GetIpForwardTable

The GetIpForwardTable function retrieves the IP forwarding table for the local computer.

This function is declared in iphlpapi.h.


PMIB_IPFORWARDTABLE GetIpForwardTable(
  [out]          PMIB_IPFORWARDTABLE pIpForwardTable,
  [in, out]      PULONG              pSize,
  [in]           BOOL                bOrder
);

Parameters

Parameter	Description
`pIpForwardTable`	A pointer to a buffer that receives the IP forwarding table as a `MIB_IPFORWARDTABLE` structure. If the buffer is not large enough, the function returns `ERROR_INSUFFICIENT_BUFFER` and this parameter is populated with the required buffer size.
`pSize`	On input, specifies the size of the buffer pointed to by `pIpForwardTable`, in bytes. On output, if the buffer is not large enough, this parameter receives the required buffer size.
`bOrder`	A boolean value that specifies whether the returned forwarding entries should be sorted. If this parameter is `TRUE`, the entries are sorted in ascending order by destination address.

Return Value

If the function succeeds, the return value is NO_ERROR.

If the buffer pointed to by pIpForwardTable is not large enough to hold the returned forwarding entries, GetIpForwardTable returns ERROR_INSUFFICIENT_BUFFER.

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

Return Code	Description
`ERROR_INVALID_PARAMETER`	One of the parameter values is invalid.
`ERROR_NOT_SUPPORTED`	This operation is not supported on the local system.

Remarks

The IP forwarding table is managed by the IP router service. The IP forwarding table contains entries that define how to forward IP datagrams between different networks.

The GetIpForwardTable function is used to retrieve the contents of the IP forwarding table. The information returned is in the form of a MIB_IPFORWARDTABLE structure.

Note: To use this function, you must have administrative privileges on the local computer.

The MIB_IPFORWARDTABLE structure contains an array of MIB_IPFORWARDROW structures. Each MIB_IPFORWARDROW structure represents an entry in the IP forwarding table.

An example of how to use GetIpForwardTable to retrieve and display the IP forwarding table:


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

#pragma comment(lib, "ws2_32.lib")
#pragma comment(lib, "iphlpapi.lib")

int main() {
    ULONG ulSize = 0;
    PMIB_IPFORWARDTABLE pIpForwardTable = NULL;
    DWORD dwRetVal = 0;

    // Get the required buffer size
    dwRetVal = GetIpForwardTable(NULL, &ulSize, TRUE);
    if (dwRetVal == ERROR_INSUFFICIENT_BUFFER) {
        pIpForwardTable = (PMIB_IPFORWARDTABLE)malloc(ulSize);
        if (pIpForwardTable == NULL) {
            printf("Error allocating memory\n");
            return 1;
        }
    } else {
        printf("Error getting buffer size: %d\n", dwRetVal);
        return 1;
    }

    // Get the IP forwarding table
    dwRetVal = GetIpForwardTable(pIpForwardTable, &ulSize, TRUE);
    if (dwRetVal == NO_ERROR) {
        printf("Successfully retrieved IP forwarding table.\n");
        printf("Number of entries: %ld\n", pIpForwardTable->dwNumEntries);

        for (int i = 0; i < pIpForwardTable->dwNumEntries; i++) {
            char destIp[INET_ADDRSTRLEN];
            char maskIp[INET_ADDRSTRLEN];
            char gateIp[INET_ADDRSTRLEN];
            char nextHopIp[INET_ADDRSTRLEN];

            inet_ntop(AF_INET, &(pIpForwardTable->table[i].dwForwardDest), destIp, INET_ADDRSTRLEN);
            inet_ntop(AF_INET, &(pIpForwardTable->table[i].dwForwardMask), maskIp, INET_ADDRSTRLEN);
            inet_ntop(AF_INET, &(pIpForwardTable->table[i].dwForwardPolicy), gateIp, INET_ADDRSTRLEN); // This might be wrong, should be gateway? Or policy related.
            inet_ntop(AF_INET, &(pIpForwardTable->table[i].dwForwardNextHop), nextHopIp, INET_ADDRSTRLEN);


            printf("\nEntry %d:\n", i + 1);
            printf("  Destination: %s\n", destIp);
            printf("  Mask: %s\n", maskIp);
            printf("  Next Hop: %s\n", nextHopIp);
            printf("  Interface: %ld\n", pIpForwardTable->table[i].dwForwardIfIndex);
            printf("  Protocol: %d\n", pIpForwardTable->table[i].dwForwardProto);
            printf("  Type: %d\n", pIpForwardTable->table[i].dwForwardType);
            printf("  Age: %ld\n", pIpForwardTable->table[i].dwForwardAge);
            printf("  Next Hop AS: %ld\n", pIpForwardTable->table[i].dwForwardNextHopAS);
            printf("  Policy: %ld\n", pIpForwardTable->table[i].dwForwardPolicy); // Assuming this represents policy
        }
    } else {
        printf("Error getting IP forwarding table: %d\n", dwRetVal);
    }

    if (pIpForwardTable) {
        free(pIpForwardTable);
    }

    return 0;
}

Requirements

Attribute	Value
Minimum supported client	Windows 2000 Professional
Minimum supported server	Windows 2000 Server
Header	`iphlpapi.h` (include `winsock2.h`)
Library	`Iphlpapi.lib`
DLL	`Iphlpapi.dll`

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