udp: Don't update ARP cache on receipt of broadcast/multicast UDP dat…

…agrams. [DC-319]

This can result in unnecessary churn of our ARP table on networks with lots of broadcast/multicast traffic.

Lib/Rabbit4000/tcpip/udp.lib

@@ -771,20 +771,21 @@ _udp_nodebug ll_prefix __far * udp_handler(ll_prefix __far * LL, byte * hdrbuf)
 
    LOCK_SOCK(s);
 
-   // Cache the hardware address, since we probably want to respond
-   //ath = arpcache_new(source, udp_datagram_info.hwa, iface);
-   //Vantive 23956: now use ARP lookup instead of sending back to source's MAC address.
-   ath = arpresolve_start_iface(source, iface);
-   if (ath > 0 && (ath = arpresolve_check(ath, source)) > 0) {
+	if(! (udp_datagram_info.flags & (UDI_BROADCAST_IP | UDI_MULTICAST_IP))) {
+		// Cache the hardware address, since we probably want to respond
+		// to unicast (but not necessarily broadcast or multicast) UDP datagrams.
+		ath = arpresolve_start_iface(source, iface);
+		if (ath > 0 && (ath = arpresolve_check(ath, source)) > 0) {
 		// Router/localhost resolved, use its MAC
 #ifdef UDP_VERBOSE
-		if (debug_on > 0) printf("UDP: ...using ARP table entry\n");
+			if (debug_on > 0) printf("UDP: ...using ARP table entry\n");
 #endif
-   }
-
-   if (ath > 0)
-   	s->sath = ath;
+		}
 
+		if (ath > 0)
+			s->sath = ath;
+	}
+
    /* process user data */
    dp = LL->payload;