Index: head/sys/netinet/tcp_input.c
===================================================================
--- head/sys/netinet/tcp_input.c
+++ head/sys/netinet/tcp_input.c
@@ -603,7 +603,7 @@
 #ifdef TCP_SIGNATURE
 	uint8_t sig_checked = 0;
 #endif
-	uint8_t iptos = 0;
+	uint8_t iptos;
 	struct m_tag *fwd_tag = NULL;
 #ifdef INET6
 	struct ip6_hdr *ip6 = NULL;
@@ -675,6 +675,7 @@
 			/* XXX stat */
 			goto drop;
 		}
+		iptos = (ntohl(ip6->ip6_flow) >> 20) & 0xff;
 	}
 #endif
 #if defined(INET) && defined(INET6)
@@ -701,6 +702,7 @@
 		th = (struct tcphdr *)((caddr_t)ip + off0);
 		tlen = ntohs(ip->ip_len) - off0;
 
+		iptos = ip->ip_tos;
 		if (m->m_pkthdr.csum_flags & CSUM_DATA_VALID) {
 			if (m->m_pkthdr.csum_flags & CSUM_PSEUDO_HDR)
 				th->th_sum = m->m_pkthdr.csum_data;
@@ -721,29 +723,20 @@
 			ipov->ih_len = htons(tlen);
 			th->th_sum = in_cksum(m, len);
 			/* Reset length for SDT probes. */
-			ip->ip_len = htons(tlen + off0);
+			ip->ip_len = htons(len);
+			/* Reset TOS bits */
+			ip->ip_tos = iptos;
+			/* Re-initialization for later version check */
+			ip->ip_v = IPVERSION;
 		}
 
 		if (th->th_sum) {
 			TCPSTAT_INC(tcps_rcvbadsum);
 			goto drop;
 		}
-		/* Re-initialization for later version check */
-		ip->ip_v = IPVERSION;
 	}
 #endif /* INET */
 
-#ifdef INET6
-	if (isipv6)
-		iptos = (ntohl(ip6->ip6_flow) >> 20) & 0xff;
-#endif
-#if defined(INET) && defined(INET6)
-	else
-#endif
-#ifdef INET
-		iptos = ip->ip_tos;
-#endif
-
 	/*
 	 * Check that TCP offset makes sense,
 	 * pull out TCP options and adjust length.		XXX