if_tuntap.c
@@ -160,12 +160,10 @@
 #define	TUN_VMIO_FLAG_MASK	0x0fff
 
 /*
- * Interface capabilities of a tap device that supports the virtio-net
- * header.
+ * Additional interface capabilities of a tap device that supports the
+ * virtio-net header.
  */
-#define TAP_VNET_HDR_CAPS	(IFCAP_HWCSUM | IFCAP_HWCSUM_IPV6	\
-				| IFCAP_VLAN_HWCSUM			\
-				| IFCAP_TSO | IFCAP_LRO			\
+#define	TAP_VNET_HDR_CAPS	(IFCAP_VLAN_HWCSUM | IFCAP_TSO	\
 				| IFCAP_VLAN_HWTSO)
 
 #define TAP_ALL_OFFLOAD		(CSUM_TSO | CSUM_TCP | CSUM_UDP |\
@@ -987,7 +985,8 @@
 	ifp->if_capabilities |= IFCAP_LINKSTATE | IFCAP_MEXTPG;
 	if ((tp->tun_flags & TUN_L2) != 0)
 		ifp->if_capabilities |=
-		    IFCAP_RXCSUM | IFCAP_RXCSUM_IPV6 | IFCAP_LRO;
+		    IFCAP_RXCSUM | IFCAP_RXCSUM_IPV6 | IFCAP_LRO |
+		    IFCAP_TXCSUM | IFCAP_TXCSUM_IPV6;
 	ifp->if_capenable |= IFCAP_LINKSTATE | IFCAP_MEXTPG;
 
 	if ((tp->tun_flags & TUN_L2) != 0) {
@@ -1767,6 +1766,13 @@
 		    vhdr.hdr.gso_size, vhdr.hdr.csum_start,
 		    vhdr.hdr.csum_offset);
 		error = uiomove(&vhdr, len, uio);
+	} else if (m->m_pkthdr.csum_flags & TAP_ALL_OFFLOAD) {
+		/*
+		 * Sender offloads TCP segmentation or TCP/UDP checksum
+		 * computation to hardware but without a virtio-net header, tap
+		 * cannot forward the offload flags.
+		 */
+		error = EOPNOTSUPP;
 	}
 	if (error == 0)
 		error = m_mbuftouio(uio, m, 0);