ethernet/arp: Use gateway for non local ipv4 routing.

Detect non local IPv4 destination addresses earlier and route them via
the gw address. Ensure that the ARP table is populated with the GW
address rather than the final destination address.

Jira: ZEP-1473

Change-Id: I3b628584148b760340ef0fea4da4e8893702c832
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>

Author: mshawcroft

subsys/net/ip/l2/arp.c

@@ -108,6 +108,7 @@ static inline struct in_addr *if_get_addr(struct net_if *iface)
 }
 
 static inline struct net_buf *prepare_arp(struct net_if *iface,
+					  struct in_addr *next_addr,
 					  struct arp_entry *entry,
 					  struct net_buf *pending)
 {
@@ -143,7 +144,7 @@ static inline struct net_buf *prepare_arp(struct net_if *iface,
 		entry->pending = net_buf_ref(pending);
 		entry->iface = net_nbuf_iface(buf);
 
-		net_ipaddr_copy(&entry->ip, &NET_IPV4_BUF(pending)->dst);
+		net_ipaddr_copy(&entry->ip, next_addr);
 
 		memcpy(&eth->src.addr,
 		       net_if_get_link_addr(entry->iface)->addr,
@@ -165,12 +166,7 @@ static inline struct net_buf *prepare_arp(struct net_if *iface,
 
 	memset(&hdr->dst_hwaddr.addr, 0x00, sizeof(struct net_eth_addr));
 
-	/* Is the destination in local network */
-	if (!net_if_ipv4_addr_mask_cmp(iface, &NET_IPV4_BUF(pending)->dst)) {
-		net_ipaddr_copy(&hdr->dst_ipaddr, &iface->ipv4.gw);
-	} else {
-		net_ipaddr_copy(&hdr->dst_ipaddr, &NET_IPV4_BUF(pending)->dst);
-	}
+	net_ipaddr_copy(&hdr->dst_ipaddr, next_addr);
 
 	memcpy(hdr->src_hwaddr.addr, eth->src.addr,
 	       sizeof(struct net_eth_addr));
@@ -203,6 +199,7 @@ struct net_buf *net_arp_prepare(struct net_buf *buf)
 	struct arp_entry *entry, *free_entry = NULL, *non_pending = NULL;
 	struct net_linkaddr *ll;
 	struct net_eth_hdr *hdr;
+	struct in_addr *addr;
 
 	if (!buf || !buf->frags) {
 		return NULL;
@@ -239,12 +236,21 @@ struct net_buf *net_arp_prepare(struct net_buf *buf)
 
 	hdr = (struct net_eth_hdr *)net_nbuf_ll(buf);
 
+	/* Is the destination in the local network, if not route via
+	 * the gateway address.
+	 */
+	if (!net_if_ipv4_addr_mask_cmp(net_nbuf_iface(buf),
+				       &NET_IPV4_BUF(buf)->dst)) {
+		addr = &net_nbuf_iface(buf)->ipv4.gw;
+	} else {
+		addr = &NET_IPV4_BUF(buf)->dst;
+	}
+
 	/* If the destination address is already known, we do not need
 	 * to send any ARP packet.
 	 */
 	entry = find_entry(net_nbuf_iface(buf),
-			   &NET_IPV4_BUF(buf)->dst,
-			   &free_entry, &non_pending);
+			   addr, &free_entry, &non_pending);
 	if (!entry) {
 		if (!free_entry) {
 			/* So all the slots are occupied, use the first
@@ -259,7 +265,7 @@ struct net_buf *net_arp_prepare(struct net_buf *buf)
 				struct net_buf *req;
 
 				req = prepare_arp(net_nbuf_iface(buf),
-						  NULL, buf);
+						  addr, NULL, buf);
 				NET_DBG("Resending ARP %p", req);
 
 				net_nbuf_unref(buf);
@@ -270,7 +276,7 @@ struct net_buf *net_arp_prepare(struct net_buf *buf)
 			free_entry = non_pending;
 		}
 
-		return prepare_arp(net_nbuf_iface(buf), free_entry, buf);
+		return prepare_arp(net_nbuf_iface(buf), addr, free_entry, buf);
 	}
 
 	ll = net_if_get_link_addr(entry->iface);