net: Work-around for brain-damaged Cisco equipment with arp-proxy

Cisco's arp-proxy feature fails to ignore the link-local address range
This means that a link-local device on a network with this Cisco
equipment will reply to ARP requests for our device (in addition to
our reply).
If we happen to reply first, the requester's ARP table will be
populated with our MAC address, and one packet will be sent to us...
shortly following this, the requester will get an ARP reply from the
Cisco equipment telling the requester to send packets their way
instead of to our device from now on.
This work-around detects this link-local condition and will delay
replying to the ARP request for 5ms so that the first packet is sent
to the Cisco equipment and all following packets are sent to our
device.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

net/arp.c

@@ -169,6 +169,20 @@ void ArpReceive(struct ethernet_hdr *et, struct ip_udp_hdr *ip, int len)
 		NetCopyIP(&arp->ar_tpa, &arp->ar_spa);
 		memcpy(&arp->ar_sha, NetOurEther, ARP_HLEN);
 		NetCopyIP(&arp->ar_spa, &NetOurIP);
+
+#ifdef CONFIG_CMD_LINK_LOCAL
+		/*
+		 * Work-around for brain-damaged Cisco equipment with
+		 *   arp-proxy enabled.
+		 *
+		 *   If the requesting IP is not on our subnet, wait 5ms to
+		 *   reply to ARP request so that our reply will overwrite
+		 *   the arp-proxy's instead of the other way around.
+		 */
+		if ((NetReadIP(&arp->ar_tpa) & NetOurSubnetMask) !=
+		    (NetReadIP(&arp->ar_spa) & NetOurSubnetMask))
+			udelay(5000);
+#endif
 		NetSendPacket((uchar *)et, eth_hdr_size + ARP_HDR_SIZE);
 		return;