Index: head/sys/netinet/ip_output.c
===================================================================
--- head/sys/netinet/ip_output.c
+++ head/sys/netinet/ip_output.c
@@ -80,6 +80,10 @@
 #include <netinet/in_var.h>
 #include <netinet/ip_var.h>
 #include <netinet/ip_options.h>
+
+#include <netinet/udp.h>
+#include <netinet/udp_var.h>
+
 #ifdef SCTP
 #include <netinet/sctp.h>
 #include <netinet/sctp_crc32.h>
@@ -920,14 +924,28 @@
 in_delayed_cksum(struct mbuf *m)
 {
 	struct ip *ip;
-	uint16_t csum, offset, ip_len;
+	struct udphdr *uh;
+	uint16_t cklen, csum, offset;
 
 	ip = mtod(m, struct ip *);
 	offset = ip->ip_hl << 2 ;
-	ip_len = ntohs(ip->ip_len);
-	csum = in_cksum_skip(m, ip_len, offset);
-	if (m->m_pkthdr.csum_flags & CSUM_UDP && csum == 0)
-		csum = 0xffff;
+
+	if (m->m_pkthdr.csum_flags & CSUM_UDP) {
+		/* if udp header is not in the first mbuf copy udplen */
+		if (offset + sizeof(struct udphdr) > m->m_len)
+			m_copydata(m, offset + offsetof(struct udphdr, uh_ulen),
+				2, (caddr_t)&cklen);
+		else {
+			uh = (struct udphdr *)((caddr_t)ip + offset);
+			cklen = ntohs(uh->uh_ulen);
+		}
+		csum = in_cksum_skip(m, cklen + offset, offset);
+		if (csum == 0)
+			csum = 0xffff;
+	} else {
+		cklen = ntohs(ip->ip_len);
+		csum = in_cksum_skip(m, cklen, offset);
+	}
 	offset += m->m_pkthdr.csum_data;	/* checksum offset */
 
 	/* find the mbuf in the chain where the checksum starts*/