don't warn for DHCP release commands

src/IPStackSimulator.cc

@@ -401,6 +401,9 @@ void IPStackSimulator::on_client_udp_frame(
     if (dhcp.magic != 0x63825363) {
       throw runtime_error("incorrect DHCP magic cookie");
     }
+    if (dhcp.opcode != 1) { // Request
+      throw runtime_error("DHCP packet is not a request");
+    }
 
     unordered_map<uint8_t, string> option_data;
     for (;;) {
@@ -412,7 +415,6 @@ void IPStackSimulator::on_client_udp_frame(
       option_data.emplace(option, r.read(size));
     }
 
-    if (dhcp.opcode == 1) { // Request
     uint8_t command = 0;
     try {
       command = option_data.at(53).at(0);
@@ -420,6 +422,10 @@ void IPStackSimulator::on_client_udp_frame(
       throw runtime_error("client did not send a DHCP command option");
     }
 
+    if (command == 7) {
+      // Release IP address (we just ignore these)
+
+    } else if ((command == 1) || (command == 3)) {
       // Populate the client's addresses
       c->mac_addr = dhcp.client_hardware_address.data();
       c->ipv4_addr = 0x0A000105; // 10.0.1.5
@@ -495,7 +501,7 @@ void IPStackSimulator::on_client_udp_frame(
       r_data = std::move(w.str());
 
     } else {
-      throw runtime_error("unknown DHCP command");
+      throw runtime_error("client sent unknown DHCP command");
     }
 
   } else if (fi.udp->dest_port == 53) { // DNS
@@ -510,6 +516,7 @@ void IPStackSimulator::on_client_udp_frame(
     throw runtime_error("UDP packet is not DHCP or DNS");
   }
 
+  if (!r_data.empty()) {
     r_ipv4.size = sizeof(IPv4Header) + sizeof(UDPHeader) + r_data.size();
     r_udp.size = sizeof(UDPHeader) + r_data.size();
     r_ipv4.checksum = FrameInfo::computed_ipv4_header_checksum(r_ipv4);
@@ -539,6 +546,7 @@ void IPStackSimulator::on_client_udp_frame(
       w.write(r_data.data(), r_data.size());
       this->log_frame(w.str());
     }
+  }
 }
 
 uint64_t IPStackSimulator::tcp_conn_key_for_connection(