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// #ifndef WIN32_LEAN_AND_MEAN
// #define WIN32_LEAN_AND_MEAN
// #endif
// #pragma warning(push)
// #pragma warning(disable: 4127)
// #include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <iphlpapi.h>
#include <stdio.h>
#define MALLOC(x) HeapAlloc(GetProcessHeap(), 0, (x))
#define FREE(x) HeapFree(GetProcessHeap(), 0, (x))
/* Note: could also use malloc() and free() */
int main()
{
// Declare and initialize variables.
/* variables used for SetIfForwardEntry */
PMIB_IPFORWARDTABLE pIpForwardTable = NULL;
PMIB_IPFORWARDROW pRow = NULL;
DWORD dwSize = 0;
BOOL bOrder = FALSE;
DWORD dwStatus = 0;
DWORD NewGateway = 0xDDBBCCAA; // this is in host order Ip Address AA.BB.CC.DD is DDCCBBAA
DWORD i;
// Find out how big our buffer needs to be.
dwStatus = GetIpForwardTable(pIpForwardTable, &dwSize, bOrder);
if (dwStatus == ERROR_INSUFFICIENT_BUFFER) {
// Allocate the memory for the table
pIpForwardTable = (PMIB_IPFORWARDTABLE) malloc(dwSize);
if (pIpForwardTable == NULL) {
printf("Unable to allocate memory for the IPFORWARDTALE\n");
exit(1);
}
// Now get the table.
dwStatus = GetIpForwardTable(pIpForwardTable, &dwSize, bOrder);
}
if (dwStatus != ERROR_SUCCESS) {
printf("getIpForwardTable failed.\n");
if (pIpForwardTable)
free(pIpForwardTable);
exit(1);
}
// Search for the row in the table we want. The default gateway has a destination
// of 0.0.0.0. Notice that we continue looking through the table, but copy only
// one row. This is so that if there happen to be multiple default gateways, we can
// be sure to delete them all.
for (i = 0; i < pIpForwardTable->dwNumEntries; i++) {
if (pIpForwardTable->table[i].dwForwardDest == 0) {
// We have found the default gateway.
if (!pRow) {
// Allocate some memory to store the row in. This is easier than filling
// in the row structure ourselves, and we can be sure to change only the
// gateway address.
pRow = (PMIB_IPFORWARDROW) malloc(sizeof (MIB_IPFORWARDROW));
if (!pRow) {
printf("Malloc failed. Out of memory.\n");
exit(1);
}
// Copy the row.
memcpy(pRow, &(pIpForwardTable->table[i]),
sizeof (MIB_IPFORWARDROW));
}
// Delete the old default gateway entry.
dwStatus = DeleteIpForwardEntry(&(pIpForwardTable->table[i]));
if (dwStatus != ERROR_SUCCESS) {
printf("Could not delete old gateway\n");
exit(1);
}
}
}
// Set the nexthop field to our new gateway. All the other properties of the route will
// be the same as they were previously.
pRow->dwForwardNextHop = NewGateway;
// Create a new route entry for the default gateway.
dwStatus = SetIpForwardEntry(pRow);
if (dwStatus == NO_ERROR)
printf("Gateway changed successfully\n");
else if (dwStatus == ERROR_INVALID_PARAMETER)
printf("Invalid parameter.\n");
else
printf("Error: %d\n", dwStatus);
// Free resources.
if (pIpForwardTable)
free(pIpForwardTable);
if (pRow)
free(pRow);
}