kernel-wfp Plugin


The kernel-wfp plugin is an interface to the IPsec backend in the Windows Filter Platform on Windows. It installs and maintains Security Associations and Policies to the Windows Kernel.

The plugin is disabled by default and can be enabled for Windows builds by adding

to the ./configure options.

The plugin has been introduced in strongSwan 5.2.0.

Supported ciphers

This kernel backend supports both AH, ESP and ESP-in-UDP for both tunnel and transport mode Security Associations.

The following integrity algorithms are supported for both AH and ESP:
  • md5
  • sha1
  • sha256
  • aes128gmac
  • aes192gmac
  • aes256gmac
the following encryption algorithms for ESP:
  • null
  • des
  • 3des
  • aes128 (using CBC)
  • aes192 (using CBC)
  • aes256 (using CBC)
and these AEAD mode ciphers for ESP:
  • aes128gcm16
  • aes192gcm16
  • aes256gcm16


The Windows Filtering platform is a firewalling framework on the Windows operating system. It supports the installation of arbitrary filters from userland applications. Windows native IPsec is managed through the WFP API by associating filters to IPsec SA information.

The kernel-wfp plugin implements an IPsec backend using the WFP API. As it seems that there is no support to register custom keying modules in Windows 7, the backend installs SA and policy information directly after negotiation. This means that strongSwan does not install Main Mode or Quick Mode policies in WFP in a way the Windows native keying modules do. Instead we maintain such information locally in our IKE daemon and just install negotiated information.

The Windows kernel handles the IPsec ruleset using so called SA contexts, managed by IPsecSaContextCreate0 and associated functions. An SA context knows exactly one inbound and one outbound IPsec SA bundle for processing traffic, associated using the IPsecSaContextAddInbound0 and IPsecSaContextAddOutbound0 functions. Each SA bundle contains algorithm and other configurations and the associated encryption and integrity protection keys.

Transport Mode

In Transport mode, an SA context is directly tied to an outbound filter rule during construction. The inbound filter rule is associated to the SA context by calling IPsecSaContextGetSpi0. Filter rules get installed using the FwpmFilterAdd0 function.

Together with the inbound and outbound filters acting as policies, the SA context with the inbound and the outbound SA bundles form the kernel-level object that got negotiated for a CHILD_SA.

Tunnel Mode

Configuring tunnel mode in WFP works very similarly to transport mode. An SA context is used to attach inbound and outbound SA bundles.

A tunnel mode, however, usually has more than just an inbound and an outbound IPsec policy. It also needs forward policies, and multiple different in, out and forward policies for different selectors are desirable.

To install the required filter set for a tunnel mode SA, the WFP API provides a FwpmIPsecTunnelAdd0 function. This function implicitly creates multiple filter rules from the passed traffic selectors, which can be associated to an SA context. The internals of this function, however, are somewhat unclear and it limits the flexibility of policy installation.

To allow the installation of multiple selectors and policies, the kernel-wfp plugin installs filter rules manually for tunnel mode. To do so, a provider context is required to associate an arbitrary number of filters to it. The provider context is then referenced by the SA context to link all filters to the SA bundles.

State and SPI management

The strongSwan IPsec kernel abstraction layer is loosely defined for the PF_KEY and similar interfaces, working on SA and policy granular level. It usually installs in- and outbound Security Associations, and then attaches IPsec policies using a unique reqid identifier.

For the WFP interface, this paradigm does not work very well. It expects the policy information first, followed by the SA context and the in- and outbound Security Associations. The WFP interface can't allocate SPIs without creating policy entries and an SA context first. In strongSwan, however, we must allocate SPIs before installing policies, as this information is negotiated in IKEv2.

To work around these interface differences, the kernel-wfp plugin caches any policy or SA addition. Once all this information has been collected, it installs it in a single batch. Instead of using kernel allocated SPIs using IPsecSaContextGetSpi0, it allocates SPIs in userland pseudo-randomly and uses IPsecSaContextSetSpi0 to set it.

All policy and SA information installed by strongSwan is non-persistent. A system reboot or a restart of the Base Filtering Engine service removes any rule installed by the plugin.

Bypass policies

When installing IPsec policies, an IKE daemon must ensure that the policies don't cover IKE packets. This makes sure that IKE packets are never encapsulated in ESP.

While WFP has built-in support for IKE bypass filters, these cover policies installed by FwpmIpsecTunnelAdd0 only. We therefore install global bypass filter rules for the IKE socket. These filter use a high priority to have precedence over any IPsec policy negotiated.

Trap policies and events

strongSwan can install IPsec policies not having Security Associations attached to it, the trap policies. If the kernel layer processes a packet matching such a policy, it raises an event to strongSwan. strongSwan then negotiates an SA for this policy, protecting traffic on demand.

In WFP, strongSwan uses drop firewall rules for trap policies and registers for firewalling events using FwpmNetEventSubscribe0 to register for drop events. If the Windows kernel raises an event for a drop rule, it informs strongSwan, which in turn looks for the corresponding trap policy and starts SA negotiation.

The firewall event subscription is also used to catch other IPsec related processing errors, which get logged by strongSwan.

ipsecdump utility

For development and debugging purposes, the plugin comes with an ipsecdump.exe utility. By default it dumps Security Associations currently installed on the system. When invoked with a filters argument, it dumps any IPsec related firewall filters.

Known issues and limitations

The following limitations apply to the kernel-wfp plugin as of 5.2.0:

Accessing Gateway internal address in a net-to-net tunnel

When a Windows Gateway offers IPsec protected access to an internal subnet in tunnel mode, an IPsec client is currently unable to access any internal IP address of the gateway itself, even if it is part of the covered subnet. The kernel raises a STATUS_IPSEC_CLEAR_TEXT_DROP event if such a packet is received. The Microsoft Knowledge Base entry KB2502685 exactly describes this issue and provides a hotfix, but is related to a Forefront TMG server. It is currently not clear if the same issue applies to the Windows 7 product family or if it can be worked around from userland.

Protocol/Port restrictions on forwarding net-to-net connections

The Windows Kernel does not parse transport layer IP packet headers when forwarding packets. Therefore it does not support protocol or port selectors in IPsec forwarding policies, but only in policies for a local address. In practice strongSwan currently just ignores protocol/port restrictions in forwarding policies, but enforces them for inbound/outbound policies. Forwarding policies therefore may cover more traffic than negotiated in IKE.

Unsolicited inbound traffic in UDP-Encapsulated ESP packets

When receiving traffic on an UDP-encapsulated ESP tunnel mode connection, any unsolicited inbound traffic gets dropped by the kernel with a STATUS_INTERNAL_ERROR event. This applies to UDP-encapsulated connections only, and does not affect traffic that the kernel can associate to a known connection. For example, the return path works fine if the Windows host initiates a TCP connection over such a tunnel.

It is currently unknown if this is a kernel related issue or if it can be worked around by configuring UDP encapsulation in a slightly different fashion.

Multiple traffic selectors for Transport Mode connections

Due the design of SA context information in the Windows kernel, in transport mode only a single inbound and outbound filter can be applied. This means that a single CHILD_SA can have a single selector only. In practice, this is usually not much of a problem, as transport mode SAs can cover a single host pair only. Defining multiple protocols or address ranges is currently not possible, though.

SA usage, lifetime and volumes

The Windows kernel does not seem to provide any means of getting per-SA statistics. IPsecGetSaStatistics0 works on a global level only, and the IPsecSaEnum0 and IPsecSaContextEnum0 return configured lifetime values only.

For strongSwan this implies that no usage counters are available, and that DPD checking and keep-alive messages can't be reduced based on IPsec SA activity.

It is currently not possible to configure volume based rekeying. Time based rekeying is managed by strongSwan in userland, but hard SA lifetimes ensure that the kernel rejects packets even if userland does not renew connections as expected.