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ndis filter driver : as install it, internet connect cut off

380nanometres380nanometres Member Posts: 9

Hello, I am trying to develop ndis filter driver working as NAT driver. I want to change destination address.

Here is my method to do it.

  1. read eth header and ip header from NB
  2. using NdisAdvanceNetBufferDataStart() function, logically remove eth header + ip header from NBL
  3. make clone of NBL using NdisAllocateCloneNetBufferList
  4. using NdisRetreatNetBufferDataStart() function, make space for header to cloned NBL (all underneath steps are 'to cloned NBL')
  5. get an starting address that eth header should be located in cloned NBL, using NdisGetDataBuffer() and add the eth header(got from 1st step)
  6. using NdisAdvanceNetBufferDataStart() and NdisGetDataBuffer, get an starting address of ip header and add the ip header(got from 1st step) to the address
  7. I want to modify dest ip from ip header, so get an dest ip address through using NdisAdvanceNetBufferDataStart() for sizeof(ipheader)-4 and NdisGetDataBuffer()
  8. overwrite dest ip in ip header (using RtlIpv4StringToAddress())
  9. in FilterSendNetBufferListsComplete, free the cloned NBL

I referenced "" to free the cloned NBL in FilterSendNetBufferListsComplete.

However, I built it and install at VM, but its internet access cut off.

So for testing, I wrote same destination ip addresses as original NBL in cloned NBL, by same methods above, and it worked normally.

So is it a problem with the one ip address that I arbitrarily overwrite? Or is it an ip checksum problem? or else?
I don't know because there is too little information to find.

After struggling a few weeks, I think I am close to what I want to make... but this problem is blocking me from progression.

Thank you


  • 380nanometres380nanometres Member Posts: 9

    I know there is TCP/IP checksum offload. nevertheless I have to calculate checksum through the code? or there is something to do after modify ip address I missed?

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