Work around IGDs that deduplicate entries based on the internal port

This is a violation of the UPnP IGD specification but we can relay through an alternate port as a workaround.

miss/relay.cpp

@@ -0,0 +1,112 @@
+#define WIN32_LEAN_AND_MEAN
+#include <Windows.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <WinSock2.h>
+#include <Ws2ipdef.h>
+
+#include "relay.h"
+
+typedef struct _UDP_TUPLE {
+    SOCKET socket;
+    unsigned short port;
+} UDP_TUPLE, *PUDP_TUPLE;
+
+DWORD
+WINAPI
+UdpRelayThreadProc(LPVOID Context)
+{
+    PUDP_TUPLE tuple = (PUDP_TUPLE)Context;
+    fd_set fds;
+    int err;
+    SOCKADDR_IN lastRemoteAddr;
+
+    RtlZeroMemory(&lastRemoteAddr, sizeof(lastRemoteAddr));
+
+    for (;;) {
+        char buffer[4096];
+        SOCKADDR_IN sourceAddr;
+        int sourceAddrLen;
+
+        FD_ZERO(&fds);
+
+        FD_SET(tuple->socket, &fds);
+
+        err = select(0, &fds, NULL, NULL, NULL);
+        if (err <= 0) {
+            break;
+        }
+
+        sourceAddrLen = sizeof(sourceAddr);
+        err = recvfrom(tuple->socket, buffer, sizeof(buffer), 0, (PSOCKADDR)&sourceAddr, &sourceAddrLen);
+        if (err == SOCKET_ERROR) {
+            continue;
+        }
+        
+        SOCKADDR_IN destinationAddr;
+        if (RtlEqualMemory(&sourceAddr.sin_addr, &in4addr_loopback, sizeof(sourceAddr.sin_addr))) {
+            // Traffic incoming from loopback interface - send it to the last remote address
+            destinationAddr = lastRemoteAddr;
+        }
+        else {
+            // Traffic incoming from the remote host - remember the source
+            lastRemoteAddr = sourceAddr;
+
+            // Send it to the normal port via the loopback adapter
+            destinationAddr = sourceAddr;
+            destinationAddr.sin_addr = in4addr_loopback;
+            destinationAddr.sin_port = htons(tuple->port);
+        }
+
+        sendto(tuple->socket, buffer, err, 0, (PSOCKADDR)&destinationAddr, sizeof(destinationAddr));
+    }
+
+    closesocket(tuple->socket);
+    free(tuple);
+    return 0;
+}
+
+int StartUdpRelay(unsigned short Port)
+{
+    SOCKET sock;
+    SOCKADDR_IN addr;
+    HANDLE thread;
+    PUDP_TUPLE tuple;
+
+    sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+    if (sock == INVALID_SOCKET) {
+        printf("socket() failed: %d\n", WSAGetLastError());
+        return WSAGetLastError();
+    }
+
+    // Bind to the alternate port
+    RtlZeroMemory(&addr, sizeof(addr));
+    addr.sin_family = AF_INET;
+    addr.sin_port = htons(Port + RELAY_PORT_OFFSET);
+    if (bind(sock, (PSOCKADDR)&addr, sizeof(addr)) == SOCKET_ERROR) {
+        printf("bind() failed: %d\n", WSAGetLastError());
+        closesocket(sock);
+        return WSAGetLastError();
+    }
+
+    tuple = (PUDP_TUPLE)malloc(sizeof(*tuple));
+    if (tuple == NULL) {
+        return ERROR_OUTOFMEMORY;
+    }
+
+    tuple->socket = sock;
+    tuple->port = Port;
+
+    thread = CreateThread(NULL, 0, UdpRelayThreadProc, tuple, 0, NULL);
+    if (thread == NULL) {
+        printf("CreateThread() failed: %d\n", GetLastError());
+        closesocket(sock);
+        return GetLastError();
+    }
+
+    CloseHandle(thread);
+
+    return 0;
+}