Windows System Software -- Consulting, Training, Development -- Unique Expertise, Guaranteed Results


Before Posting...

Please check out the Community Guidelines in the Announcements and Administration Category.

More Info on Driver Writing and Debugging

The free OSR Learning Library has more than 50 articles on a wide variety of topics about writing and debugging device drivers and Minifilters. From introductory level to advanced. All the articles have been recently reviewed and updated, and are written using the clear and definitive style you've come to expect from OSR over the years.

Check out The OSR Learning Library at:

How to update source ip address in UDP packets

chetanchauhan111992chetanchauhan111992 Member Posts: 9
edited May 16 in NTDEV

I am trying to update the source IP address of the UDP and TCP packets which are captured at layer FWPM_LAYER_ALE_CONNECT_REDIRECT_V4and FWPM_LAYER_ALE_AUTH_CONNECT_V4.
In case of TCP packets source IP address is successfully updated but in the case of UDP packets source IP address is not updated.
I am checking the IP address used in the packet using Wireshark.

UINT64 classifyHandle = 0;
NTSTATUS status = FwpsAcquireClassifyHandle0(
if (!NT_SUCCESS(status))
    DbgPrintEx(DPFLTR_IHVNETWORK_ID, DPFLTR_ERROR_LEVEL, "Could not get the classify handle %x\r\n", status);
status = FwpsAcquireWritableLayerDataPointer(
if (!NT_SUCCESS(status))
    DbgPrintEx(DPFLTR_IHVNETWORK_ID, DPFLTR_ERROR_LEVEL, "FwpsAcquireWritableLayerDataPointer Failed %x\r\n", status);
SOCKADDR_IN* sourceAddr = (SOCKADDR_IN*)(&(writableLayerData->localAddressAndPort));
if(sourceAddr ==NULL)
    DbgPrintEx(DPFLTR_IHVNETWORK_ID, DPFLTR_ERROR_LEVEL, "socket address is NULL\r\n");
    goto Exit;
sourceAddr->sin_addr.S_un.S_addr = RtlUlongByteSwap(*(unsigned long*)&vip);

    if (inFixedValues->layerId == FWPS_LAYER_ALE_CONNECT_REDIRECT_V4)
        writableLayerData->localRedirectHandle = gRedirectConnectHandlev4;
    else if (inFixedValues->layerId == FWPS_LAYER_ALE_AUTH_CONNECT_V4)
        writableLayerData->localRedirectHandle = gAuthConnectHandlev4;

    writableLayerData->localRedirectContext = NULL;
    writableLayerData->localRedirectContextSize = 0;
    classifyOut->actionType = FWP_ACTION_PERMIT;

Please let me know how to correctly update the source IP address in UDP packets.


Post edited by Peter_Viscarola_(OSR) on


  • Jason_StephensonJason_Stephenson Member Posts: 120

    Changing the local address and port of a flow is only supported in the bind-redirect layer. This is not supported in the connect-redirect layer.

  • chetanchauhan111992chetanchauhan111992 Member Posts: 9

    Thanks Jason,

    I am able to update the UDP packet using FWPM_LAYER_ALE_BIND_REDIRECT_V4 layer.

  • the_penthe_pen Member Posts: 1

    Dear chetanchauhan111992,

    How did you get the classifyContext "option" turned on?

    When I try a classifyFn in the FWPM_LAYER_ALE_AUTH_CONNECT_V4 myself, the classifyContext variable passed comes in as NULL. There is no documentation on this that I can find. What operating system does your code target, only Win 8 and above, or did you have this working in Win 7?

    Did you declare something extra when you registered the callout?

    Thank you in advance for sharing.



  • Peter_Viscarola_(OSR)Peter_Viscarola_(OSR) Administrator Posts: 8,986

    First post... at it's to a thread that's almost a year old.

    You know, of course, from reading the forum rules before posting, that posting to such an old thread is not allowed.

    Soooo... I'm locking this thread. If you want to ask a question, start a new thread.

    Please... DO try to follow the rules, OK?

    Peter Viscarola

This discussion has been closed.

Howdy, Stranger!

It looks like you're new here. Sign in or register to get started.

Upcoming OSR Seminars
OSR has suspended in-person seminars due to the Covid-19 outbreak. But, don't miss your training! Attend via the internet instead!
Writing WDF Drivers 12 September 2022 Live, Online
Internals & Software Drivers 23 October 2022 Live, Online
Kernel Debugging 14 November 2022 Live, Online
Developing Minifilters 5 December 2022 Live, Online