Export (0) Print
Expand All
Expand Minimize
3 out of 8 rated this helpful - Rate this topic

CreateIpForwardEntry function

The CreateIpForwardEntry function creates a route in the local computer's IPv4 routing table.

Syntax


DWORD CreateIpForwardEntry(
  _In_  PMIB_IPFORWARDROW pRoute
);

Parameters

pRoute [in]

A pointer to a MIB_IPFORWARDROW structure that specifies the information for the new route. The caller must specify values for all members of this structure. The caller must specify MIB_IPPROTO_NETMGMT for the dwForwardProto member of MIB_IPFORWARDROW.

Return value

The function returns NO_ERROR (zero) if the function is successful.

If the function fails, the return value is one of the following error codes.

Return codeDescription
ERROR_ACCESS_DENIED

Access is denied. This error is returned on Windows Vista and Windows Server 2008 under several conditions that include the following: the user lacks the required administrative privileges on the local computer or the application is not running in an enhanced shell as the built-in Administrator (RunAs administrator).

ERROR_INVALID_PARAMETER

An input parameter is invalid, no action was taken. This error is returned if the pRoute parameter is NULL, the dwForwardProto member of MIB_IPFORWARDROW was not set to MIB_IPPROTO_NETMGMT, the dwForwardMask member of the PMIB_IPFORWARDROW structure is not a valid IPv4 subnet mask, or one of the other members of the MIB_IPFORWARDROW structure is invalid.

ERROR_NOT_SUPPORTED

The IPv4 transport is not configured on the local computer.

Other

Use FormatMessage to obtain the message string for the returned error.

 

Remarks

The dwForwardProto member of MIB_IPFORWARDROW structure pointed to by the route parameter must be set to MIB_IPPROTO_NETMGMT otherwise CreateIpForwardEntry will fail. Routing protocol identifiers are used to identify route information for the specified routing protocol. For example, MIB_IPPROTO_NETMGMT is used to identify route information for IP routing set through network management such as the Dynamic Host Configuration Protocol (DHCP), the Simple Network Management Protocol (SNMP), or by calls to the CreateIpForwardEntry, DeleteIpForwardEntry, or SetIpForwardEntry functions.

On Windows Vista and Windows Server 2008, the route metric specified in the dwForwardMetric1 member of the MIB_IPFORWARDROW structure pointed to by pRoute parameter represents a combination of the route metric added to the interface metric specified in the Metric member of the MIB_IPINTERFACE_ROW structure of the associated interface. So the dwForwardMetric1 member of the MIB_IPFORWARDROW structure should be equal to or greater than Metric member of the associated MIB_IPINTERFACE_ROW structure. If an application would like to set the route metric to 0, then the dwForwardMetric1 member of the MIB_IPFORWARDROW structure should be set equal to the value of the interface metric specified in the Metric member of the associated MIB_IPINTERFACE_ROW structure. An application can retrieve the interface metric by calling the GetIpInterfaceEntry function.

On Windows Vista and Windows Server 2008, the CreateIpForwardEntry only works on interfaces with a single sub-interface (where the interface LUID and subinterface LUID are the same). The dwForwardIfIndex member of the MIB_IPFORWARDROW structure specifies the interface.

A number of members of the MIB_IPFORWARDROW structure pointed to by the route parameter are not currently used by CreateIpForwardEntry. These members include dwForwardPolicy, dwForwardType, dwForwardAge, dwForwardNextHopAS, dwForwardMetric2, dwForwardMetric3, dwForwardMetric4, and dwForwardMetric5.

A new route created by CreateIpForwardEntry will automatically have a default value for dwForwardAge of INFINITE.

To modify an existing route in the IPv4 routing table, use the SetIpForwardEntry function. To retrieve the IPv4 routing table, call the GetIpForwardTable function.

On Windows Vista and later, the CreateIpForwardEntry function can only be called by a user logged on as a member of the Administrators group. If CreateIpForwardEntry is called by a user that is not a member of the Administrators group, the function call will fail and ERROR_ACCESS_DENIED is returned.

The CreateIpForwardEntry function can also fail because of user account control (UAC) on Windows Vista and later. If an application that contains this function is executed by a user logged on as a member of the Administrators group other than the built-in Administrator, this call will fail unless the application has been marked in the manifest file with a requestedExecutionLevel set to requireAdministrator. If the application lacks this manifest file, a user logged on as a member of the Administrators group other than the built-in Administrator must then be executing the application in an enhanced shell as the built-in Administrator (RunAs administrator) for this function to succeed.

Note  On Windows NT 4.0 and Windows 2000 and later, this function executes a privileged operation. For this function to execute successfully, the caller must be logged on as a member of the Administrators group or the NetworkConfigurationOperators group.

Examples

The following example demonstrates how to change the default gateway to NewGateway. Simply calling GetIpForwardTable, changing the gateway, and then calling SetIpForwardEntry will not change the route, but rather will just add a new one. If for some reason there are multiple default gateways present, this code will delete them. Note that the new gateway must be viable; otherwise, TCP/IP will ignore the change.

Note  Executing this code will change your IP routing tables and will likely cause network activity to fail.


#ifndef WIN32_LEAN_AND_MEAN
#define WIN32_LEAN_AND_MEAN
#endif

#include <windows.h>
#include <winsock2.h>
#include <iphlpapi.h>
#include <stdio.h>
#include <stdlib.h>

#pragma comment(lib, "iphlpapi.lib")

int main()
{
    // Declare and initialize variables

    PMIB_IPFORWARDTABLE pIpForwardTable = NULL;
    PMIB_IPFORWARDROW pRow = NULL;
    DWORD dwSize = 0;
    BOOL bOrder = FALSE;
    DWORD dwStatus = 0;
    DWORD NewGateway = 0xDDCCBBAA;  // this is in host order Ip Address AA.BB.CC.DD is DDCCBBAA
    
    unsigned int 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
        if (!(pIpForwardTable = (PMIB_IPFORWARDTABLE) malloc(dwSize))) {
            printf("Malloc failed. Out of memory.\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 we 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 = CreateIpForwardEntry(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);

    exit(0);
}


Requirements

Minimum supported client

Windows 2000 Professional [desktop apps only]

Minimum supported server

Windows 2000 Server [desktop apps only]

Header

Iphlpapi.h

Library

Iphlpapi.lib

DLL

Iphlpapi.dll

See also

DeleteIpForwardEntry
GetIpForwardTable
GetIpInterfaceEntry
IP Helper Start Page
IP Helper Function Reference
MIB_IPFORWARDROW
SetIpForwardEntry

 

 

Build date: 11/16/2013

Community Additions

ADD
Show:
© 2014 Microsoft. All rights reserved.