learn the client's l2 address from an incomping tcp syn

Author: gasbytes

src/wolfip.c

@@ -1322,6 +1322,9 @@ struct arp_pending_entry {
 
 static int arp_lookup(struct wolfIP *s, unsigned int if_idx, ip4 ip, uint8_t *mac);
 static void arp_request(struct wolfIP *s, unsigned int if_idx, ip4 tip);
+static void arp_store_neighbor(struct wolfIP *s, unsigned int if_idx, ip4 ip,
+                               const uint8_t *mac);
+static int arp_neighbor_index(struct wolfIP *s, unsigned int if_idx, ip4 ip);
 #if WOLFIP_ENABLE_FORWARDING
 static void wolfIP_forward_packet(struct wolfIP *s, unsigned int out_if,
                                   struct wolfIP_ip_packet *ip, uint32_t len,
@@ -5203,6 +5206,13 @@ static void tcp_input(struct wolfIP *S, unsigned int if_idx,
                     t->sock.tcp.snd_una = t->sock.tcp.seq;
                     t->dst_port = ee16(tcp->src_port);
                     t->remote_ip = ee32(tcp->ip.src);
+                    {
+                        unsigned int nh_if = if_idx;
+                        ip4 nh = wolfIP_select_nexthop_ex(S, &nh_if, t->remote_ip);
+                        if (!wolfIP_ll_is_non_ethernet(S, nh_if) &&
+                                arp_neighbor_index(S, nh_if, nh) < 0)
+                            arp_store_neighbor(S, nh_if, nh, tcp->ip.eth.src);
+                    }
                     t->events |= CB_EVENT_READABLE; /* Keep flag until application calls accept */
                     tcp_process_ts(t, tcp, frame_len);
                     tcp_send_syn(t, TCP_FLAG_SYN | TCP_FLAG_ACK);