diff --git a/CHANGES b/CHANGES
index e0c725d3e8..99a3773f6c 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,6 @@
+2444.	[port]		Linux, FreeBSD, AIX: Turn of path mtu discovery
+			(clear DF) for UDP responses and requests.
+
 2443.	[bug]		win32: UDP connect() would not generate an event,
 			and so connected UDP sockets would never clean up.
 			Fix this by doing an immediate WSAConnect() rather
diff --git a/lib/isc/unix/socket.c b/lib/isc/unix/socket.c
index a895405bea..531bac1a46 100644
--- a/lib/isc/unix/socket.c
+++ b/lib/isc/unix/socket.c
@@ -15,7 +15,7 @@
  * PERFORMANCE OF THIS SOFTWARE.
  */
 
-/* $Id: socket.c,v 1.302 2008/09/04 05:56:43 marka Exp $ */
+/* $Id: socket.c,v 1.303 2008/09/23 02:20:21 marka Exp $ */
 
 /*! \file */
 
@@ -2031,6 +2031,27 @@ opensocket(isc_socketmgr_t *manager, isc_socket_t *sock) {
 #endif /* ISC_PLATFORM_HAVEIPV6 */
 #endif /* defined(USE_CMSG) */
 
+#if defined(IP_MTU_DISCOVER) && defined(IP_PMTUDISC_DONT)
+		/*
+		 * Turn off Path MTU discovery on IPv4/UDP sockets.
+		 */
+		if (sock->pf == AF_INET) {
+			int action = IP_PMTUDISC_DONT;
+			(void)setsockopt(sock->fd, IPPROTO_IP, IP_MTU_DISCOVER,
+					 &action, sizeof(action));
+		}
+#endif
+#if defined(IP_DONTFRAG)
+		/*
+		 * Turn off Path MTU discovery on IPv4/UDP sockets.
+		 */
+		if (sock->pf == AF_INET) {
+			int off = 0;
+			(void)setsockopt(sock->fd, IPPROTO_IP, IP_DONTFRAG,
+					 &off, sizeof(off));
+		}
+#endif
+
 #if defined(SO_RCVBUF)
 		optlen = sizeof(size);
 		if (getsockopt(sock->fd, SOL_SOCKET, SO_RCVBUF,