Add ARP Responder module

Address code review comments,
1) Drop ARP packets in case no cache entry is found and
not an ARP request
2) Update TODO's
3) Use 0 as the default incoming gate

Author: GalSagie

bessctl/conf/samples/arp.bess

@@ -1,11 +1,11 @@
 import scapy.all as scapy
 
 eth_header = scapy.Ether(src='02:1e:67:9f:4d:ae', dst='ff:ff:ff:ff:ff:ff')
-arp_header = scapy.ARP(op=1, pdst='1.1.1.1')
+arp_header = scapy.ARP(op=1, pdst='1.2.3.4')
 pkt = eth_header/arp_header
 packets = [str(pkt)]
 
-arp = ArpResponder(name="arp")
-arp.add(ip='1.1.1.1', mac_addr='11:22:33:44:55:66')
+arp::ArpResponder()
+arp.add(ip='1.2.3.4', mac_addr='A0:22:33:44:55:66')
 
 Source() -> Rewrite(templates=packets) -> arp -> Sink()

core/modules/arp_responder.cc

@@ -31,13 +31,12 @@ CommandResponse ArpResponder::CommandAdd(const bess::pb::ArpResponderArg &arg) {
 
 void ArpResponder::ProcessBatch(bess::PacketBatch *batch) {
   gate_idx_t out_gates[bess::PacketBatch::kMaxBurst];
-  gate_idx_t incoming_gate = get_igate();
 
   int cnt = batch->cnt();
   for (int i = 0; i < cnt; i++) {
     bess::Packet *pkt = batch->pkts()[i];
 
-    out_gates[i] = incoming_gate;
+    out_gates[i] = 0;
 
     Ethernet *eth = pkt->head_data<Ethernet *>();
     if (eth->ether_type != be16_t(Ethernet::Type::kArp)) {
@@ -64,11 +63,17 @@ void ArpResponder::ProcessBatch(bess::PacketBatch *batch) {
 
         arp->target_ip_addr = arp->sender_ip_addr;
         arp->sender_ip_addr = entry.ip_addr;
+      } else {
+        // Did not find an ARP entry in cache, drop packet
+        // TODO(galsagie) Optinally continue packet to next module here
+        out_gates[i] = DROP_GATE;
       }
     } else if (arp->opcode == be16_t(Arp::Opcode::kReply)) {
       // TODO(galsagie) When learn is added, learn SRC MAC here
+      out_gates[i] = DROP_GATE;
     } else {
       // TODO(galsagie) Other opcodes are not handled yet.
+      out_gates[i] = DROP_GATE;
     }
   }
 

protobuf/module_msg.proto

@@ -974,7 +974,8 @@ message WildcardMatchArg {
 }
 
 /**
- * The ARP Responder module is responding to ARP requests and can optionally learn new MAC's-IP's mapping
+ * The ARP Responder module is responding to ARP requests
+ * TODO: Dynamic learn new MAC's-IP's mapping
  *
  * __Input Gates__: 1
  * __Output Gates__: 1