WinDivert 2.2: Windows Packet Divert

Table of Contents

• 1. Introduction

• 2. Building

  • 2.1 Driver Signing

• 3. Installing

• 4. Uninstalling

• 5. Programming API

  • 5.1 WINDIVERT_LAYER
  • 5.2 WINDIVERT_EVENT
  • 5.3 WINDIVERT_ADDRESS
  • 5.4 WinDivertOpen
  • 5.5 WinDivertRecv
  • 5.6 WinDivertRecvEx
  • 5.7 WinDivertSend
  • 5.8 WinDivertSendEx
  • 5.9 WinDivertShutdown
  • 5.19 WinDivertClose
  • 5.11 WinDivertSetParam
  • 5.12 WinDivertGetParam

• 6. Helper Programming API

  • 6.1 WINDIVERT_IPHDR
  • 6.2 WINDIVERT_IPV6HDR
  • 6.3 WINDIVERT_ICMPHDR
  • 6.4 WINDIVERT_ICMPV6HDR
  • 6.5 WINDIVERT_TCPHDR
  • 6.6 WINDIVERT_UDPHDR
  • 6.7 WinDivertHelperParsePacket
  • 6.8 WinDivertHelperHashPacket
  • 6.9 WinDivertHelperParseIPv4Address
  • 6.10 WinDivertHelperParseIPv6Address
  • 6.11 WinDivertHelperFormatIPv4Address
  • 6.12 WinDivertHelperFormatIPv6Address
  • 6.13 WinDivertHelperCalcChecksums
  • 6.14 WinDivertHelperDecrementTTL
  • 6.15 WinDivertHelperCompileFilter
  • 6.16 WinDivertHelperEvalFilter
  • 6.17 WinDivertHelperFormatFilter
  • 6.18 WinDivertHelperNtoh*
  • 6.19 WinDivertHelperHton*

• 7. Filter Language

  • 7.1 Filter Examples
  • 7.2 Filter Usage

• 8. Performance

• 9. Samples

• 10. Known Issues

• 11. License

1. Introduction

WinDivert is a powerful user-mode capture/sniffing/modification/blocking/re-injection package for Windows 7, Windows 8 and Windows 10. WinDivert can be used to implement user-mode packet filters, packet sniffers, firewalls, NAT, VPNs, tunneling applications, etc., without the need to write kernel-mode code.

The main features of the WinDivert are:

• User-mode packet capture, sniffing, dropping, filtering, modification, re-injection, etc.
• Simple, high-level, programming API.
• Fully documented with sample programs.
• Full IPv6 support.
• Full loopback (localhost) support.
• A modern WDF/WFP driver implementation.
• Open source; Licensed under GNU Lesser General Public License (LGPL) version 3. See the License for more information.

WinDivert provides similar functionality to divert sockets from FreeBSD/MacOS, NETLINK sockets from Linux.

2. Building

Note that pre-built WinDivert binary distributions are available from the WinDivert website. Most users do not need to build their own version of WinDivert from source.

The source code for WinDivert is available for download at:

https://github.com/basil00/Divert

To build the WinDivert drivers from source:

1. Download and install Windows Driver Kit 7.1.0.
2. Open a x86 Free Build Environment console.
3. In the WinDivert package root directory, run the command:

wddk-build.bat

This will build the install\WDDK\i386\WinDivert32.sys driver. 4. Next, open a x64 Free Build Environment console. 5. Re-run the wddk-build.bat command to build the install\WDDK\amd64\WinDivert64.sys driver.

To build the WinDivert user-mode library (WinDivert.dll) and sample programs:

1. First, build the WinDivert drivers by running the wddk-build.bat command described above.
2. In Linux (with the MinGW cross-compilers installed) and in the WinDivert package root directory, run the command:

sh mingw-build.sh

This will the user-mode library and sample programs which will be placed in the install\MINGW subdirectory.

The generated WinDivert.dll/WinDivert.lib files should be compatible with all major compilers, including both MinGW and Visual Studio.

2.1 Driver Signing

If you built your own WinDivert32.sys/WinDivert64.sys drivers, they must be digitally signed before they can be used. See Driver Signing Requirements for Windows for more information.

Note that the pre-built WinDivert32.sys/WinDivert64.sys drivers from the official WinDivert distribution are already digitally signed.

3. Installing

WinDivert does not require any special installation. Depending on your target configuration, simply place the following files in your application's home directory:

The WinDivert driver is automatically (and silently) installed on demand whenever your application calls WinDivertOpen(). The calling application must have Administrator privileges.

4. Uninstalling

To uninstall, simply delete the WinDivert.dll, WinDivert32.sys, and WinDivert64.sys files. If already running, the WinDivert driver will be automatically uninstalled during the next machine reboot. The WinDivert driver can also be manually removed by (1) terminating all processes that are using WinDivert, and (2) issuing the following commands at the command prompt

sc stop WinDivert
    sc delete WinDivert

Alternatively, the WinDivert driver can be removed by using the windivertctl.exe sample program by issuing the following command:

windivertctl uninstall

5. Programming API

To use the WinDivert package, a program/application must:

1. Include the windivert.h header file

#include "windivert.h"

2. Link against or dynamically load the WinDivert.dll dynamic link library.

5.1 WINDIVERT_LAYER

|

typedef enum
{
    WINDIVERT_LAYER_NETWORK = 0,
    WINDIVERT_LAYER_NETWORK_FORWARD,
    WINDIVERT_LAYER_FLOW,
    WINDIVERT_LAYER_SOCKET,
    WINDIVERT_LAYER_REFLECT,
} **WINDIVERT\_LAYER** , ***PWINDIVERT\_LAYER** ;

|

Remarks

WinDivert supports several layers for diverting or capturing network packets/events. Each layer has its own capabilities, such as the ability to block events or to inject new events, etc. The list of supported WinDivert layers is summarized below:

Here, the layer capabilities are:

• (Block?) the layer can block events/packets;
• (Inject?) the layer can inject new events/packets;
• (Data?) whether the layer returns packets/data or not; and
• (PID?) whether the ID for the process associated with an event/packet is available at this layer, or not.

The WINDIVERT_LAYER_NETWORK and WINDIVERT_LAYER_NETWORK_FORWARD layers allow the user application to capture/block/inject network packets passing to/from (and through) the local machine. Due to technical limitations, process ID information is not available at these layers.

The WINDIVERT_LAYER_FLOW layer captures information about network flow establishment/deletion events. Here, a flow represents either (1) a TCP connection, or (2) an implicit flow created by the first sent/received packet for non-TCP traffic, e.g., UDP. Old flows are deleted when the corresponding connection is closed (for TCP), or based on an activity timeout (non-TCP). Flow-related events can be captured, but not blocked nor injected. Process ID information is also available at this layer. Due to technical limitations, the WINDIVERT_LAYER_FLOW layer cannot capture flow events that occurred before the handle was opened.

The WINDIVERT_LAYER_SOCKET layer can capture or block events corresponding to socket operations, such as bind(), connect(), listen(), etc., or the termination of socket operations, such as a TCP socket disconnection. Unlike the flow layer, most socket-related events can be blocked. However, it is not possible to inject new or modified socket events. Process ID information (of the process responsible for the socket operation) is available at this layer. Due to technical limitations, this layer cannot capture events that occurred before the handle was opened.

Finally, the WINDIVERT_LAYER_REFLECT layer can capture events relating to WinDivert itself, such as when another process opens a new WinDivert handle, or closes an old WinDivert handle. WinDivert events can be captured but not injected nor blocked. Process ID information (of the process responsible for opening the WinDivert handle) is available at this layer. This layer also returns data in the form of an object representation of the filter string used to open the handle. The object representation can be converted back into a human-readable filter string using the WinDivertHelperFormatFilter() function. This layer can also capture events that occurred before the handle was opened. This layer cannot capture events related to other WINDIVERT_LAYER_REFLECT-layer handles.

5.2 WINDIVERT_EVENT

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typedef enum
{
    WINDIVERT_EVENT_NETWORK_PACKET,
    WINDIVERT_EVENT_FLOW_ESTABLISHED,
    WINDIVERT_EVENT_FLOW_DELETED,
    WINDIVERT_EVENT_SOCKET_BIND,
    WINDIVERT_EVENT_SOCKET_CONNECT,
    WINDIVERT_EVENT_SOCKET_LISTEN,
    WINDIVERT_EVENT_SOCKET_ACCEPT,
    WINDIVERT_EVENT_SOCKET_CLOSE,
    WINDIVERT_EVENT_REFLECT_OPEN,
    WINDIVERT_EVENT_REFLECT_CLOSE,
} **WINDIVERT\_EVENT** , ***PWINDIVERT\_EVENT** ;

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Remarks

Each layer supports one or more events summarized below:

• WINDIVERT_LAYER_NETWORK and WINDIVERT_LAYER_NETWORK_FORWARD : Only a single event is supported:

• WINDIVERT_LAYER_FLOW : Two events are supported:

• WINDIVERT_LAYER_SOCKET : The following events are supported:

• WINDIVERT_LAYER_REFLECT : Two events are supported:

5.3 WINDIVERT_ADDRESS

|

typedef struct
{
    UINT32 IfIdx;
    UINT32 SubIfIdx;
} **WINDIVERT\_DATA\_NETWORK** , ***PWINDIVERT\_DATA\_NETWORK** ;

typedef struct
{
    UINT64 Endpoint;
    UINT64 ParentEndpoint;
    UINT32 ProcessId;
    UINT32 LocalAddr[4];
    UINT32 RemoteAddr[4];
    UINT16 LocalPort;
    UINT16 RemotePort;
    UINT8 Protocol;
} **WINDIVERT\_DATA\_FLOW** , ***PWINDIVERT\_DATA\_FLOW** ;

typedef struct
{
    UINT64 Endpoint;
    UINT64 ParentEndpoint;
    UINT32 ProcessId;
    UINT32 LocalAddr[4];
    UINT32 RemoteAddr[4];
    UINT16 LocalPort;
    UINT16 RemotePort;
    UINT8 Protocol;
} **WINDIVERT\_DATA\_SOCKET** , ***PWINDIVERT\_DATA\_SOCKET** ;

typedef struct
{
    INT64 Timestamp;
    UINT32 ProcessId;
    WINDIVERT_LAYER Layer;
    UINT64 Flags;
    INT16 Priority;
} **WINDIVERT\_DATA\_REFLECT** , ***PWINDIVERT\_DATA\_REFLECT** ;

typedef struct
{
    INT64 Timestamp;
    UINT64 Layer:8;
    UINT64 Event:8;
    UINT64 Sniffed:1;
    UINT64 Outbound:1;
    UINT64 Loopback:1;
    UINT64 Impostor:1;
    UINT64 IPv6:1;
    UINT64 IPChecksum:1;
    UINT64 TCPChecksum:1;
    UINT64 UDPChecksum:1;
    union
    {
        WINDIVERT_DATA_NETWORK Network;
        WINDIVERT_DATA_FLOW Flow;
        WINDIVERT_DATA_SOCKET Socket;
        WINDIVERT_DATA_REFLECT Reflect;
    };
} **WINDIVERT\_ADDRESS** , ***PWINDIVERT\_ADDRESS** ;

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Fields

• Timestamp: A timestamp indicating when event occurred.
• Layer: The handle's layer (WINDIVERT_LAYER_*).
• Event: The captured event (WINDIVERT_EVENT_*).
• Sniffed: Set to 1 if the event was sniffed (i.e., not blocked), 0 otherwise..
• Outbound: Set to 1 for outbound packets/event, 0 for inbound or otherwise.
• Loopback: Set to 1 for loopback packets, 0 otherwise
• Impostor: Set to 1 for impostor packets, 0 otherwise.
• IPv6: Set to 1 for IPv6 packets/events, 0 otherwise
• IPChecksum: Set to 1 if the IPv4 checksum is valid, 0 otherwise.
• TCPChecksum: Set to 1 if the TCP checksum is valid, 0 otherwise.
• UDPChecksum: Set to 1 if the UDP checksum is valid, 0 otherwise.
• Network.IfIdx: The interface index on which the packet arrived (for inbound packets), or is to be sent (for outbound packets).
• Network.SubIfIdx: The sub-interface index for IfIdx.
• Flow.EndpointId: The endpoint ID of the flow.
• Flow.ParentEndpointId: The parent endpoint ID of the flow.
• Flow.ProcessId: The ID of the process associated with the flow.
• Flow.LocalAddr, Flow.RemoteAddr, Flow.LocalPort, Flow.RemotePort, and Flow.Protocol: The network 5-tuple associated with the flow.
• Socket.EndpointId: The endpoint ID of the socket operation.
• Socket.ParentEndpointId: The parent endpoint ID of the socket operation.
• Socket.ProcessId: The ID of the process associated with the socket operation.
• Socket.LocalAddr, Socket.RemoteAddr, Socket.LocalPort, Socket.RemotePort, and Socket.Protocol: The network 5-tuple associated with the socket operation.
• Reflect.Timestamp: A timestamp indicating when the handle was opened.
• Reflect.ProcessId: The ID of the process that opened the handle.
• Reflect.Layer, Reflect.Flags, and Reflect.Priority: The WinDivertOpen() parameters of the opened handle.

Remarks
The WINDIVERT_ADDRESS structure represents the "address" of a captured or injected packet. The address includes the packet's timestamp, layer, event, flags, and layer-specific data. All fields are set by WinDivertRecv() when the packet/event is captured. Only some fields are used by WinDivertSend() when a packet is injected.

The Timestamp indicates when the packet/event was first captured by WinDivert. It uses the same clock as QueryPerformanceCounter().

The Layer indicates the layer parameter (WINDIVERT_LAYER_*) that was passed to WinDivertOpen(). It is included in the address to make the structure self-contained.

The Event indicates the layer-specific event (WINDIVERT_EVENT_*) that was captured.

The Outbound flag is set for outbound packets/events, and is cleared for inbound or direction-less packets/events.

The Loopback flag is set for loopback packets. Note that Windows considers any packet originating from, and destined to, the current machine to be a loopback packet, so loopback packets are not limited to localhost addresses. Note that WinDivert considers loopback packets to be outbound only, and will not capture loopback packets on the inbound path.

The Impostor flag is set for impostor packets. An impostor packet is any packet injected by another driver rather than originating from the network or Windows TCP/IP stack. Impostor packets are problematic since they can cause infinite loops, where a packet injected by WinDivertSend() is captured again by WinDivertRecv(). For more information, see WinDivertSend().

The IPv6 flag is set for IPv6 packets/events, and cleared for IPv4 packets/events.

The *Checksum flags indicate whether the packet has valid checksums or not. When IP/TCP/UDP checksum offloading is enabled, it is possible that captured packets do not have valid checksums. Invalid checksums may be arbitrary values.

The Network.* fields are only valid at the WINDIVERT_LAYER_NETWORK and WINDIVERT_LAYER_NETWORK_FORWARD layers. The Network.IfIdx/Network.SubIfIdx indicate the packet's network adapter (a.k.a. interface) index. These values are ignored for outbound packets.

The Flow.* fields are only valid at the WINDIVERT_LAYER_FLOW layer. The Flow.ProcessId is the ID of the process that created the flow (for outbound), or receives the flow (for inbound). The (Flow.LocalAddr, Flow.LocalPort, Flow.RemoteAddr, Flow.RemotePort, Flow.Protocol) fields form the network 5-tuple associated with the flow. For IPv4, the Flow.LocalAddr and Flow.RemoteAddr fields will be IPv4-mapped IPv6 addresses, e.g. the IPv4 address X.Y.Z.W will be represented by ::ffff:X.Y.Z.W.

The Socket.* fields are only valid at the WINDIVERT_LAYER_SOCKET layer. The Socket.ProcessId is the ID of the process that executed the socket operation. The (Socket.LocalAddr, Socket.LocalPort, Socket.RemoteAddr, Socket.RemotePort, Socket.Protocol) fields form the network 5-tuple associated with the operation. For IPv4, the Socket.LocalAddr and Socket.RemoteAddr fields will be IPv4-mapped IPv6 addresses. The WINDIVERT_EVENT_SOCKET_BIND and WINDIVERT_EVENT_SOCKET_LISTEN events can occur before a connection attempt has been made, meaning that the Socket.RemoteAddr and Socket.RemotePort fields for these events will be zero.

The Reflect.* fields are only valid at the WINDIVERT_LAYER_REFLECT layer. The Reflect.ProcessId is the ID of the process that opened the WinDivert handle. The Reflect.Timestamp field is a timestamp indicating when the handle was opened, using the same clock as QueryPerformanceCounter(). The Reflect.Layer, Reflect.Flags, and Reflect.Priority fields correspond to the WinDivertOpen() parameters of the opened handle.

Most address fields are ignored by WinDivertSend(). The exceptions are Outbound (for WINDIVERT_LAYER_NETWORK only), Impostor, IPChecksum, TCPChecksum, UDPChecksum, Network.IfIdx and Network.SubIfIdx.

5.4 WinDivertOpen

|

HANDLE **WinDivertOpen** (
    __in const char *filter,
    __in WINDIVERT_LAYER layer,
    __in INT16 priority,
    __in UINT64 flags
);

|

Parameters

• filter: A packet filter string specified in the WinDivert filter language.
• layer: The layer.
• priority: The priority of the handle.
• flags: Additional flags.

Return Value
A valid WinDivert handle on success, or INVALID_HANDLE_VALUE if an error occurred. Use GetLastError() to get the reason for the error. Common errors include:

1. the WinDivert driver is blocked by security software; or
2. you are using a virtualization environment that does not support drivers.

| | EPT_S_NOT_REGISTERED | 1753 | This error occurs when the Base Filtering Engine service has been disabled. |

Remarks
Opens a WinDivert handle for the given filter. Unless otherwise specified by flags, any packet or event that matches the filter will be diverted to the handle. Diverted packets/events can be read by the application with WinDivertRecv().

A typical application is only interested in a subset of all network traffic or events. In this case the filter should match as closely as possible to the subset of interest. This avoids unnecessary overheads introduced by diverting packets to the user-mode application. See the filter language section for more information.

The layer of the WinDivert handle is determined by the layer parameter. See WINDIVERT_LAYER for more information. Currently the following layers are supported:

Different WinDivert handles can be assigned different priorities by the priority parameter. Packets are diverted to higher priority handles before lower priority handles. Packets injected by a handle are then diverted to the next priority handle, and so on, provided the packet matches the handle's filter. A packet is only diverted once per priority level, so handles should not share priority levels unless they use mutually exclusive filters. Otherwise it is not defined which handle will receive the packet first. Higher priority values represent higher priorities, with WINDIVERT_PRIORITY_HIGHEST being the highest priority, 0 the middle (and a good default) priority, and WINDIVERT_PRIORITY_LOWEST the lowest priority.

Different flags affect how the opened handle behaves. The following flags are supported:

Note that any combination of (WINDIVERT_FLAG_SNIFF | WINDIVERT_FLAG_DROP) or (WINDIVERT_FLAG_RECV_ONLY | WINDIVERT_FLAG_SEND_ONLY) are considered invalid.

Some layers have mandatory flags, as listed below:

5.5 WinDivertRecv

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BOOL **WinDivertRecv** (
    __in HANDLE handle,
    __out_opt PVOID pPacket,
    __in UINT packetLen,
    __out_opt UINT *pRecvLen,
    __out_opt WINDIVERT_ADDRESS *pAddr
);

|

Parameters

• handle: A valid WinDivert handle created by WinDivertOpen().
• pPacket: An optional buffer for the captured packet.
• packetLen: The length of the pPacket buffer.
• pRecvLen: The total number of bytes written to pPacket. Can be NULL if this information is not required.
• pAddr: An optional buffer for the address of the captured packet/event.

Return Value
TRUE if a packet/event was successfully received, or FALSE if an error occurred. Use GetLastError() to get the reason for the error.

Common errors include:

Remarks
Receives a single captured packet/event matching the filter passed to WinDivertOpen(). The received packet/event is guaranteed to match the filter.

Only some layers can capture packets/data, as summarized below:

For layers that do support capturing, the captured packet/data will be written to the pPacket buffer. If non-NULL, then the total number of bytes written to pPacket will be written to pRecvLen. If the pPacket buffer is too small, the packet will be truncated and the operation will fail with the ERROR_INSUFFICIENT_BUFFER error code. This error can be ignored if the application only intends to receive part of the packet, e.g., the IP headers only. For layers that do not capture packets/data, the pPacket parameter should be NULL and packetLen should be zero.

If non-NULL, the address of the packet/event will be written to the pAddr buffer.

An application should call WinDivertRecv()as soon as possible after a successful call to WinDivertOpen(). When a WinDivert handle is open, any packet/event that matches the filter will be captured and queued until handled by WinDivertRecv(). Packets/events are not queued indefinitely, and if not handled in a timely manner, data may be lost. The amount of time a packet/event is queued can be controlled using the WinDivertSetParam() function.

Captured packets are guaranteed to have correct checksums or have the corresponding *Checksum flag unset (see WINDIVERT_ADDRESS).

WinDivertRecv() should not be used on any WinDivert handle created with the WINDIVERT_FLAG_DROP set.

5.6 WinDivertRecvEx

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BOOL **WinDivertRecvEx** (
    __in HANDLE handle,
    __out VOID *pPacket,
    __in UINT packetLen,
    __out_opt UINT *pRecvLen,
    __in UINT64 flags,
    __out_opt WINDIVERT_ADDRESS *pAddr,
    __inout_opt UINT *pAddrLen,
    __inout_opt LPOVERLAPPED lpOverlapped
);

|

Parameters

• handle: A valid WinDivert handle created by WinDivertOpen().
• pPacket: A buffer for the captured packet(s).
• packetLen: The length of the pPacket buffer in bytes.
• pRecvLen: The total number of bytes written to pPacket. Can be NULL if this information is not required.
• flags: Reserved, set to zero.
• pAddr: The WINDIVERT_ADDRESS of the captured packet(s).
• pAddrLen: Initially, a pointer to the length of the pAddr buffer in bytes. This value is updated to the total bytes written to pAddr. If NULL, a fixed length of sizeof(WINDIVERT_ADDRESS) is assumed.
• lpOverlapped: An optional pointer to a OVERLAPPED structure.

Return Value
TRUE if a packet was successfully received, or FALSE otherwise. Use GetLastError() to get the reason. The error code ERROR_IO_PENDING indicates that the overlapped operation has been successfully initiated and that completion will be indicated at a later time. All other codes indicate an error.

Remarks
This function is equivalent to WinDivertRecv() except:

• Overlapped I/O is supported via the lpOverlapped parameter.
• Batched I/O (i.e., receiving multiple packets at once) is supported.

Batched I/O makes it possible to receive up to WINDIVERT_BATCH_MAX packets at once using a single operation, reducing the number of kernel/user-mode context switches and improving performance. To enable batched I/O:

1. pass an array of more than one WINDIVERT_ADDRESS to pAddr;
2. set pAddrLen to be the total size (in bytes) of the pAddr buffer; and
3. ensure that pPacket points to a sufficiently large buffer capable of receiving multiple packets.

For example:

UINT8 packets[10 * MTU]; // Space for up to 10 packets
    WINDIVERT_ADDRESS addr[10]; // Addresses for up to 10 packets
    UINT addr_len = sizeof(addr);
    BOOL result = WinDivertRecvEx(handle, packets, ..., addr, &addr_len, ...);

upon successful completion, the value pointed to by pAddrLen is updated to the total number of address bytes actually received. For example, if a total of 5 packets were received, then the value pointed to by pAddrLen will be set to (5*sizeof(WINDIVERT_ADDRESS)). The received packets are packed contiguously (i.e., no gaps) into the pPacket buffer.

5.7 WinDivertSend

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BOOL **WinDivertSend** (
    __in HANDLE handle,
    __in const VOID *pPacket,
    __in UINT packetLen,
    __out_opt UINT *pSendLen,
    __in const WINDIVERT_ADDRESS *pAddr
);

|

Parameters

• handle: A valid WinDivert handle created by WinDivertOpen().
• pPacket: A buffer containing a packet to be injected.
• packetLen: The total length of the pPacket buffer.
• pSendLen: The total number of bytes injected. Can be NULL if this information is not required.
• pAddr: The address of the injected packet.

Return Value
TRUE if a packet was successfully injected, or FALSE if an error occurred. Use GetLastError() to get the reason for the error.

Common errors include:

Remarks
Injects a packet into the network stack. The injected packet may be one received from WinDivertRecv(), or a modified version, or a completely new packet. Injected packets can be captured and diverted again by other WinDivert handles with lower priorities.

Only the WINDIVERT_LAYER_NETWORK and WINDIVERT_LAYER_NETWORK_FORWARD layers support packet injection, as summarized below:

For the WINDIVERT_LAYER_NETWORK layer the pAddr->Outbound value determines which direction the packet is injected. If the pAddr->Outbound field is 1, the packet will be injected into the outbound path (i.e. a packet leaving the local machine). Else, if pAddr->Outbound is 0, the packet is injected into the inbound path (i.e. a packet arriving to the local machine). Note that only the Outbound field, and not the IP addresses in the injected packet, determines the packet's direction.

For packets injected into the inbound path, the pAddr->Network.IfIdx and pAddr->Network.SubIfIdx fields are assumed to contain valid interface numbers. These may be retrieved from WinDivertRecv() (for packet modification), or from the IP Helper API.

For outbound injected packets, the IfIdx and SubIfIdx fields are currently ignored and may be arbitrary values. Injecting an inbound packet on the outbound path may work (for some types of packets), however this should be considered "undocumented" behavior, and may be changed in the future.

For impostor packets (where pAddr->Impostor is set to 1) WinDivert will automatically decrement the ip.TTL or ipv6.HopLimit fields before reinjection. This is to mitigate infinite loops since WinDivert cannot prevent impostor packets from being captured again by WinDivertRecv().

Injected packets must have the correct checksums or have the corresponding pAddr->*Checksum flag unset. A packet/address pair captured by WinDivertRecv() is guaranteed to satisfy this condition, so can be reinjected unmodified without recalculating checksums. Otherwise, if a modification is necessary, checksums can be recalculated using the WinDivertHelperCalcChecksums() function.

5.8 WinDivertSendEx

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BOOL **WinDivertSendEx** (
    __in HANDLE handle,
    __in const VOID *pPacket,
    __in UINT packetLen,
    __out_opt UINT *pSendLen,
    __in UINT64 flags,
    __in const WINDIVERT_ADDRESS *pAddr,
    __in UINT addrLen,
    __inout_opt LPOVERLAPPED lpOverlapped
);

|

Parameters

• handle: A valid WinDivert handle created by WinDivertOpen().
• pPacket: A buffer containing the packet(s) to be injected.
• packetLen: The total length of the buffer pPacket.
• pSendLen: The total number of bytes injected. Can be NULL if this information is not required.
• flags: Reserved, set to zero.
• pAddr: The address(es) of the injected packet(s).
• addrLen: The total length (in bytes) of the pAddr buffer.
• lpOverlapped: An optional pointer to a OVERLAPPED structure.

Return Value
TRUE if a packet was successfully injected, or FALSE otherwise. Use GetLastError() to get the reason. The error code ERROR_IO_PENDING indicates that the overlapped operation has been successfully initiated and that completion will be indicated at a later time. All other codes indicate an error.

Remarks
This function is equivalent to WinDivertSend() except:

• Overlapped I/O is supported via the lpOverlapped parameter.
• Batched I/O (i.e., sending multiple packets at once) is supported.

Batched I/O makes it possible to send up to WINDIVERT_BATCH_MAX packets at once using a single operation, reducing the number of kernel/user-mode context switches and improving performance. To use batched I/O:

1. pack N packets into the pPacket buffer (with no gaps between packets);
2. set packetLen to be the total sum of the N packet lengths;
3. pack the corresponding N WINDIVERT_ADDRESS address structures into the pAddr buffer; and
4. set addrLen to be the total size (in bytes) of the pAddr buffer.

5.9 WinDivertShutdown

|

BOOL **WinDivertShutdown** (
    __in HANDLE handle,
    __in WINDIVERT_SHUTDOWN how);
);

|

Parameters

• handle: A valid WinDivert handle created by WinDivertOpen().
• how: A WINDIVERT_SHUTDOWN value to indicate how the handle should be shutdown.

Return Value
TRUE if successful, FALSE if an error occurred. Use GetLastError() to get the reason for the error.

Remarks
This operation causes all or part of a WinDivert handle to be shut down. The possible values for how are:

Note that previously queued packets can still be received after WINDIVERT_SHUTDOWN_RECV. When the packet queue is empty, WinDivertRecv() will fail with ERROR_NO_DATA.

5.10 WinDivertClose

|

BOOL **WinDivertClose** (
    __in HANDLE handle
);

|

Parameters

• handle: A valid WinDivert handle created by WinDivertOpen().

Return Value
TRUE if successful, FALSE if an error occurred. Use GetLastError() to get the reason for the error.

Remarks
Closes a WinDivert handle created by WinDivertOpen().

5.11 WinDivertSetParam

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BOOL **WinDivertSetParam** (
    __in HANDLE handle,
    __in WINDIVERT_PARAM param,
    __in UINT64 value);

|

Parameters

• handle: A valid WinDivert handle created by WinDivertOpen().
• param: A WinDivert parameter name.
• value: The parameter's new value.

Return Value
TRUE if successful, FALSE if an error occurred. Use GetLastError() to get the reason for the error.

Remarks
Sets a WinDivert parameter. Currently, the following WinDivert parameters are defined.

5.12 WinDivertGetParam

|

BOOL **WinDivertGetParam** (
    __in HANDLE handle,
    __in WINDIVERT_PARAM param,
    __out UINT64 *pValue);

|

Parameters

• handle: A valid WinDivert handle created by WinDivertOpen().
• param: A WinDivert parameter name.
• value: The parameter's current value.

Return Value
TRUE if successful, FALSE if an error occurred. Use GetLastError() to get the reason for the error.

Remarks
Gets a WinDivert parameter. This function supports all the parameters from WinDivertSetParam(), and the following additional read-only parameters:

6. Helper Programming API

The WinDivert helper programming API is a collection of definitions and functions designed to make writing WinDivert applications easier. The use of the helper API is optional.

6.1 WINDIVERT_IPHDR

|

typedef struct
{
    UINT8 HdrLength:4;
    UINT8 Version:4;
    UINT8 TOS;
    UINT16 Length;
    UINT16 Id;
    UINT16_..._;
    UINT8 TTL;
    UINT8 Protocol;
    UINT16 Checksum;
    UINT32 SrcAddr;
    UINT32 DstAddr;
} **WINDIVERT\_IPHDR** , ***PWINDIVERT\_IPHDR** ;

|

Fields
See here for more information.

Remarks
IPv4 header definition.

The following fields can only be get/set using the following macro definitions:

• FragOff with WINDIVERT_IPHDR_GET_FRAGOFF(hdr) and WINDIVERT_IPHDR_SET_FRAGOFF(hdr, val)
• MF with WINDIVERT_IPHDR_GET_MF(hdr) and WINDIVERT_IPHDR_SET_MF(hdr, val)
• DF with WINDIVERT_IPHDR_GET_DF(hdr) and WINDIVERT_IPHDR_SET_DF(hdr, val)
• Reserved with WINDIVERT_IPHDR_GET_RESERVED(hdr) and WINDIVERT_IPHDR_SET_RESERVED(hdr, val)

6.2 WINDIVERT_IPV6HDR

|

typedef struct
{
    UINT32 Version:4;
    UINT32 ...:28;
    UINT16 Length;
    UINT8 NextHdr;
    UINT8 HopLimit;
    UINT32 SrcAddr[4];
    UINT32 DstAddr[4];
} **WINDIVERT\_IPV6HDR** , ***PWINDIVERT\_IPV6HDR** ;

|

Fields
See here for more information.

Remarks
IPv6 header definition.

The following fields can only be get/set using the following macro definitions:

• TrafficClass with WINDIVERT_IPV6HDR_GET_TRAFFICCLASS(hdr) and WINDIVERT_IPV6HDR_SET_TRAFFICCLASS(hdr, val)
• FlowLabel with WINDIVERT_IPV6HDR_GET_FLOWLABEL(hdr) and WINDIVERT_IPV6HDR_SET_FLOWLABEL(hdr, val)

6.3 WINDIVERT_ICMPHDR

|

typedef struct
{
    UINT8 Type;
    UINT8 Code;
    UINT16 Checksum;
    UINT32 Body;
} **WINDIVERT\_ICMPHDR** , ***PWINDIVERT\_ICMPHDR** ;

|

Fields
See here for more information.

Remarks
ICMP header definition.

6.4 WINDIVERT_ICMPV6HDR

|

typedef struct
{
    UINT8 Type;
    UINT8 Code;
    UINT16 Checksum;
    UINT32 Body;
} **WINDIVERT\_ICMPV6HDR** , ***PWINDIVERT\_ICMPV6HDR** ;

|

Fields
See here for more information.

Remarks
ICMPv6 header definition.

6.5 WINDIVERT_TCPHDR

|

typedef struct
{
    UINT16 SrcPort;
    UINT16 DstPort;
    UINT32 SeqNum;
    UINT32 AckNum;
    UINT16 Reserved1:4;
    UINT16 HdrLength:4;
    UINT16 Fin:1;
    UINT16 Syn:1;
    UINT16 Rst:1;
    UINT16 Psh:1;
    UINT16 Ack:1;
    UINT16 Urg:1;
    UINT16 Reserved2:2;
    UINT16 Window;
    UINT16 Checksum;
    UINT16 UrgPtr;
} **WINDIVERT\_TCPHDR** , ***PWINDIVERT\_TCPHDR** ;

|

Fields
See here for more information.

Remarks
TCP header definition.

6.6 WINDIVERT_UDPHDR

|

typedef struct
{
    UINT16 SrcPort;
    UINT16 DstPort;
    UINT16 Length;
    UINT16 Checksum;
} **WINDIVERT\_UDPHDR** , ***PWINDIVERT\_UDPHDR** ;

|

Fields
See here for more information.

Remarks
UDP header definition.

6.7 WinDivertHelperParsePacket

|

BOOL **WinDivertHelperParsePacket** (
    __in PVOID pPacket,
    __in UINT packetLen,
    __out_opt PWINDIVERT_IPHDR *ppIpHdr,
    __out_opt PWINDIVERT_IPV6HDR *ppIpv6Hdr,
    __out_opt UINT8 *pProtocol,
    __out_opt PWINDIVERT_ICMPHDR *ppIcmpHdr,
    __out_opt PWINDIVERT_ICMPV6HDR *ppIcmpv6Hdr,
    __out_opt PWINDIVERT_TCPHDR *ppTcpHdr,
    __out_opt PWINDIVERT_UDPHDR *ppUdpHdr,
    __out_opt PVOID *ppData,
    __out_opt UINT *pDataLen,
    __out_opt PVOID *ppNext,
    __out_opt UINT *pNextLen
);

|

Parameters

• pPacket: The packet(s) to be parsed.
• packetLen: The total length of the packet(s) pPacket.
• ppIpHdr: Output pointer to a WINDIVERT_IPHDR.
• ppIpv6Hdr: Output pointer to a WINDIVERT_IPV6HDR.
• pProtocol: Output transport protocol.
• ppIcmpHdr: Output pointer to a WINDIVERT_ICMPHDR.
• ppIcmpv6Hdr: Output pointer to a WINDIVERT_ICMPV6HDR.
• ppTcpHdr: Output pointer to a WINDIVERT_TCPHDR.
• ppUdpHdr: Output pointer to a WINDIVERT_UDPHDR.
• ppData: Output pointer to the packet's data/payload.
• pDataLen: Output data/payload length.
• ppNext: Output pointer to the next packet (if present).
• pNextLen: Output next packet length.

Return Value
TRUE if successful, FALSE if an error occurred.

Remarks
Parses a raw packet or batch of packets (e.g. from WinDivertRecv()) into the various packet headers and/or payloads that may or may not be present.

Each output parameter may be NULL or non-NULL. For non-NULL parameters, this function will write the pointer to the corresponding header/payload if it exists, or will write NULL otherwise. Any non-NULL pointer that is returned:

1. Is a pointer into the original pPacket packet buffer; and
2. There is enough space in pPacket to fit the header.

This function does not do any verification of the header/payload contents beyond checking the header length and any other minimal information required for parsing. This function will always succeed provided the pPacket buffer contains at least one IPv4 or IPv6 header and the packetLen is correct.

By default this function will parse a single packet. However, if either ppNext or pNextLen are non-NULL, then the pPacket parameter can point to a batch (>1) of packets (and packetLen can be the total length of the batch). In this case, the function will parse the first packet, and a pointer to the remaining packet(s) will be written to ppNext, and the remaining length will be written to pNextLen. This makes it convenient to loop over every packet in the batch as follows:

while (WinDivertHelperParsePacket(pPacket, packetLen, ..., &pPacket, &packetLen))
    {
        ...
    }

6.8 WinDivertHelperHashPacket

|

UINT64 **WinDivertHelperHashPacket** (
    __in const VOID *pPacket,
    __in UINT packetLen,
    __in UINT64 seed = 0
);

|

Parameters

• pPacket: The packet to be hashed.
• packetLen: The total length of the packet pPacket.
• seed: An optional seed value.

Return Value
A 64bit hash value.

Remarks
Calculates a 64bit hash value of the given packet. Note that the hash function depends on the packet's IP and transport headers only, and not the payload of the packet. That said, a weak dependency on the payload will exist if the TCP/UDP checksums are valid. The hash function itself is based on the xxHash algorithm and is not cryptographic.

The optional seed value is also incorporated into the hash.

6.9 WinDivertHelperParseIPv4Address

|

BOOL **WinDivertHelperParseIPv4Address** (
    __in const char *addrStr,
    __out_opt UINT32 *pAddr
);

|

Parameters

• addrStr: The address string.
• pAddr: Output address.

Return Value
TRUE if successful, FALSE if an error occurred. Use GetLastError() to get the reason for the error.

Remarks
Parses an IPv4 address stored in addrStr. If pAddr is non-NULL, the result is be stored in host-byte-order. Use WinDivertHelperHtonl() to convert the result into network-byte-order.

6.10 WinDivertHelperParseIPv6Address

|

BOOL **WinDivertHelperParseIPv6Address** (
    __in const char *addrStr,
    __out_opt UINT32 *pAddr
);

|

Parameters

• addrStr: The address string.
• pAddr: Output address.

Return Value
TRUE if successful, FALSE if an error occurred. Use GetLastError() to get the reason for the error.

Remarks
Parses an IPv6 address stored in addrStr. If pAddr is non-NULL, the buffer assumed to be large enough to hold a 16-byte IPv6 address. The result is stored in host-byte-order. Use WinDivertHelperHtonIPv6Address() to convert the result into network-byte-order.

6.11 WinDivertHelperParseIPv4Address

|

BOOL **WinDivertHelperFormatIPv4Address** (
    __in UINT32 addr,
    __out char *buffer,
    __in UINT bufLen
);

|

Parameters

• addr: The IPv4 address in host-byte order.
• buffer: The buffer to store the formatted string.
• bufLen: The length of buffer.

Return Value
TRUE if successful, FALSE if an error occurred. Use GetLastError() to get the reason for the error.

Remarks
Convert an IPv4 address into a string.

6.12 WinDivertHelperParseIPv6Address

|

BOOL **WinDivertHelperFormatIPv6Address** (
    __in const UINT32 *pAddr,
    __out char *buffer,
    __in UINT bufLen
);

|

Parameters

• pAddr: The IPv6 address in host-byte order.
• buffer: The buffer to store the formatted string.
• bufLen: The length of buffer.

Return Value
TRUE if successful, FALSE if an error occurred. Use GetLastError() to get the reason for the error.

Remarks
Convert an IPv6 address into a string.

6.13 WinDivertHelperCalcChecksums

|

BOOL **WinDivertHelperCalcChecksums** (
    __inout VOID *pPacket,
    __in UINT packetLen,
    __out_opt WINDIVERT_ADDRESS *pAddr,
    __in UINT64 flags
);

|

Parameters

• pPacket: The packet to be modified.
• packetLen: The total length of the packet pPacket.
• pAddr: Optional pointer to a WINDIVERT_ADDRESS structure.
• flags: One or more of the following flags:
  • WINDIVERT_HELPER_NO_IP_CHECKSUM: Do not calculate the IPv4 checksum.
  • WINDIVERT_HELPER_NO_ICMP_CHECKSUM: Do not calculate the ICMP checksum.
  • WINDIVERT_HELPER_NO_ICMPV6_CHECKSUM: Do not calculate the ICMPv6 checksum.
  • WINDIVERT_HELPER_NO_TCP_CHECKSUM: Do not calculate the TCP checksum.
  • WINDIVERT_HELPER_NO_UDP_CHECKSUM: Do not calculate the UDP checksum.

Return Value
TRUE if successful, FALSE if an error occurred.

Remarks
(Re)calculates the checksum for any IPv4/ICMP/ICMPv6/TCP/UDP checksum present in the given packet. Individual checksum calculations may be disabled via the appropriate flag. Typically this function should be invoked on a modified packet before it is injected with WinDivertSend().

By default this function will calculate each checksum from scratch, even if the existing checksum is correct. This may be inefficient for some applications. For better performance, incremental checksum calculations should be used instead (not provided by this API).

If pAddr is non-NULL, this function sets the corresponding *Checksum flag (see WINDIVERT_ADDRESS). Normally, pAddr should point to the address passed to WinDivertSend() for packet injection.

6.14 WinDivertHelperDecrementTTL

|

BOOL **WinDivertHelperDecrementTTL** (
    __inout VOID *packet,
    __in packetLen
);

|

Parameters

• pPacket: The packet to be modified.
• packetLen: The total length of the packet pPacket.

Return Value
TRUE if successful, FALSE if an error occurred. Returns FALSE if the ip.TTL or ipv6.HopHimit fields go to 0.

Remarks
Decrements the ip.TTL or ipv6.HopHimit field by 1, and returns TRUE only if the result is non-zero. This is useful for applications where packet loops may be a problem.

For IPv4, this function will preserve the validity of the IPv4 checksum. That is, if the packet had a valid checksum before the operation, the resulting checksum will also be valid after the operation. This function updates the checksum field incrementally.

6.15 WinDivertHelperCompileFilter

|

BOOL **WinDivertHelperCompileFilter** (
    __in const char *filter,
    __in WINDIVERT_LAYER layer,
    __out_opt char *object,
    __in UINT objLen,
    __out_opt const char **errorStr,
    __out_opt UINT *errorPos
);

|

Parameters

• filter: The packet filter string to be checked.
• layer: The layer.
• object: The compiled filter object.
• objLen: The length of the object buffer.
• errorStr: The error description.
• errorPos: The error position.

Return Value
TRUE if the packet filter compilation is successful, FALSE otherwise.

Remarks
Compiles the given packet filter string into a compact object representation that is optionally stored in object if non-NULL. The object representation is a valid null terminated C string, but is otherwise opaque and not meant to be human readable. The object representation can be passed to all WinDivert functions, such as WinDivertOpen(), in place of the human-readable filter string equivalent.

The compilation operation will succeed if the given filter string is valid with respect to the filter language. Otherwise, if the filter is invalid, then a human readable description of the error is returned by errorStr (if non-NULL), and the error's position is returned by errorPos (if non-NULL).

Note that all strings returned through errorStr are global static objects, and therefore do not need to be deallocated.

6.16 WinDivertHelperEvalFilter

|

BOOL **WinDivertHelperEvalFilter** (
    __in const char *filter,
    __in const VOID *pPacket,
    __in UINT packetLen,
    __in const WINDIVERT_ADDRESS *pAddr
);

|

Parameters

• filter: The packet filter string to be evaluated.
• pPacket: The packet.
• packetLen: The total length of the packet pPacket.
• pAddr: The WINDIVERT_ADDRESS of the packet pPacket.

Return Value
TRUE if the packet matches the filter string, FALSE otherwise.

Remarks
Evaluates the given packet against the given packet filter string. This function returns TRUE if the packet matches, and returns FALSE otherwise.

This function also returns FALSE if an error occurs, in which case GetLastError() can be used to get the reason for the error. Otherwise, if no error occurred, GetLastError() will return 0.

Note that this function is relatively slow since the packet filter string will be (re)compiled for each call. This overhead can be minimized by pre-compiling the filter string into the object representation using the WinDivertHelperCompileFilter() function.

6.17 WinDivertHelperFormatFilter

|

BOOL **WinDivertHelperEvalFilter** (
    __in const char *filter,
    __in WINDIVERT_LAYER layer,
    __out char *buffer,
    __in UINT bufLen
);

|

Parameters

• filter: The packet filter string to be evaluated.
• layer: The layer.
• buffer: A buffer for the formatted filter.
• bufLen: The length of buffer.

Return Value
TRUE if successful, FALSE if an error occurred. Use GetLastError() to get the reason for the error.

Remarks
Formats the given filter string or object. This function is mainly useful for decompiling the filter object representation back into a human-readable filter string representation. One application is the WINDIVERT_LAYER_REFLECT layer, where the filter object associated with the reflection event is returned by WinDivertRecv().

6.18 WinDivertHelperNtoh*

|

UINT16 **WinDivertHelperNtohs** (
    __in UINT16 x
);
UINT32 **WinDivertHelperNtohl** (
    __in UINT32 x
);
UINT64 **WinDivertHelperNtohll** (
    __in UINT64 x
);
void **WinDivertHelperNtohIPv6Address** (
    __in const UINT *inAddr,
    __out UINT *outAddr
);

|

Parameters

• x: The input value in network byte-order.
• inAddr: The input IPv6 address in network byte-order.
• outAddr: A buffer for the output IPv6 address in host byte-order.

Return Value
The output value in host byte order.

Remarks
Converts a value/IPv6-address from network to host byte-order.

6.19 WinDivertHelperHton*

|

UINT16 **WinDivertHelperHtons** (
    __in UINT16 x
);
UINT32 **WinDivertHelperHtonl** (
    __in UINT32 x
);
UINT64 **WinDivertHelperHtonll** (
    __in UINT64 x
);
void **WinDivertHelperHtonIPv6Address** (
    __in const UINT *inAddr,
    __out UINT *outAddr
);

|

Parameters

• x: The input value in host byte-order.
• inAddr: The input IPv6 address in host byte-order.
• outAddr: A buffer for the output IPv6 address in network byte-order.

Return Value
The output value in network byte order.

Remarks
Converts a value/IPv6-address from host to network byte-order.

7. Filter Language

The WinDivertOpen() function accepts a string containing a filter. Only packets/events that match the filter will be blocked and/or captured. All other non-matching packets/events will be allowed to continue as normal.

The filter allows an application to select only a subset traffic that is of interest. For example, a HTTP blacklist filter is only interested in packets that might contain URLs. This could be achieved using the following filter.

HANDLE handle = WinDivertOpen(
    "outbound and "
    "tcp.PayloadLength > 0 and "
    "tcp.DstPort == 80", 0, 0, 0);

This filter selects only the subset of all traffic that is:

1. outbound;
2. contains a non-empty payload; and
3. has TCP destination port 80 (i.e. HTTP web traffic).

A filter is a Boolean expression of the form:

_FILTER_:= true | false |_FILTER_and_FILTER_|_FILTER_or_FILTER_| (_FILTER_) | (_FILTER_?_FILTER_:_FILTER_) |_TEST_

C-style syntax &&, ||, and ! may also be used instead of and, or, and not, respectively. C-style conditional operators are also supported, where the expression (A? B: C) evaluates to:

• B if A evaluates to true; or
• C if A evaluates to false.

A test is of the following form:

_TEST_:=_TEST0_| not_TEST0__TEST0_:=_FIELD_|_FIELD_op_VAL_

where op is one of the following:

and VAL is a decimal number, hexadecimal number, IPv4 address, IPv6 address or a layer-specific macro. If the op VAL is missing, the test is implicitly FIELD != 0.

Finally, a field is some layer-specific property matching the packet or event. The possible fields are:

A test will also fails if the field is not relevant. For example, the test tcp.DstPort == 80 will fail if the packet does not contain a TCP header.

The processId field matches the ID of the process associated to an event. Due to technical limitations, this field is not supported by the WINDIVERT_LAYER_NETWORK* layers. That said, it is usually possible to associate process IDs to network packets matching the same network 5-tuple. Note that a fundamental race condition exists between the processId and the termination of the corresponding process, see the know issues listed below.

The packet*[i], tcp.Payload*[i] and udp.Payload*[i] fields take an index parameter (i). The following indexing schemes are supported:

• Undecorated integer (e.g., packet32[10]): evaluates to the ith word from the start of the packet/payload. This is essentially C-style array indexing;
• Negative decorated integer (e.g., packet32[-10]): evaluates to the ith word from the end of the packet/payload. Here the index (-1) is the first full word that fits; and
• Byte decorated (negative) integer (e.g., packet32[10b] or packet32[-10b]): evaluated to the word offset by i bytes from the start (or end) of the packet/payload.

These fields can be used to match filters against the contents of packets/payloads in addition to address/header information. Words are assumed to be in network-byte ordering. If the index is out-of-bounds then the corresponding test is deemed to have failed.

The random* fields are not really random but use a deterministic hash value calculated using the WinDivertHelperHashPacket() function.

Layer-specific macros make it possible to match events and layers symbolically, e.g., event == CONNECT or layer == SOCKET. The possible macros are:

7.1 Filter Examples

1. Divert all outbound (non-local) web traffic:

HANDLE handle = WinDivertOpen(
        "outbound and !loopback and "
        "(tcp.DstPort == 80 or udp.DstPort == 53)",
        0, 0, 0
    );

2. Divert all inbound TCP SYNs:

HANDLE handle = WinDivertOpen(
        "inbound and "
        "tcp.Syn",
        0, 0, 0
    );

3. Divert all traffic:

HANDLE handle = WinDivertOpen("true", 0, 0, 0);

4. Divert no traffic:

HANDLE handle = WinDivertOpen("false", 0, 0, 0);

This is useful for packet injection.

7.2 Filter Usage

The purpose of the filter is to help applications select the subset of all network traffic that the application is interested in. Ideally the filter should be

1. As short as possible; and
2. As selective as possible.

For some applications these two objectives can conflict. That is, a selective filter is not short, and a short filter is not selective. For such applications the developer should experiment with different filter configurations and carefully measure the performance impact to find the optimal solution.

8. Performance

Using WinDivert to redirect network traffic to/from a user application incurs performance overheads, such as copying packet data and user/kernel mode context switching. Under heavy load (≥1Gbps) these overheads can be significant. The following techniques can be used to reduce overheads (in order of importance):

1. Selective Filter: Only select the subset of network traffic the user application is interested in. Non-matching traffic will continue to use the default path without incurring additional overheads.
2. Batch Mode: The WinDivertRecvEx() and WinDivertSendEx() functions support batching that allows several packets to be received/sent at once. This can significantly reduce the overheads relating to user/kernel mode context switching.
3. Multi-threading: It is possible to spread packet processing over multiple threads ensuring that the user application does not become a bottleneck. That said, sometimes spawning too many threads can degrade performance.
4. Small Buffers: Large buffers generally incur more overhead compared to smaller buffers. In general, the buffer size should reflect the expected usage as closely as possible.
5. Simple Filters: Currently WinDivert does not optimize the filter compilation, so it is up to the user application to ensure the filter is simple/optimized.
6. Overlapped I/O: This allows the user application to do additional tasks at the same time as receive/send operations, which may improve performance for some applications. It is also possible for a single thread to initiate several receive/send operations at once. However, using overlapped I/O can be tricky, and it is important that all buffers passed to WinDivertRecvEx() or WinDivertSendEx() (including the OVERLAPPED structure) are not modified by the user application until the operation completes.
7. Queue length/size/time: If these values are too small then some packets may be dropped under heavy load. These values can be controlled using the WinDivertSetParam() function.

The passthru.exe sample program can be used to experiment with different batch sizes and thread counts.

9. Samples

Some samples have been provided to demonstrate the WinDivert API. The sample programs are:

• webfilter.exe: A simple URL blacklist filter. This program monitors outbound HTTP traffic. If it finds a URL request that matches the blacklist, it hijacks the TCP connection, reseting the connection at the server's end, and sending a simple block-page to the browser. The blacklist(s) are specified at the command-line.
• netdump.exe: A simple packet sniffer based on the WinDivert filter language. This program takes a filter specified at the command line, and prints information about any packet that matches the filter. This example uses WinDivert in "packet sniffing" mode, similar to winpcap. However, unlike winpcap, WinDivert can see local (loopback) packets.
• netfilter.exe: A simple firewall based on the WinDivert filter language. This program takes a filter specified at the command line, and blocks any packet that matches the filter. It blocks TCP by sending a TCP reset, UDP by an ICMP message, and all other traffic it simply drops. This is similar to the Linux iptables command with the -j REJECT option.
• passthru.exe: A simple program that simply re-injects every packet it captures. This example has a configurable batch-size and thread count, and so is useful for performance testing or as a starting point for more interesting applications.
• streamdump.exe: A simple program that demonstrates how to handle streams using WinDivert. The basic idea is to divert outbound TCP connections to a local proxy server which can capture or manipulate the stream.
• flowtrack.exe: A program that tracks all network flows to and from the local machine, including information such as the ID of the responsible process. The flowtrack sample demonstrates the WINDIVERT_LAYER_FLOW layer.
• socketdump.exe: Dumps socket operations (bind(), connect(), etc.) and the ID of the responsible process. The socketdump sample demonstrates the WINDIVERT_LAYER_SOCKET layer.
• windivertctl.exe allows the user to query which processes are using WinDivert via the list or watch commands, or to terminate all such processes using the kill command. The windivertctl.exe can also forcibly remove the WinDivert driver using the uninstall command. The windivertctl sample demonstrates the WINDIVERT_LAYER_REFLECT layer.

The samples are intended for educational purposes only, and are not fully-featured applications.

The following basic template for a WinDivert application using the WINDIVERT_LAYER_NETWORK layer. The basic idea is to open a WinDivert handle, then enter a capture-modify-reinject loop:

HANDLE handle; // WinDivert handle
    WINDIVERT_ADDRESS addr; // Packet address
    char packet[MAXBUF]; // Packet buffer
    UINT packetLen;

    // Open some filter
    handle = WinDivertOpen("...", WINDIVERT_LAYER_NETWORK, 0, 0);
    if (handle == INVALID_HANDLE_VALUE)
    {
        // Handle error
        exit(1);
    }

    // Main capture-modify-inject loop:
    while (TRUE)
    {
        if (!WinDivertRecv(handle, packet, sizeof(packet), &packetLen, &addr))
        {
            // Handle recv error
            continue;
        }

        // Modify packet.

        WinDivertHelperCalcChecksums(packet, packetLen, &addr, 0);
        if (!WinDivertSend(handle, packet, packetLen, NULL, &addr))
        {
            // Handle send error
            continue;
        }
    }

For applications that do not need to modify the packet, a better approach is to open the WinDivert handle with the WINDIVERT_FLAG_SNIFF flag set, and not re-inject the packet with WinDivertSend(). See the netdump.exe sample program for an example of this usage.

10. Known Issues

WinDivert has some known limitations listed below:

• Injecting inbound ICMP/ICMPv6 messages: Calling WinDivertSend() will fail with an error for certain types of inbound ICMP/ICMPv6 messages. This is probably because the Windows TCP/IP stack does not handle such messages. Such errors are harmless and can be ignored.
• The forward layer does not interact well with the Windows NAT: It is not possible to block packets pre-NAT with WinDivert. As a general principle, you should not try and mix WinDivert at the forward layer with the Windows NAT implementation.
• Re-injecting unmodified packets can lead to infinite loops: If two or more Windows Filtering Platform (WFP) callout drivers (including WinDivert applications) block and inject unmodified copies of packets then this can lead to an infinite loop. If such a loop occurs, WinDivertSend() will eventually fail with error ERROR_HOST_UNREACHABLE. Unfortunately, such errors are not easy to fix. Some crude solutions include: (1) removing the incompatible driver, or (2) ignoring all packets with ip.TTL or ipv6.HopLimit less than the Windows DefaultTTL registry value. See GitHub issue #41 for more information.
• WinDivert can cause the MSVC x86_64 debugger to deadlock: The deadlock occurs because the debugger uses local sockets. Thus: the debugger pauses the WinDivert application, which stops packets from being processed, which causes the debugger wait forever on input from a socket. The deadlock can be avoided by ignoring loopback traffic. See GitHub issue #26 for more information.
• WinDivert can cause packets to be out-of-order: Simply running the passthru.exe sample program can cause packets to become out-of-order. This is not a bug, since there is no requirement for packets to remain in-order. However, this may affect other buggy software (e.g. some buggy NAT implementations) that incorrectly assume packets to be in-order.
• A race condition exists between addr.*.processId and process termination. By the time an event is received using WinDivertRecv(), it is possible that the process responsible for the event has already terminated. Furthermore, it is theoretically possible that the processId has been reassigned to an unrelated process. This problem can be partly mitigated by comparing the timestamp (addr.Timestamp) with the creation time of the process. If the process is newer, then the ID has been reassigned. This race condition does not affect the WINDIVERT_EVENT_REFLECT_OPEN event. In this special case, the addr.Reflect.processId is guaranteed to be valid until the corresponding WINDIVERT_EVENT_REFLECT_CLOSE event is received by the user application or is dropped (filter mismatch or timeout).

11. License

WinDivert is dual-licensed under your choice of either the GNU Lesser General Public License (LGPL) Version 3 or the GNU General Public License (GPL) Version 2. Please see the notices below:

LGPL version 3 :

WinDivert is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.

GPL version 2 :

WinDivert is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License along
with this program; if not, write to the Free Software Foundation, Inc.,
51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.