Index: sys/netinet/udp_usrreq.c
===================================================================
--- sys/netinet/udp_usrreq.c	(revision 316739)
+++ sys/netinet/udp_usrreq.c	(working copy)
@@ -372,6 +372,9 @@ udp_append(struct inpcb *inp, struct ip *ip, struc
 		append_sa = (struct sockaddr *)&udp_in[0];
 	m_adj(n, off);
 
+	/* Clear any h/w csum flags as they are no longer valid. */
+	n->m_pkthdr.csum_flags &= ~(CSUM_DATA_VALID | CSUM_IP_VALID);
+
 	so = inp->inp_socket;
 	SOCKBUF_LOCK(&so->so_rcv);
 	if (sbappendaddr_locked(&so->so_rcv, append_sa, n, opts) == 0) {
Index: sys/netinet6/udp6_usrreq.c
===================================================================
--- sys/netinet6/udp6_usrreq.c	(revision 316739)
+++ sys/netinet6/udp6_usrreq.c	(working copy)
@@ -187,6 +187,9 @@ udp6_append(struct inpcb *inp, struct mbuf *n, int
 	}
 	m_adj(n, off + sizeof(struct udphdr));
 
+	/* Clear any h/w csum flags as they are no longer valid. */
+	n->m_pkthdr.csum_flags &= ~CSUM_DATA_VALID;
+
 	so = inp->inp_socket;
 	SOCKBUF_LOCK(&so->so_rcv);
 	if (sbappendaddr_locked(&so->so_rcv, (struct sockaddr *)&fromsa[0], n,