Index: ip_input.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_input.c,v
retrieving revision 1.385
diff -u -p -r1.385 ip_input.c
--- ip_input.c	18 May 2023 09:59:43 -0000	1.385
+++ ip_input.c	5 Aug 2023 10:04:21 -0000
@@ -1529,6 +1529,12 @@ ip_forward(struct mbuf *m, struct ifnet 
 	}
 
 	ip->ip_ttl -= IPTTLDEC;
+	{
+		uint32_t sum = ip->ip_sum;
+		sum += htons(IPTTLDEC << 8);
+		sum += sum >> 16;
+		ip->ip_sum = sum;
+	}
 
 	/*
 	 * If forwarding packet using same interface that it came in on,
Index: ip_output.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_output.c,v
retrieving revision 1.390
diff -u -p -r1.390 ip_output.c
--- ip_output.c	7 Jul 2023 08:05:02 -0000	1.390
+++ ip_output.c	5 Aug 2023 10:04:21 -0000
@@ -1790,7 +1806,12 @@ ip_mloopback(struct ifnet *ifp, struct m
 void
 in_hdr_cksum_out(struct mbuf *m, struct ifnet *ifp)
 {
-	struct ip *ip = mtod(m, struct ip *);
+	struct ip *ip;
+
+	if (ISSET(m->m_pkthdr.csum_flags, M_IPV4_CSUM_IN_OK))
+		return;
+
+	ip = mtod(m, struct ip *);
 
 	ip->ip_sum = 0;
 	if (in_ifcap_cksum(m, ifp, IFCAP_CSUM_IPv4)) {