Redirection (Gateway)

[ed101]

Hi,
if I use a Windows computer as an internet-gateway (Internet Connection Sharing), is it possible to redirect some users (with a specific IP) to another Website than the requested?
Is it possible in Visual Basic ?
Please add some relevant source-code.
Thanks a lot in advance.
Ed

[Vadim Smirnov]

Yes, this is possible to implement in Visual basic using WinpkFilter library, however I can’t provide you with ready to use code in VB (as I’m not a VB coder). If you can use the C code I think I can provide some.

Also you can use port/protocol mapping avalable in NeT Firewall (set up redirector rules on the internal LAN interface).

[ed101]

Hi Serpent,
first of all, thanks for the information.
But I have a few further questions:

The WinpkFilter library you mentioned, is it free or do I have to purchase it? Is it the free downloadable ndisapi.dll ?

Can you provide some code samples for the redirection in C or another programming language?

Thanks in advance.
Ed

[Vadim Smirnov]

The WinpkFilter library you mentioned, is it free or do I have to purchase it? Is it the free downloadable ndisapi.dll ?

WinpkFilter is free for non-commercial use and available for download from here (http://www.ntkernel.com/w&p.php?id=7). Package includes drivers(ndisrd.sys/ndisrd.vxd), API DLL (ndisapi.dll) and several simple samples.

If you register WinpkFilter (any type of subscription starting from 95$) you also get access to the Internet Gateway source code (http://www.ntkernel.com/w&p.php?id=31) which implements NAT and gives a good clue to how to redirect packets.

Some code samples can be also found by looking through this forum. I can also C&P a redirect relative portion of code from NeT Firewall source, but it may appear a bit difficult to understand.

[kkkerem]

@SerpentFly wrote:

Yes, this is possible to implement in Visual basic using WinpkFilter library, however I can’t provide you with ready to use code in VB (as I’m not a VB coder). If you can use the C code I think I can provide some.

Also you can use port/protocol mapping avalable in NeT Firewall (set up redirector rules on the internal LAN interface).

Can you provide some C code for this purpose ?

[Vadim Smirnov]

Can you provide some C code for this purpose ?

The routine below is taken from the Internet Gateway source and implements NAT processing:

unsigned __stdcall CsnatDlg::StartNAT ( void* pArguments )

{

CsnatDlg*			pDlg = (CsnatDlg*)pArguments;

HANDLE				hEvents[ADAPTER_LIST_SIZE + 1];

CNetworkInterface*	hAdapters [ADAPTER_LIST_SIZE + 1];

CNetworkInterface	*pNetCard, *pProviderCard;

unsigned			dwActiveAdaptersCount = 1;

ADAPTER_MODE		Mode;

ETH_REQUEST			Request;

INTERMEDIATE_BUFFER PacketBuffer;

DWORD				dwWait, dwIndex;

ether_header*		pEthHeader;

iphdr*				pIpHeader;

tcphdr*				pTcpHeader;

udphdr*				pUdpHeader;



Mode.dwFlags = MSTCP_FLAG_SENT_TUNNEL|MSTCP_FLAG_RECV_TUNNEL;



hEvents[0] = pDlg->m_hNATTerminateEvent;



// Walk adapters list and initialize provider and clients interfaces

POSITION pos = pDlg->m_NetCardsList.GetHeadPosition();



for (unsigned i = 0; i < pDlg->m_dwAdapterCount; ++i)

{

pNetCard = (CNetworkInterface*)pDlg->m_NetCardsList.GetNext(pos);



if ((pNetCard->m_NATState == CLIENT) || (pNetCard->m_NATState == PROVIDER))

{

hAdapters[dwActiveAdaptersCount] = pNetCard;

hEvents[dwActiveAdaptersCount] = ::CreateEvent(NULL, TRUE, FALSE, NULL);

pDlg->m_NdisApi.SetPacketEvent(pNetCard->m_hAdapter, hEvents[dwActiveAdaptersCount]);

Mode.hAdapterHandle = pNetCard->m_hAdapter;

pDlg->m_NdisApi.SetAdapterMode(&Mode);

dwActiveAdaptersCount++;



if(pNetCard->m_NATState == PROVIDER)

pProviderCard = pNetCard;

}

}



// Initialize Request

ZeroMemory ( &Request, sizeof(ETH_REQUEST) );

ZeroMemory ( &PacketBuffer, sizeof(INTERMEDIATE_BUFFER) );

Request.EthPacket.Buffer = &PacketBuffer;



do

{

dwWait = ::WaitForMultipleObjects(

dwActiveAdaptersCount,

hEvents,

FALSE,

INFINITE

);



dwIndex = dwWait - WAIT_OBJECT_0;



if (!dwIndex)

continue;



::ResetEvent(hEvents[dwIndex]);



Request.hAdapterHandle = hAdapters[dwIndex]->m_hAdapter;



// Read all queued packets from the specified interface

while(pDlg->m_NdisApi.ReadPacket(&Request))

{

pEthHeader = (ether_header*)PacketBuffer.m_IBuffer;

if ( ntohs(pEthHeader->h_proto) == ETH_P_IP )

{

pIpHeader = (iphdr*)(PacketBuffer.m_IBuffer + ETHER_HEADER_LENGTH);



// Check if connection is established from local system (we don't do NAT processing

// for local system)

BOOL bIsLocalAddress = hAdapters[dwIndex]->IsLocalAddress(&pIpHeader->ip_src);



if (bIsLocalAddress && (PacketBuffer.m_dwDeviceFlags == PACKET_FLAG_ON_SEND))

{

// Place packet on the network interface

pDlg->m_NdisApi.SendPacketToAdapter(&Request);



continue;

}



// TCP packet processing

if (pIpHeader->ip_p == IPPROTO_TCP)

{

// This is TCP packet, get TCP header pointer

pTcpHeader = (tcphdr*)(((PUCHAR)pIpHeader) + sizeof(DWORD)*pIpHeader->ip_hl);



// Outgoing TCP packets processing

if ((hAdapters[dwIndex]->m_NATState == PROVIDER)&&

(PacketBuffer.m_dwDeviceFlags == PACKET_FLAG_ON_SEND))

{

CPortNATEntry* pTcpNE = NULL;



if (pTcpHeader->th_flags == TH_SYN)

{

// New TCP connnection established, allocate dynamic NAT entry

pTcpNE = pDlg->m_TcpNatTable.Allocate(pIpHeader->ip_src, pTcpHeader->th_sport, pIpHeader->ip_dst, pTcpHeader->th_dport);



if(pTcpNE)

{

pTcpNE->m_IpNAT = hAdapters[dwIndex]->m_NATIp;

}

}

else

{

// Try to locate xisting NAT entry

pTcpNE = pDlg->m_TcpNatTable.Find(pIpHeader->ip_src, pTcpHeader->th_sport, pIpHeader->ip_dst, pTcpHeader->th_dport);

}



if (pTcpNE)

{

// If NAT entry is found perform NAT processing

pIpHeader->ip_src.S_un.S_addr = htonl(pTcpNE->m_IpNAT.S_un.S_addr);

pTcpHeader->th_sport = htons(pTcpNE->m_usNATPort);

// Recalculate checksums

RecalculateTCPChecksum (&PacketBuffer);

RecalculateIPChecksum(pIpHeader);

}

}



// Incoming TCP packets processing

if ((hAdapters[dwIndex]->m_NATState == PROVIDER)&&

(PacketBuffer.m_dwDeviceFlags == PACKET_FLAG_ON_RECEIVE))

{

// Map connection to the NAT entry if the one exists

CPortNATEntry* pTcpNE = pDlg->m_TcpNatTable.Map(pTcpHeader->th_dport);

if (pTcpNE)

{

// NAT entry exists, make NAT processing

if (htonl(pTcpNE->m_IpDst.S_un.S_addr) == pIpHeader->ip_src.S_un.S_addr)

{

pIpHeader->ip_dst.S_un.S_addr = htonl(pTcpNE->m_IpSrc.S_un.S_addr);

pTcpHeader->th_dport = htons(pTcpNE->m_usSrcPort);

RecalculateTCPChecksum (&PacketBuffer);

RecalculateIPChecksum(pIpHeader);

}

}



}

}

// UDP packets processing

if (pIpHeader->ip_p == IPPROTO_UDP)

{

// This is UDP packet, get UDP header pointer

pUdpHeader = (udphdr*)(((PUCHAR)pIpHeader) + sizeof(DWORD)*pIpHeader->ip_hl);



// DNS hook

// If we receive DNS packet on the NAT client adapter then we redirect it

// to this system configured DNS server

if ((hAdapters[dwIndex]->m_NATState == CLIENT)&&

(PacketBuffer.m_dwDeviceFlags == PACKET_FLAG_ON_RECEIVE))

{

if (ntohs(pUdpHeader->th_dport) == 53/*DNS port*/)

{

// Save the DNS IP used by the NAT client system

hAdapters[dwIndex]->m_LocalDNS.S_un.S_addr = ntohl(pIpHeader->ip_dst.S_un.S_addr);

pIpHeader->ip_dst.S_un.S_addr = pDlg->m_DNSIp.S_un.S_addr;

RecalculateIPChecksum(pIpHeader);

}

}

// DNS reply came, substitute source IP back to the original DNS address

if ((hAdapters[dwIndex]->m_NATState == CLIENT)&&

(PacketBuffer.m_dwDeviceFlags == PACKET_FLAG_ON_SEND))

{

if (ntohs(pUdpHeader->th_sport) == 53/*DNS port*/)

{

pIpHeader->ip_src.S_un.S_addr = htonl(hAdapters[dwIndex]->m_LocalDNS.S_un.S_addr);

RecalculateIPChecksum(pIpHeader);

}

}

// Outgoing UDP NAT processing

if ((hAdapters[dwIndex]->m_NATState == PROVIDER)&&

(PacketBuffer.m_dwDeviceFlags == PACKET_FLAG_ON_SEND))

{

CPortNATEntry* pUdpNE = NULL;

// Try to find existing entry

pUdpNE = pDlg->m_UdpNatTable.Find(pIpHeader->ip_src, pUdpHeader->th_sport, pIpHeader->ip_dst, pUdpHeader->th_dport);

// If not found -> allocate a new one

if (!pUdpNE)

{

pUdpNE = pDlg->m_UdpNatTable.Allocate(pIpHeader->ip_src, pUdpHeader->th_sport, pIpHeader->ip_dst, pUdpHeader->th_dport);



if(pUdpNE)

{

pUdpNE->m_IpNAT = hAdapters[dwIndex]->m_NATIp;

}

}

// NAT processing

if (pUdpNE)

{

pIpHeader->ip_src.S_un.S_addr = htonl(pUdpNE->m_IpNAT.S_un.S_addr);

pUdpHeader->th_sport = htons(pUdpNE->m_usNATPort);

RecalculateUDPChecksum (&PacketBuffer);

RecalculateIPChecksum(pIpHeader);

}

}

// Incoming UDP packets processing

if ((hAdapters[dwIndex]->m_NATState == PROVIDER)&&

(PacketBuffer.m_dwDeviceFlags == PACKET_FLAG_ON_RECEIVE))

{

CPortNATEntry* pUdpNE = pDlg->m_UdpNatTable.Map(pUdpHeader->th_dport);

if (pUdpNE)

{

if (htonl(pUdpNE->m_IpDst.S_un.S_addr) == pIpHeader->ip_src.S_un.S_addr)

{

pIpHeader->ip_dst.S_un.S_addr = htonl(pUdpNE->m_IpSrc.S_un.S_addr);

pUdpHeader->th_dport = htons(pUdpNE->m_usSrcPort);

RecalculateUDPChecksum (&PacketBuffer);

RecalculateIPChecksum(pIpHeader);

}

}



}

}



}



// Reinject packet into the stack

if (PacketBuffer.m_dwDeviceFlags == PACKET_FLAG_ON_SEND)

{

// Place packet on the network interface

pDlg->m_NdisApi.SendPacketToAdapter(&Request);

}

else

{

// Indicate packet to MSTCP

pDlg->m_NdisApi.SendPacketToMstcp(&Request);

}

}



}while (dwIndex);



// Free all NAT entries

pDlg->m_TcpNatTable.RemoveAll();

pDlg->m_UdpNatTable.RemoveAll();



for (unsigned i = 1; i < dwActiveAdaptersCount; ++i)

{

Mode.dwFlags = 0;

Mode.hAdapterHandle = hAdapters->m_hAdapter;



// Set NULL event to release previously set event object

pDlg->m_NdisApi.SetPacketEvent(hAdapters->m_hAdapter, NULL);



// Close Event

if (hEvents)

CloseHandle ( hEvents );



// Set default adapter mode

pDlg->m_NdisApi.SetAdapterMode(&Mode);



// Empty adapter packets queue

pDlg->m_NdisApi.FlushAdapterPacketQueue (hAdapters->m_hAdapter);

}



_endthreadex( 0 );

return 0;

}

Complete source code for the Internet Gateway is available to registered WinpkFilter customers.

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