diff --git a/sys/netinet/libalias/alias_ftp.c b/sys/netinet/libalias/alias_ftp.c
--- a/sys/netinet/libalias/alias_ftp.c
+++ b/sys/netinet/libalias/alias_ftp.c
@@ -754,7 +754,7 @@
 		/* Compute TCP checksum for revised packet */
 		tc->th_sum = 0;
 #ifdef _KERNEL
-		tc->th_x2 = 1;
+		tc->th_x2 = (TH_RES1 >> 8);
 #else
 		tc->th_sum = TcpChecksum(pip);
 #endif
diff --git a/sys/netinet/libalias/alias_irc.c b/sys/netinet/libalias/alias_irc.c
--- a/sys/netinet/libalias/alias_irc.c
+++ b/sys/netinet/libalias/alias_irc.c
@@ -458,7 +458,7 @@
 		/* Compute TCP checksum for revised packet */
 		tc->th_sum = 0;
 #ifdef _KERNEL
-		tc->th_x2 = 1;
+		tc->th_x2 = (TH_RES1 >> 8);
 #else
 		tc->th_sum = TcpChecksum(pip);
 #endif
diff --git a/sys/netinet/libalias/alias_proxy.c b/sys/netinet/libalias/alias_proxy.c
--- a/sys/netinet/libalias/alias_proxy.c
+++ b/sys/netinet/libalias/alias_proxy.c
@@ -368,7 +368,7 @@
 
 	tc->th_sum = 0;
 #ifdef _KERNEL
-	tc->th_x2 = 1;
+	tc->th_x2 = (TH_RES1 >> 8);
 #else
 	tc->th_sum = TcpChecksum(pip);
 #endif
diff --git a/sys/netinet/libalias/alias_skinny.c b/sys/netinet/libalias/alias_skinny.c
--- a/sys/netinet/libalias/alias_skinny.c
+++ b/sys/netinet/libalias/alias_skinny.c
@@ -216,7 +216,7 @@
 
 	tc->th_sum = 0;
 #ifdef _KERNEL
-	tc->th_x2 = 1;
+	tc->th_x2 = (TH_RES1 >> 8);
 #else
 	tc->th_sum = TcpChecksum(pip);
 #endif
@@ -259,7 +259,7 @@
 
 	tc->th_sum = 0;
 #ifdef _KERNEL
-	tc->th_x2 = 1;
+	tc->th_x2 = (TH_RES1 >> 8);
 #else
 	tc->th_sum = TcpChecksum(pip);
 #endif
@@ -289,7 +289,7 @@
 
 	tc->th_sum = 0;
 #ifdef _KERNEL
-	tc->th_x2 = 1;
+	tc->th_x2 = (TH_RES1 >> 8);
 #else
 	tc->th_sum = TcpChecksum(pip);
 #endif
diff --git a/sys/netinet/libalias/alias_smedia.c b/sys/netinet/libalias/alias_smedia.c
--- a/sys/netinet/libalias/alias_smedia.c
+++ b/sys/netinet/libalias/alias_smedia.c
@@ -404,7 +404,7 @@
 
 	tc->th_sum = 0;
 #ifdef _KERNEL
-	tc->th_x2 = 1;
+	tc->th_x2 = (TH_RES1 >> 8);
 #else
 	tc->th_sum = TcpChecksum(pip);
 #endif
@@ -451,7 +451,7 @@
 				/* Compute TCP checksum for revised packet */
 				tc->th_sum = 0;
 #ifdef _KERNEL
-				tc->th_x2 = 1;
+				tc->th_x2 = (TH_RES1 >> 8);
 #else
 				tc->th_sum = TcpChecksum(pip);
 #endif
diff --git a/sys/netinet/tcp.h b/sys/netinet/tcp.h
--- a/sys/netinet/tcp.h
+++ b/sys/netinet/tcp.h
@@ -72,6 +72,9 @@
 #define	TH_ECE	0x40
 #define	TH_CWR	0x80
 #define	TH_AE	0x100			/* maps into th_x2 */
+#define	TH_RES3	0x200
+#define	TH_RES2	0x400
+#define	TH_RES1	0x800
 #define	TH_FLAGS	(TH_FIN|TH_SYN|TH_RST|TH_PUSH|TH_ACK|TH_URG|TH_ECE|TH_CWR)
 #define	PRINT_TH_FLAGS	"\20\1FIN\2SYN\3RST\4PUSH\5ACK\6URG\7ECE\10CWR\11AE"
 
diff --git a/sys/netpfil/ipfw/ip_fw_nat.c b/sys/netpfil/ipfw/ip_fw_nat.c
--- a/sys/netpfil/ipfw/ip_fw_nat.c
+++ b/sys/netpfil/ipfw/ip_fw_nat.c
@@ -418,7 +418,7 @@
 		struct tcphdr 	*th;
 
 		th = (struct tcphdr *)(ip + 1);
-		if (th->th_x2)
+		if (th->th_x2 & (TH_RES1 >> 8))
 			ldt = 1;
 	}
 
@@ -438,7 +438,7 @@
 			 * Maybe it was set in
 			 * libalias...
 			 */
-			th->th_x2 = 0;
+			th->th_x2 &= ~(TH_RES1 >> 8);
 			th->th_sum = cksum;
 			mcl->m_pkthdr.csum_data =
 			    offsetof(struct tcphdr, th_sum);