Wintun Network Adapter

TUN Device Driver for Windows

This is a layer 3 TUN driver for Windows 7, 8, 8.1, and 10. Originally created for WireGuard, it is intended to be useful to a wide variety of projects that require layer 3 tunneling devices with implementations primarily in userspace.

Installation

The following snippet may be used inside of a WiX <Product> element for including Wintun inside of a Windows Installer. Note that MSI is the only supported method of installing Wintun; if you're using a different install system (such as NSIS) and cannot bother to switch to MSI/WiX, then simply bundle an embedded MSI that you can execute with msiexec.exe.

<DirectoryRef Id="INSTALLFOLDER">
    <Merge Id="WintunMergeModule" Language="0" DiskId="1" SourceFile="path\to\wintun-x.y-amd64.msm" />
</DirectoryRef>
<Feature Id="WintunFeature" Title="Wintun" Level="1">
    <MergeRef Id="WintunMergeModule" />
</Feature>

It is advisable to use the prebuilt and Microsoft-signed MSM files from Wintun.net.

Usage

After loading the driver and creating a network interface the typical way using SetupAPI, open the NDIS device object associated with the PnPInstanceId, enabling all forms of file sharing:

TCHAR *InterfaceList = NULL;
for (;;) {
    free(InterfaceList);
    DWORD RequiredBytes;
    if (CM_Get_Device_Interface_List_Size(&RequiredBytes, (LPGUID)&GUID_DEVINTERFACE_NET,
        InstanceId, CM_GET_DEVICE_INTERFACE_LIST_PRESENT) != CR_SUCCESS)
        return FALSE;
    InterfaceList = calloc(sizeof(*InterfaceList), RequiredBytes);
    if (!InterfaceList)
        return FALSE;
    CONFIGRET Ret = CM_Get_Device_Interface_List((LPGUID)&GUID_DEVINTERFACE_NET, InstanceId,
                    InterfaceList, RequiredBytes, CM_GET_DEVICE_INTERFACE_LIST_PRESENT);
    if (Ret == CR_SUCCESS)
        break;
    if (Ret != CR_BUFFER_SMALL) {
        free(InterfaceList);
        return FALSE;
    }
}
HANDLE WintunHandle = CreateFile(InterfaceList, GENERIC_READ | GENERIC_WRITE,
                                 FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
                                 NULL, OPEN_EXISTING, 0, NULL);
free(InterfaceList);
...

Ring layout

You must allocate two ring structs, one for receiving and one for sending:

typedef struct _TUN_RING {
    volatile ULONG Head;
    volatile ULONG Tail;
    volatile LONG Alertable;
    UCHAR Data[];
} TUN_RING;

Head: Byte offset of the first packet in the ring. Its value must be a multiple of 4 and less than ring capacity.
Tail: Byte offset of the start of free space in the ring. Its value must be multiple of 4 and less than ring capacity.
Alertable: Zero when the consumer is processing packets, non-zero when the consumer has processed all packets and is waiting for TailMoved event.
Data: The ring data.

In order to determine the size of the Data array:

Pick a ring capacity ranging from 128kiB to 64MiB bytes. This capacity must be a power of two (e.g. 1MiB). The ring can hold up to this much data.
Add 0x10000 trailing bytes to the capacity, in order to allow for always-contigious packet segments.

The total ring size memory is then sizeof(TUN_RING) + capacity + 0x10000.

Each packet is stored in the ring aligned to sizeof(ULONG) as:

typedef struct _TUN_PACKET {
    ULONG Size;
    UCHAR Data[];
} TUN_PACKET;

Size: Size of packet (max 0xFFFF).
Data: Layer 3 IPv4 or IPv6 packet.

Registering rings

In order to register the two TUN_RINGs, prepare a registration struct as:

typedef struct _TUN_REGISTER_RINGS {
    struct {
        ULONG RingSize;
        TUN_RING *Ring;
        HANDLE TailMoved;
    } Send, Receive;
} TUN_REGISTER_RINGS;

Send.RingSize, Receive.RingSize: Sizes of the rings (sizeof(TUN_RING) + capacity + 0x10000, as above).
Send.Ring, Receive.Ring: Pointers to the rings.
Send.TailMoved: A handle to an auto-reset event created by the client that Wintun signals after it moves the Tail member of the send ring.
Receive.TailMoved: A handle to an auto-reset event created by the client that the client will signal when it changes Receive.Ring->Tail and Receive.Ring->Alertable is non-zero.

With events created, send and receive rings allocated, and registration struct populated, DeviceIoControl(TUN_IOCTL_REGISTER_RINGS: 0xca6ce5c0) with pointer and size of descriptor struct specified as lpInBuffer and nInBufferSize parameters. You may call TUN_IOCTL_REGISTER_RINGS on one handle only.

Writing to and from rings

Reading packets from the send ring may be done as:

for (;;) {
    TUN_PACKET *Next = PopFromRing(Rings->Send.Ring);
    if (!Next) {
        Rings->Send.Ring->Alertable = TRUE;
        Next = PopFromRing(Rings->Send.Ring);
        if (!Next) {
            WaitForSingleObject(Rings->Send.TailMoved, INFINITE);
            Rings->Send.Ring->Alertable = FALSE;
            continue;
        }
        Rings->Send.Ring->Alertable = FALSE;
        ResetEvent(Rings->Send.TailMoved);
    }
    SendToClientProgram(Next);
}

It may be desirable to spin a short amount of time before waiting on the TailMoved event, in order to reduce latency.

When closing the handle, Wintun will set the Tail to 0xFFFFFFFF and set the TailMoved event to unblock the waiting user process.

Writing packets to the receive ring may be done as:

for (;;) {
    TUN_PACKET *Next = ReceiveFromClientProgram();
    WriteToRing(Rings->Receive.Ring, Next);
    if (Rings->Receive.Ring->Alertable)
        SetEvent(Rings->Recieve.TailMoved);
}

Wintun will abort reading the receive ring on invalid Head or Tail or on a bogus packet. In this case, Wintun will set the Head to 0xFFFFFFFF. In order to restart it, reopen the handle and call TUN_IOCTL_REGISTER_RINGS again. However, it should be entirely possible to avoid feeding Wintun bogus packets and invalid offsets.

Building

Do not distribute drivers named "Wintun", as they will most certainly clash with official deployments. Instead distribute the signed MSMs from Wintun.net.

General requirements:

wintun.sln may be opened in Visual Studio for development and building. Be sure to run bcdedit /set testsigning on before to enable unsigned driver loading. The default run sequence (F5) in Visual Studio will build and insert Wintun.