diff --git a/CHANGES b/CHANGES
index 296fcac1ca..b1b55177a8 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,8 @@
+4435.	[tuning]	Only set IPV6_USE_MIN_MTU for UDP when the message
+			will not fit into a single IPv4 encapsulated IPv6
+			UDP packet when transmitted over a Ethernet link.
+			[RT #42871]
+
 4434.	[protocol]	Return EDNS EXPIRE option for master zones in addition
 			to slave zones. [RT #43008]
 
diff --git a/bin/named/client.c b/bin/named/client.c
index 3faf6d2e11..20963eb46f 100644
--- a/bin/named/client.c
+++ b/bin/named/client.c
@@ -923,6 +923,15 @@ client_sendpkg(ns_client_t *client, isc_buffer_t *buffer) {
 
 	isc_buffer_usedregion(buffer, &r);
 
+	/*
+	 * If this is a UDP client and the IPv6 packet can't be
+	 * encapsulated without generating a PTB on a 1500 octet
+	 * MTU link force fragmentation at 1280 if it is a IPv6
+	 * response.
+	 */
+	if (!TCP_CLIENT(client) && r.length > 1432)
+		client->sendevent->attributes |= ISC_SOCKEVENTATTR_USEMINMTU;
+
 	CTRACE("sendto");
 
 	result = isc_socket_sendto2(sock, &r, client->task,
diff --git a/lib/isc/include/isc/socket.h b/lib/isc/include/isc/socket.h
index 1acfed5239..d8956740ef 100644
--- a/lib/isc/include/isc/socket.h
+++ b/lib/isc/include/isc/socket.h
@@ -251,6 +251,7 @@ struct isc_socket_connev {
  * _PKTINFO:	The pktinfo member is valid.
  * _MULTICAST:	The UDP packet was received via a multicast transmission.
  * _DSCP:	The UDP DSCP value is valid.
+ * _USEMINMTU:	Set the per packet IPV6_USE_MIN_MTU flag.
  */
 #define ISC_SOCKEVENTATTR_ATTACHED		0x80000000U /* internal */
 #define ISC_SOCKEVENTATTR_TRUNC			0x00800000U /* public */
@@ -259,6 +260,7 @@ struct isc_socket_connev {
 #define ISC_SOCKEVENTATTR_PKTINFO		0x00100000U /* public */
 #define ISC_SOCKEVENTATTR_MULTICAST		0x00080000U /* public */
 #define ISC_SOCKEVENTATTR_DSCP			0x00040000U /* public */
+#define ISC_SOCKEVENTATTR_USEMINMTU		0x00020000U /* public */
 /*@}*/
 
 #define ISC_SOCKEVENT_ANYEVENT  (0)
diff --git a/lib/isc/unix/socket.c b/lib/isc/unix/socket.c
index 2496bd1f61..47606b2997 100644
--- a/lib/isc/unix/socket.c
+++ b/lib/isc/unix/socket.c
@@ -6,8 +6,6 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  */
 
-/* $Id$ */
-
 /*! \file */
 
 #include <config.h>
@@ -1505,12 +1503,12 @@ build_msghdr_send(isc__socket_t *sock, isc_socketevent_t *dev,
 	msg->msg_control = NULL;
 	msg->msg_controllen = 0;
 	msg->msg_flags = 0;
-#if defined(USE_CMSG) && defined(ISC_PLATFORM_HAVEIN6PKTINFO)
-	if ((sock->type == isc_sockettype_udp)
-	    && ((dev->attributes & ISC_SOCKEVENTATTR_PKTINFO) != 0)) {
-#if defined(IPV6_USE_MIN_MTU)
-		int use_min_mtu = 1;	/* -1, 0, 1 */
-#endif
+#if defined(USE_CMSG)
+
+#if defined(ISC_PLATFORM_HAVEIN6PKTINFO)
+	if ((sock->type == isc_sockettype_udp) &&
+	    ((dev->attributes & ISC_SOCKEVENTATTR_PKTINFO) != 0))
+	{
 		struct in6_pktinfo *pktinfop;
 
 		socket_log(sock, NULL, TRACE,
@@ -1528,12 +1526,15 @@ build_msghdr_send(isc__socket_t *sock, isc_socketevent_t *dev,
 		cmsgp->cmsg_len = cmsg_len(sizeof(struct in6_pktinfo));
 		pktinfop = (struct in6_pktinfo *)CMSG_DATA(cmsgp);
 		memmove(pktinfop, &dev->pktinfo, sizeof(struct in6_pktinfo));
+	} 
+#endif
+
 #if defined(IPV6_USE_MIN_MTU)
-		/*
-		 * Set IPV6_USE_MIN_MTU as a per packet option as FreeBSD
-		 * ignores setsockopt(IPV6_USE_MIN_MTU) when IPV6_PKTINFO
-		 * is used.
-		 */
+	if ((sock->type == isc_sockettype_udp) &&
+	    ((dev->attributes & ISC_SOCKEVENTATTR_USEMINMTU) != 0))
+	{
+		int use_min_mtu = 1;	/* -1, 0, 1 */
+
 		cmsgp = (struct cmsghdr *)(sock->sendcmsgbuf +
 					   msg->msg_controllen);
 		msg->msg_controllen += cmsg_space(sizeof(use_min_mtu));
@@ -1543,8 +1544,8 @@ build_msghdr_send(isc__socket_t *sock, isc_socketevent_t *dev,
 		cmsgp->cmsg_type = IPV6_USE_MIN_MTU;
 		cmsgp->cmsg_len = cmsg_len(sizeof(use_min_mtu));
 		memmove(CMSG_DATA(cmsgp), &use_min_mtu, sizeof(use_min_mtu));
-#endif
 	}
+#endif
 
 	if (isc_dscp_check_value > -1) {
 		if (sock->type == isc_sockettype_udp)
@@ -1622,7 +1623,7 @@ build_msghdr_send(isc__socket_t *sock, isc_socketevent_t *dev,
 		}
 #endif
 	}
-#endif /* USE_CMSG && ISC_PLATFORM_HAVEIPV6 */
+#endif /* USE_CMSG */
 #else /* ISC_NET_BSD44MSGHDR */
 	msg->msg_accrights = NULL;
 	msg->msg_accrightslen = 0;
@@ -2756,7 +2757,8 @@ opensocket(isc__socketmgr_t *manager, isc__socket_t *sock,
 	/*
 	 * Use minimum mtu if possible.
 	 */
-	use_min_mtu(sock);
+	if (sock->type == isc_sockettype_tcp)
+		use_min_mtu(sock);
 
 #if defined(USE_CMSG) || defined(SO_RCVBUF)
 	if (sock->type == isc_sockettype_udp) {