Index: ip_divert.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_divert.c,v
retrieving revision 1.64
diff -u -p -r1.64 ip_divert.c
--- ip_divert.c	16 Nov 2020 06:38:20 -0000	1.64
+++ ip_divert.c	5 Apr 2021 08:57:54 -0000
@@ -213,6 +213,42 @@ divert_packet(struct mbuf *m, int dir, u
 			break;
 		}
 		if_put(ifp);
+	} else {
+		struct ip *ip;
+		int min_hdrlen, off;
+		int flags = 0;
+
+		ip = mtod(m, struct ip *);
+		off = ip->ip_hl << 2;
+
+		switch (ip->ip_p) {
+		case IPPROTO_TCP:
+			min_hdrlen = sizeof(struct tcphdr);
+			flags = M_TCP_CSUM_OUT;
+			break;
+		case IPPROTO_UDP:
+			min_hdrlen = sizeof(struct udphdr);
+			flags = M_UDP_CSUM_OUT;
+			break;
+		case IPPROTO_ICMP:
+			min_hdrlen = ICMP_MINLEN;
+			flags = M_ICMP_CSUM_OUT;
+			break;
+		default:
+			min_hdrlen = 0;
+			break;
+		}
+		if (off + min_hdrlen >= m->m_pkthdr.len)
+			m->m_pkthdr.csum_flags |= flags;
+
+		/*
+		 * Calculate IP and protocol checksums for the outbound packet
+		 * since the kernel may be expecting the NIC to calculate it,
+		 * or pf has modified the packet.
+		 */
+		ip->ip_sum = 0;
+		ip->ip_sum = in_cksum(m, off);
+		in_proto_cksum_out(m, NULL);
 	}
 
 	if (inp) {