Index: in.h
===================================================================
RCS file: /cvs/src/sys/netinet/in.h,v
retrieving revision 1.137
diff -u -p -r1.137 in.h
--- in.h	4 Nov 2019 23:52:28 -0000	1.137
+++ in.h	13 Jul 2020 05:02:27 -0000
@@ -807,6 +807,7 @@ struct sockaddr;
 struct sockaddr_in;
 struct ifaddr;
 struct in_ifaddr;
+struct ip;
 
 void	   ipv4_input(struct ifnet *, struct mbuf *);
 
@@ -814,6 +815,7 @@ int	   in_broadcast(struct in_addr, u_in
 int	   in_canforward(struct in_addr);
 int	   in_cksum(struct mbuf *, int);
 int	   in4_cksum(struct mbuf *, u_int8_t, int, int);
+uint16_t   ip_cksum(const struct ip *);
 void	   in_proto_cksum_out(struct mbuf *, struct ifnet *);
 void	   in_ifdetach(struct ifnet *);
 int	   in_mask2len(struct in_addr *);
Index: ip_input.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_input.c,v
retrieving revision 1.348
diff -u -p -r1.348 ip_input.c
--- ip_input.c	12 Apr 2020 11:56:52 -0000	1.348
+++ ip_input.c	13 Jul 2020 05:02:27 -0000
@@ -267,7 +267,7 @@ ip_input_if(struct mbuf **mp, int *offp,
 		}
 
 		ipstat_inc(ips_inswcsum);
-		if (in_cksum(m, hlen) != 0) {
+		if (ip_cksum(ip) != 0x0000) {
 			ipstat_inc(ips_badsum);
 			goto bad;
 		}
@@ -1796,4 +1796,27 @@ ip_send(struct mbuf *m)
 {
 	mq_enqueue(&ipsend_mq, m);
 	task_add(net_tq(0), &ipsend_task);
+}
+
+uint16_t
+ip_cksum(const struct ip *ip)
+{
+	const uint32_t *w = (const uint32_t *)ip;
+	uint64_t s;
+	unsigned int i;
+
+	s  = w[0];
+	s += w[1];
+	s += w[2];
+	s += w[3];
+	s += w[4];
+
+	for (i = 5; i < ip->ip_hl; i++)
+		s += (uint64_t)w[i];
+
+	do {
+		s = (s >> 16) + (s & 0xffff);
+	} while (s >> 16);
+
+	return (~s);
 }
Index: ip_output.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_output.c,v
retrieving revision 1.357
diff -u -p -r1.357 ip_output.c
--- ip_output.c	24 Jun 2020 22:03:43 -0000	1.357
+++ ip_output.c	13 Jul 2020 05:02:27 -0000
@@ -462,7 +462,7 @@ sendit:
 			m->m_pkthdr.csum_flags |= M_IPV4_CSUM_OUT;
 		else {
 			ipstat_inc(ips_outswcsum);
-			ip->ip_sum = in_cksum(m, hlen);
+			ip->ip_sum = ip_cksum(ip);
 		}
 
 		error = ifp->if_output(ifp, m, sintosa(dst), ro->ro_rt);
@@ -721,7 +721,7 @@ ip_fragment(struct mbuf *m, struct ifnet
 			m->m_pkthdr.csum_flags |= M_IPV4_CSUM_OUT;
 		else {
 			ipstat_inc(ips_outswcsum);
-			mhip->ip_sum = in_cksum(m, mhlen);
+			mhip->ip_sum = ip_cksum(mhip);
 		}
 		ipstat_inc(ips_ofragments);
 		fragments++;
@@ -742,7 +742,7 @@ ip_fragment(struct mbuf *m, struct ifnet
 		m->m_pkthdr.csum_flags |= M_IPV4_CSUM_OUT;
 	else {
 		ipstat_inc(ips_outswcsum);
-		ip->ip_sum = in_cksum(m, hlen);
+		ip->ip_sum = ip_cksum(ip);
 	}
 sendorfree:
 	if (error) {
@@ -1709,7 +1709,7 @@ ip_mloopback(struct ifnet *ifp, struct m
 		 */
 		ip = mtod(copym, struct ip *);
 		ip->ip_sum = 0;
-		ip->ip_sum = in_cksum(copym, ip->ip_hl << 2);
+		ip->ip_sum = ip_cksum(ip);
 		if_input_local(ifp, copym, dst->sin_family);
 	}
 }