Icmp ttl expire to be answered with gw ip

For traceroute requests from vm, when ttl expires for them, Agent diag
module should respond with gateway address of network. Currently,
agent is responding with its underlay Ip address. This is modified
by responding with gateway address for traceroute requests.

Change-Id: Iee194c4f46b38f5acc1b7618a7ce770a3cd058a4
Closes-Bug: #1616430
(cherry picked from commit e6883b0)

src/vnsw/agent/diag/diag_pkt_handler.cc

@@ -107,14 +107,30 @@ void DiagPktHandler::SendTimeExceededPacket() {
     // Retain the agent-header before ethernet header
     uint16_t len = (char *)pkt_info_->eth - (char *)pkt_info_->pkt;
 
+    Ip4Address src_ip(0);
+    VmInterface *vm_itf = dynamic_cast<VmInterface *>
+        (agent()->interface_table()->FindInterface(GetInterfaceIndex()));
+    if (vm_itf == NULL) {
+        src_ip = agent()->router_id();
+    } else {
+        if (vm_itf->vn() == NULL) {
+            return;
+        }
+        const VnIpam *ipam = vm_itf->vn()->GetIpam(pkt_info_->ip_saddr.to_v4());
+        if (ipam == NULL) {
+            return;
+        }
+        src_ip = ipam->default_gw.to_v4();
+    }
+
     // Form ICMP Packet with EthHdr - IP Header - ICMP Header
     len += EthHdr(ptr + len, buf_len - len, agent()->vhost_interface()->mac(),
                   MacAddress(pkt_info_->eth->ether_shost), ETHERTYPE_IP,
                   VmInterface::kInvalidVlanId);
 
     uint16_t ip_len = sizeof(iphdr) + 8 + icmp_len;
     len += IpHdr(ptr + len, buf_len - len, ip_len,
-                 htonl(agent()->router_id().to_ulong()),
+                 htonl(src_ip.to_ulong()),
                  htonl(pkt_info_->ip_saddr.to_v4().to_ulong()),
                  IPPROTO_ICMP, DEFAULT_IP_ID, DEFAULT_IP_TTL);