bonding: send arp requests even if there's no route to them

Currently we're only sending arp requests if we have a route to the target
(and, thus, can find out the source ip address).

There are some use cases, however, where we don't want/need to set an ip
address (or set up a specific route) for bonding to use arp monitoring *for
traffic generation*. We can easily send arp probes (arp requests with src
ip == 0) to generate arp broadcast responses from the target ip and use
them for determining if the target is up.

This, obviously, won't work with arp validation - because we don't have the
ip address set and, thus, will filter out the responses. So in that case -
print a warning.

CC: François CACHEREUL <f.cachereul@alphalink.fr>
CC: Zhenjie Chen <zhchen@redhat.com>
CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -2155,8 +2155,13 @@ static void bond_arp_send_all(struct bonding *bond, struct slave *slave)
 		rt = ip_route_output(dev_net(bond->dev), targets[i], 0,
 				     RTO_ONLINK, 0);
 		if (IS_ERR(rt)) {
-			pr_debug("%s: no route to arp_ip_target %pI4\n",
-				 bond->dev->name, &targets[i]);
+			/* there's no route to target - try to send arp
+			 * probe to generate any traffic (arp_validate=0)
+			 */
+			if (bond->params.arp_validate && net_ratelimit())
+				pr_warn("%s: no route to arp_ip_target %pI4 and arp_validate is set\n",
+					bond->dev->name, &targets[i]);
+			bond_arp_send(slave->dev, ARPOP_REQUEST, targets[i], 0, 0);
 			continue;
 		}