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	4 Jul 2020 12:14:33 -0000
@@ -216,6 +216,28 @@ ipv4_input(struct ifnet *ifp, struct mbu
 	KASSERT(nxt == IPPROTO_DONE);
 }
 
+uint16_t
+ip_cksum(const struct ip *ip)
+{
+	const uint32_t *w = (const uint32_t *)ip;
+	uint64_t s;
+	unsigned int i;
+
+	s = (uint64_t)w[0] +
+	    (uint64_t)w[1] +
+	    (uint64_t)w[2] +
+	    (uint64_t)w[3] +
+	    (uint64_t)w[4];
+
+	for (i = 5; i < ip->ip_hl; i++)
+		s += (uint64_t)w[i];
+
+	s = (s >> 16) + (s & 0xffff);
+	s = (s >> 16) + (s & 0xffff);
+
+	return (~s);
+}
+
 int
 ip_input_if(struct mbuf **mp, int *offp, int nxt, int af, struct ifnet *ifp)
 {
@@ -267,7 +289,7 @@ 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;
 		}