Reply To: Reading packets

Home Forums Discussions Support Portal Reading packets Reply To: Reading packets

#6508

Vadim Smirnov
Moderator

The wwwcensor.cpp below blocks HTTP packets which contain the specified string pattern. You can change/extend this code to filter ports different from TCP:80 (you have to know which ports are used by each IM you’d like to support) of even just drop everyTCP packet which contain the specified pattenr by one simple modification – remove the following check:

//
// Check if this HTTP packet (destined to remote system port 80, or received from it)
//

if (((pTcpHeader->th_dport == htons (80))&&(PacketBuffer.m_dwDeviceFlags == PACKET_FLAG_ON_SEND))||
((pTcpHeader->th_sport == htons (80))&&(PacketBuffer.m_dwDeviceFlags == PACKET_FLAG_ON_RECEIVE)))
{

/*************************************************************************/
/* Copyright (c) 2000-2007 NT Kernel Resources. */
/* All Rights Reserved. */
/* http://www.ntkernel.com */
/* ndisrd@ntkernel.com */
/* */
/* Module Name: wwwcensor.cpp */
/* */
/* Abstract: Defines the entry point for the console application */
/* */
/*************************************************************************/

#include "stdafx.h"

USHORT ntohs( USHORT netshort )
{
PUCHAR pBuffer;
USHORT nResult;

nResult = 0;
pBuffer = (PUCHAR )&netshort;

nResult = ( (pBuffer[ 0 ] << 8) & 0xFF00 )
| ( pBuffer[ 1 ] & 0x00FF );

return( nResult );
}

#define htons ntohs

int main(int argc, char* argv[])
{
TCP_AdapterList AdList;
CNdisApi api;
ETH_REQUEST Request;
INTERMEDIATE_BUFFER PacketBuffer;
ether_header_ptr pEthHeader = NULL;
iphdr_ptr pIpHeader = NULL;
tcphdr_ptr pTcpHeader = NULL;
HANDLE hEvent[256];
DWORD dwAdIndex = 0;
char szTempString[1500];
char szPattern[256];
BOOL bDrop = FALSE;


if (argc < 2)
{
printf ("Command line syntax:ntwwwcensor.exe pattern ntpattern - phrase or word to block HTTP packets with.n");
return 0;
}

if(!api.IsDriverLoaded())
{
printf ("Driver not installed on this system of failed to load.n");
return 0;
}

if ( strlen(argv[1]) > 255 )
{
printf ("Pattern is too,long, please use one with maximum of 255 characters.n");
return 0;
}

//
// Get pattern in upper case
//
ZeroMemory ( szPattern, 256 );
strcpy ( szPattern, argv[1] );
for ( unsigned i = 0; i < strlen (szPattern); ++i )
{
if (isalpha(((UCHAR)szPattern)))
szPattern
= (char)toupper((UCHAR)szPattern);
}

//
// Get system installed network interfaces
//
api.GetTcpipBoundAdaptersInfo ( &AdList );

//
// Initialize common ADAPTER_MODE structure (all network interfaces will operate in the same mode)
//
ADAPTER_MODE Mode;
Mode.dwFlags = MSTCP_FLAG_SENT_TUNNEL|MSTCP_FLAG_RECV_TUNNEL;

//
// Create notification events and initialize the driver to pass packets thru us
//
for (dwAdIndex = 0; dwAdIndex < AdList.m_nAdapterCount; ++dwAdIndex)
{
hEvent[dwAdIndex] = CreateEvent(NULL, TRUE, FALSE, NULL);

if (!hEvent[dwAdIndex])
{
printf("Failed to create notification event for network interface n");
return 0;
}

Mode.hAdapterHandle = (HANDLE)AdList.m_nAdapterHandle[dwAdIndex];

//
// Set MSTCP_FLAG_SENT_TUNNEL|MSTCP_FLAG_RECV_TUNNEL for the network interface
//
api.SetAdapterMode(&Mode);

//
// Set packet notification event for the network interface
//
api.SetPacketEvent((HANDLE)AdList.m_nAdapterHandle[dwAdIndex], hEvent[dwAdIndex]);
}


// Initialize common part of ETH_REQUEST
ZeroMemory ( &Request, sizeof(ETH_REQUEST) );
ZeroMemory ( &PacketBuffer, sizeof(INTERMEDIATE_BUFFER) );
Request.EthPacket.Buffer = &PacketBuffer;

//
// Go into the endless loop (this is just a sample application)
//
while (TRUE)
{
//
// Wait before any of the interfaces is ready to indicate the packet
//
dwAdIndex = WaitForMultipleObjects ( AdList.m_nAdapterCount, hEvent, FALSE, INFINITE ) - WAIT_OBJECT_0;

//
// Complete initialization of ETH_REQUEST
//

Request.hAdapterHandle = (HANDLE)AdList.m_nAdapterHandle[dwAdIndex];

//
// Read packet from the interface until there are any
//
while(api.ReadPacket(&Request))
{
//
// Get Ethernet header
//
pEthHeader = (ether_header_ptr)PacketBuffer.m_IBuffer;

//
// Check if Ethernet frame contains IP packet
//
if(ntohs(pEthHeader->h_proto) == ETH_P_IP)
{
//
// Get IP header
//
pIpHeader = (iphdr_ptr)(pEthHeader + 1);

//
// Check if IP packet contains TCP packet
//
if (pIpHeader->ip_p == IPPROTO_TCP)
{
//
// Get TCP header pointer
//
pTcpHeader = (tcphdr_ptr)((PUCHAR)pIpHeader + pIpHeader->ip_hl*4);

//
// Check if this HTTP packet (destined to remote system port 80, or received from it)
//

if (((pTcpHeader->th_dport == htons (80))&&(PacketBuffer.m_dwDeviceFlags == PACKET_FLAG_ON_SEND))||
((pTcpHeader->th_sport == htons (80))&&(PacketBuffer.m_dwDeviceFlags == PACKET_FLAG_ON_RECEIVE)))
{
//
// Get data size in the packet and pointer to the data
//

DWORD dwDataLength = PacketBuffer.m_Length - (sizeof(ether_header) + pIpHeader->ip_hl*4 + pTcpHeader->th_off*4);
PCHAR pData = (PCHAR)pEthHeader + (sizeof(ether_header) + pIpHeader->ip_hl*4 + pTcpHeader->th_off*4);

// If packet contains any data - process it
if (dwDataLength)
{
//
// Copy packet payload into the temporary string, replace all 0 bytes with 0x20, convert string to upper case and place at the end
//
memcpy (szTempString, pData, dwDataLength);
for (unsigned t = 0; t < dwDataLength; ++t)
{
if (szTempString[t] == 0)
szTempString[t] = 0x20;

if (isalpha((UCHAR)szTempString[t]))
szTempString[t] = (char)toupper((UCHAR)szTempString[t]);
}
szTempString[dwDataLength] = 0;

//
// Check if this packet payload contains user supplied pattern in ASCII code
//

if (strstr ( szTempString, szPattern ))
bDrop = TRUE;
}
}

}
}

if(bDrop)
{
printf ("TCP %d.%d.%d.%d:%d -> %d.%d.%d.%d:%d pattern found & packet dropped n",
pIpHeader->ip_src.S_un.S_un_b.s_b1, pIpHeader->ip_src.S_un.S_un_b.s_b2, pIpHeader->ip_src.S_un.S_un_b.s_b3, pIpHeader->ip_src.S_un.S_un_b.s_b4, ntohs(pTcpHeader->th_sport),
pIpHeader->ip_dst.S_un.S_un_b.s_b1, pIpHeader->ip_dst.S_un.S_un_b.s_b2, pIpHeader->ip_dst.S_un.S_un_b.s_b3, pIpHeader->ip_dst.S_un.S_un_b.s_b4, ntohs (pTcpHeader->th_dport));
bDrop = FALSE;
}
else
if (PacketBuffer.m_dwDeviceFlags == PACKET_FLAG_ON_SEND)
{
// Place packet on the network interface
api.SendPacketToAdapter(&Request);
}
else
{
// Indicate packet to MSTCP
api.SendPacketToMstcp(&Request);
}
}

//
// Reset signalled event
//
ResetEvent(hEvent[dwAdIndex]);

}

return 0;
}