Index: sys/mbuf.h
===================================================================
RCS file: /cvs/src/sys/sys/mbuf.h,v
retrieving revision 1.234
diff -u -p -r1.234 mbuf.h
--- sys/mbuf.h	9 Feb 2018 02:26:33 -0000	1.234
+++ sys/mbuf.h	10 Feb 2018 09:38:40 -0000
@@ -224,12 +224,14 @@ struct mbuf {
 #define	M_ICMP_CSUM_OUT		0x0200	/* ICMP/ICMPv6 checksum needed */
 #define	M_ICMP_CSUM_IN_OK	0x0400	/* ICMP/ICMPv6 checksum verified */
 #define	M_ICMP_CSUM_IN_BAD	0x0800	/* ICMP/ICMPv6 checksum bad */
+#define M_IPV6_DF_OUT		0x1000	/* don't fragment outgoing IPv6 */
 
 #ifdef _KERNEL
 #define MCS_BITS \
     ("\20\1IPV4_CSUM_OUT\2TCP_CSUM_OUT\3UDP_CSUM_OUT\4IPV4_CSUM_IN_OK" \
     "\5IPV4_CSUM_IN_BAD\6TCP_CSUM_IN_OK\7TCP_CSUM_IN_BAD\10UDP_CSUM_IN_OK" \
-    "\11UDP_CSUM_IN_BAD\12ICMP_CSUM_OUT\13ICMP_CSUM_IN_OK\14ICMP_CSUM_IN_BAD")
+    "\11UDP_CSUM_IN_BAD\12ICMP_CSUM_OUT\13ICMP_CSUM_IN_OK\14ICMP_CSUM_IN_BAD" \
+    "\15IPV6_NODF_OUT")
 #endif
 
 /* mbuf types */
Index: netinet6/ip6_output.c
===================================================================
RCS file: /cvs/src/sys/netinet6/ip6_output.c,v
retrieving revision 1.232
diff -u -p -r1.232 ip6_output.c
--- netinet6/ip6_output.c	1 Sep 2017 15:05:31 -0000	1.232
+++ netinet6/ip6_output.c	10 Feb 2018 09:38:40 -0000
@@ -656,7 +656,10 @@ reroute:
 	 */
 	tlen = m->m_pkthdr.len;
 
-	if (opt && (opt->ip6po_flags & IP6PO_DONTFRAG))
+	if (ISSET(m->m_pkthdr.csum_flags, M_IPV6_DF_OUT)) {
+		CLR(m->m_pkthdr.csum_flags, M_IPV6_DF_OUT);
+		dontfrag = 1;
+	} else if (opt && ISSET(opt->ip6po_flags, IP6PO_DONTFRAG))
 		dontfrag = 1;
 	else
 		dontfrag = 0;