[Vadim Smirnov]

IMHO, in theory (hardly likely that it will ever happen in real life) it is possible that application running on NAT system access the same IP/port as another application running behind the NAT and the same source port used in NAT table and for local application running on NAT system. In this case it is not possible to distinguish if packet should be NAT’ed or not. But even in this rare case you still can use sequence and aknowledgement fields in TCP header to determine wheather the NAT should be applied (it will work for TCP connection only).

[Vadim Smirnov]

You can use IP/port/protocol information gathered on the TDI level and match it against information extracted from IP packet. If it is the same then the packet is probably originated from the application you intercepted on the TDI level.

[Vadim Smirnov]

The latest WinpkFilter release is fully compatible with Windows XP SP2.

[Vadim Smirnov]

So what about if some application use this port number at the NAT computer? Is there any conflicts if use same port number ( with some application at NAT computer )?

The conflict is possible, you will just bypass those applications (they may be not working properly with your NAT application, but it won’t make any problems to NAT). However, you can just reserve required port range by your own application.

Another question: how to get binding IP address for each interface adapter, and how to get windows route table?

IP Helper API

[Vadim Smirnov]

When I replace the Client(IP, PORT) of the inner packet with the NAT External(ip, port), what port number should I used for the NAT external? The same with the Client Port or a new one? Is that must be a free number that no one uses it? How do I generate this port number.

When you substitute inner source IP with external one but don’t substitute inner port this is called static NAT. When you substitute inner source port in addition to IP this is called dynamic NAT. You can use any free port number (an example just take some rarely used port range like 10000-64000 and allocate ports from there).

PS: where can I get the code of Checksum Calculation?

That depends from the progarmming environment you use, an example some C and Delhi code is available on this board.

[Vadim Smirnov]

If you want to realize your own packet forwarding you can do it with API’s you have mentioned. If you filter two adapters and you can read packet from one and send them to another, here is no crime. Just don’t forget that MTU for the interfaces can be different.

[Vadim Smirnov]

One of the approaches (the easiest from my point of view) is filtering Server(WAN) interface substituting the original IP (if it is from LAN) in the outgoing packet with Server WAN card IP (port substitution also should be done in order to perform backward mapping). For incoming packets on the Server (WAN) interface you should match against NAT table and perform the reversed operation. Please note that packet forwarding should be enabled on the Server. You don’t need to route packets youself, TCP/IP will do it for you.

[Vadim Smirnov]

What environment do you use for the development? There is a C header file iphlp.h which has some sample protocol header definitions (IP header, TCP header, UDP header and etc…). If you are using Delphi then you can use the ones below:

TIPHeaderPtr = ^TIPHeader;

TIPHeader = packed record

VerLen: Byte;   //HL

TOS: Byte;

TotalLen: Word;

Identifer: Word;

FragOffsets: Word;

TTL: Byte;

Protocol: Byte;

CheckSum: Word;

SourceIp: DWORD;

DestIp: DWORD;

// Options: DWORD;

end;



TTCPHeaderPtr = ^TTCPHeader;

TTCPHeader = packed record

SourcePort:Word;

DestPort:Word;

SequenceNumber:DWord;

AcknowledgementNumber:DWord;

Offset:Byte; //only left 4 bits. Header length in 32-bit segments

Flags:Byte;

Window:Word;

Checksum:Word;  //includes speudo header instead of TCP header.

UrgentPointer:Word;

end;



pTCPData: PChar; 

And probably it makes sense for you to look at this topic:
http://ntkernel.com/forum/viewtopic.php?t=114

We are trying to provide as good support as we can 🙄

[Vadim Smirnov]

Is it possible to use both packet level and application level filtering with in the same program ie. inspect packet using winPKfilter then a LSP/TDI filter?

Yes, sure.

Does your company provide a application level filter?

We provide localhost monitor API, which is TDI filter for Windows NT/2000/XP/2003. LSP sample is avalaible as a part of MSDN. Simple TDI filter for Windows 9x can be found in the VTOOLSD samples (from Compuware).

[Vadim Smirnov]

WinpkFilter allows you implementing packet filtering technologies in both user or kernel mode (the one you choose depends from your expirience and requirements). But this is packet level filtering (it is not possible to determine application context on the NDIS level). For application level filtering you would need LSP or TDI filter.

[Vadim Smirnov]

Visual Basic samples for WinpkFilter 2.4 released. You can download it from WinpkFilter product homepage.

[Vadim Smirnov]

1. How can I use WinpkFilter for capturing packets from multiple adapters ?

You can start the dedicated thread for each network interface to capture and process packets from it, just like PassThru sample does. Another way is setting up events for each interface and using WaitForMultipleObjects.

2. Dose my WinpkFilter Appication NOT conflict with any other NDIS hooking drivers or WinpkFilter Applications ?

We can’t guarantee the compatibility with any other hooking drivers, but WinpkFilter is compatible with the majority of firewalls on the market. In order to avoid conflicts with other WinpkFilter applications you would need the custom build (with customized names for drivers and devices) of WinpkFilter drivers (free for the Developer license).

[Vadim Smirnov]

LSP is user mode solution, but TDI is kernel one. So, LSP is much more easier way. I don’t think that you should intercept clients who work with TDI directly, so probably LSP fits you.

[Vadim Smirnov]

Is it possible to use virtual network interface? Or even virtual NIC is not allowed? In the last case you have the only choice – TDI filter driver.

[Vadim Smirnov]

Just follow the online order link for the WinpkFilter Source Code (click Order Now) available at: http://www.ntkernel.com/products/winpkfilter.shtml

Here the exact link for your convinience:

http://secure.emetrix.com/order/product.asp?PID=38895814

If you have any problems with it please contact support@ntkernel.com.

Thanks for your interest in WinpkFilter.

[Author]

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