UDP packet sent out the wrong interface

[Bitblit11]

I wrote code to proxy UDP packets from a routable subnet to a nonroutable subnet.* Everything works great in the debugger.* I see my packet copied to the nonroutable subnet with a new source IP address.* But when I run the executable outside the debugger it sends the packet out the wrong interface.* It sends it back out the routable subnet.*

This is a sequence of when it works correctly (in debugger).

1. Packet from x.x.x.147 to x.x.x.146 (NIC 1)
2. Copied and sent** 192.168.0.10* to* 192.168.0.20 (NIC 2)

This is a sequence of when it does not work correctly (outside of debugger)

1. Packet from x.x.x.147 to x.x.x.146 (NIC 1)
2. Copied and sent** x.x.x.146* to* 192.168.0.20* (NIC 1)

My code does not change.* Why does it behave different when I'm not running in the debugger?

// udpproxy.cpp : main project file.

#include "stdafx.h"

#pragma comment(lib,"ws2_32.lib")
#include<Winsock2.h>
#include<stdio.h>
#include<stdlib.h>
#define BUFFER_LENGTH 1500
#define MAX_PORT_COUNT 20
#define DONT_FRAGMENT false

using namespace System;
using namespace System::Net;
using namespace System::Net::Sockets;

int main(int argc, char *argv[]) {
	WSADATA wsaData;
	Socket ^sSocket;
	int iLen;
	int iSend;
	int iRecv;
	int bytesSent = 0;
	int srvPort = 0;
	int dstPort = 0;
	char recv_buf[BUFFER_LENGTH];
	array<UdpClient^>^ destination = gcnew array<UdpClient^>(MAX_PORT_COUNT);
	int iAddressCount = 0;
	char *address;
	char *srcAddressChar;
	int i = 0;

	struct sockaddr_in ser,cli;

	if (argc < 4) {                  // Test for correct number of parameters
		printf("Usage: UDPProxy <Server Port> <Dest Port> <Dest IP Address>[,<IP Address>,<IP Address>,...] ");
		return -5;
	}
	
	if(WSAStartup(MAKEWORD(2,2),&wsaData)!=0)
	{
		printf("Failed to load Winsock.\n");
		return -1;
	}

	srvPort = atoi(argv[1]);
	printf("--------------------------\n");
	printf("Listening to port %i\n", srvPort);

	dstPort = atoi(argv[2]);
	printf("Forwarding to port %i:\n", dstPort);
	
	printf("Targets:\n");
	address = strtok(argv[3], ",");
	while(address != 0)
	{
		System::String ^dstAddress = gcnew System::String(address);
		destination[iAddressCount] = gcnew UdpClient(dstAddress, dstPort);
		printf("     %s\n", dstAddress);
		iAddressCount++;
		address = strtok(0, ",");
	} 
    printf("--------------------------\n");

	UdpClient^ client = gcnew UdpClient();
	client->DontFragment = DONT_FRAGMENT;
	
    Socket^ socket = gcnew Socket(AddressFamily::InterNetwork, SocketType::Dgram, ProtocolType::Udp);
    IPEndPoint^ ipep = gcnew IPEndPoint(IPAddress::Any, srvPort);

    socket->Bind(ipep);

    while(true) 
    {
        array<unsigned char>^ message = gcnew array<unsigned char>(BUFFER_LENGTH);
        EndPoint^ Remote = (EndPoint^) gcnew IPEndPoint(IPAddress::Any, 0);

        int recv = socket->ReceiveFrom(message, Remote);
		
		//printf(".");

		for(int i=0; i < iAddressCount; ++i)
		{
			bytesSent = destination[i]->Send(message, recv);
			if(bytesSent == recv)
			{
				printf(".");
			}
		}
    }

	WSACleanup();

	return 0;
}

[VictorN]

Your code has nothing to do with this Visual C++ Forum. Nor is it a valid native C++.
If it is something like managed C++ code then please, ask your question in the corresponding Managed C++/CLI forum.

[Igor Vartanov]

Why does it behave different when I'm not running in the debugger?

Usually the most probable reason is: your code has some error.