Index: in_cksum.c
===================================================================
RCS file: /cvs/src/usr.sbin/tcpdump/in_cksum.c,v
retrieving revision 1.1
diff -u -p -r1.1 in_cksum.c
--- in_cksum.c	20 Jun 2014 04:01:42 -0000	1.1
+++ in_cksum.c	6 Jul 2018 04:31:15 -0000
@@ -86,3 +86,40 @@ in_cksum_shouldbe(u_int16_t sum, u_int16
 	shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
 	return shouldbe;
 }
+
+uint32_t
+in_cksum_add(const void *buf, size_t len, uint32_t sum)
+{
+	const uint16_t *words = buf;
+
+	while (len > 1) {
+		sum += *words++;
+		len -= sizeof(*words);
+	}
+
+	if (len == 1) {
+		uint8_t byte = *(const uint8_t *)words;
+		sum += htons(byte << 8);
+	}
+
+	return (sum);
+}
+
+uint16_t
+in_cksum_fini(uint32_t sum)
+{
+	sum = (sum >> 16) + (sum & 0xffff);	/* add hi 16 to low 16 */
+	sum += (sum >> 16);			/* add carry */
+
+	return (~sum);
+}
+
+/*
+ * compute an IP header checksum.
+ * don't modifiy the packet.
+ */
+uint16_t
+in_cksum(const void *addr, size_t len, uint32_t sum)
+{
+	return (in_cksum_fini(in_cksum_add(addr, len, sum)));
+}
Index: interface.h
===================================================================
RCS file: /cvs/src/usr.sbin/tcpdump/interface.h,v
retrieving revision 1.72
diff -u -p -r1.72 interface.h
--- interface.h	10 Feb 2018 10:00:32 -0000	1.72
+++ interface.h	6 Jul 2018 04:31:15 -0000
@@ -295,5 +295,7 @@ extern void ospf6_print(const u_char *, 
 extern void dhcp6_print(const u_char *, u_int, u_short, u_short);
 #endif /*INET6*/
 
-extern u_short in_cksum(const u_short *addr, int len, int csum);
+extern uint32_t in_cksum_add(const void *, size_t, uint32_t);
+extern uint16_t in_cksum_fini(uint32_t);
+extern uint16_t in_cksum(const void *, size_t, uint32_t);
 extern u_int16_t in_cksum_shouldbe(u_int16_t, u_int16_t);
Index: print-ip.c
===================================================================
RCS file: /cvs/src/usr.sbin/tcpdump/print-ip.c,v
retrieving revision 1.49
diff -u -p -r1.49 print-ip.c
--- print-ip.c	10 Feb 2018 10:00:32 -0000	1.49
+++ print-ip.c	6 Jul 2018 04:31:15 -0000
@@ -312,40 +312,6 @@ trunc:
 }
 
 /*
- * compute an IP header checksum.
- * don't modifiy the packet.
- */
-u_short
-in_cksum(const u_short *addr, int len, int csum)
-{
-	int nleft = len;
-	const u_short *w = addr;
-	u_short answer;
-	int sum = csum;
-
- 	/*
-	 *  Our algorithm is simple, using a 32 bit accumulator (sum),
-	 *  we add sequential 16 bit words to it, and at the end, fold
-	 *  back all the carry bits from the top 16 bits into the lower
-	 *  16 bits.
- 	 */
-	while (nleft > 1)  {
-		sum += *w++;
-		nleft -= 2;
-	}
-	if (nleft == 1)
-		sum += htons(*(u_char *)w<<8);
-
-	/*
-	 * add back carry outs from top 16 bits to low 16 bits
-	 */
-	sum = (sum >> 16) + (sum & 0xffff);	/* add hi 16 to low 16 */
-	sum += (sum >> 16);			/* add carry */
-	answer = ~sum;				/* truncate to 16 bits */
-	return (answer);
-}
-
-/*
  * print an IP datagram.
  */
 void