Index: sys/netinet/ip_fastfwd.c
===================================================================
--- sys/netinet/ip_fastfwd.c
+++ sys/netinet/ip_fastfwd.c
@@ -397,6 +397,31 @@
 	}
 
 	/*
+	 * Translate Layer 4 inbound checksum offloading flags to outbound
+	 */
+	if ((m->m_pkthdr.csum_flags & (CSUM_DATA_VALID | CSUM_PSEUDO_HDR)) ==
+	    (CSUM_DATA_VALID | CSUM_PSEUDO_HDR) &&
+	    m->m_pkthdr.csum_data == 0xffff) {
+		/* L4 checksum was correct, but it's not present. */
+		switch(ip->ip_p) {
+		case IPPROTO_TCP:
+			m->m_pkthdr.csum_flags |= CSUM_TCP;
+			break;
+		case IPPROTO_UDP:
+			m->m_pkthdr.csum_flags |= CSUM_UDP;
+			break;
+		case IPPROTO_SCTP:
+			m->m_pkthdr.csum_flags |= CSUM_SCTP;
+			break;
+		default:
+			/*
+			 * No checksum offloading support
+			 */
+			break;
+		}
+	}
+
+	/*
 	 * Check if packet fits MTU or if hardware will fragment for us
 	 */
 	if (ro.ro_rt->rt_mtu)