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Kernel backend plugins are responsible for installing IPsec Security Associations (SAs) and Security Policies (SPs) into the operating system, and for managing routes for tunneled traffic. Exactly one kernel interface plugin must be loaded at runtime.

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Available kernel plugins

PluginPlatformInterfaceNotes
kernel-netlinkLinuxXFRM via NetlinkDefault for Linux; supports XFRM interfaces
kernel-pfkeyLinux, BSDPF_KEY socketLegacy Linux; native on BSD kernels
kernel-pfrouteFreeBSD, macOSPF_ROUTE socketHandles routing on BSD; used alongside kernel-pfkey
kernel-libipsecAnyUser-space libipsecSoftware IPsec via TUN device; no XFRM required
kernel-iphWindowsIP Helper APISA/SP management on Windows
kernel-wfpWindowsWindows Filtering PlatformPolicy enforcement on Windows
kernel-netlink is the standard plugin for Linux. It communicates with the kernel’s XFRM framework via Netlink sockets to install xfrm_state (SAs) and xfrm_policy (SPs).

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XFRM interfaces

XFRM interfaces (available since Linux 4.19) provide a virtual network interface for IPsec traffic, avoiding the need for route-based workarounds. Configure them in swanctl.conf using if_id_in and if_id_out:
swanctl.conf
connections {
  example {
    if_id_in  = 10
    if_id_out = 10
    children {
      net {
        # traffic selectors ...
      }
    }
  }
}
To also install routes for SAs that reference XFRM interfaces, set install_routes_xfrmi = yes in the plugin configuration:
/etc/strongswan.d/charon/kernel-netlink.conf
plugins {
  kernel-netlink {
    load = yes
    install_routes_xfrmi = yes
  }
}

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Firewall marks

Use fwmark to direct traffic to a specific routing table while exempting IKE traffic:
/etc/strongswan.d/charon/kernel-netlink.conf
plugins {
  kernel-netlink {
    load = yes
    fwmark = !0x42   # Apply routing rule to packets NOT marked 0x42
  }
}

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Key configuration options

OptionDefaultDescription
fwmark—Firewall mark on the routing rule (format: [!]mark[/mask])
install_routes_xfrminoInstall routes for SAs using XFRM interfaces
mtu0MTU on installed routes (0 = kernel default)
mss0MSS clamping on installed routes (0 = disabled)
roam_eventsyesTrigger roam events on interface/address/route changes
receive_buffer_size8388608Netlink socket receive buffer (bytes)
xfrm_acq_expires165Lifetime of XFRM acquire state (seconds)
port_bypassnoUse port-based instead of socket-based IKE bypass policies

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kernel-pfkey (BSD / legacy Linux)

kernel-pfkey uses the PF_KEY socket interface (RFC 2367) to manage SAs. On Linux it is superseded by kernel-netlink but remains useful for BSD kernels. On FreeBSD and macOS, load kernel-pfkey together with kernel-pfroute:
strongswan.conf
charon {
  load_modular = yes
}
/etc/strongswan.d/charon/kernel-pfkey.conf
plugins {
  kernel-pfkey {
    load = yes
    route_via_internal = no
  }
}
/etc/strongswan.d/charon/kernel-pfroute.conf
plugins {
  kernel-pfroute {
    load = yes
    mtu = 1400
  }
}
OptionDefaultDescription
route_via_internalnoRoute via the internal (traffic selector) interface rather than the external interface
events_buffer_size0Receive buffer size for the event socket (0 = OS default)

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kernel-pfroute (FreeBSD / macOS)

kernel-pfroute handles routing table management on BSD-family systems using the PF_ROUTE socket. It is always paired with kernel-pfkey.
OptionDefaultDescription
mtu1400MTU set on TUN devices created for virtual IPs
vip_wait1000Milliseconds to wait for virtual IP addresses to appear or disappear

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kernel-libipsec (user-space)

kernel-libipsec implements IPsec entirely in user space using strongSwan’s libipsec library. It creates a TUN device and performs ESP encapsulation/decapsulation without involving the kernel XFRM or PF_KEY interfaces. This is the appropriate choice for:
  • Android (XFRM not available to unprivileged apps)
  • Platforms without kernel IPsec support
  • Testing and development environments
User-space IPsec has lower throughput than kernel-based implementations. Do not use kernel-libipsec in performance-sensitive production deployments on Linux unless XFRM is unavailable.
/etc/strongswan.d/charon/kernel-libipsec.conf
plugins {
  kernel-libipsec {
    load = yes
    allow_peer_ts = no   # Disallow TS that equals the IKE peer (avoids routing loops)
    raw_esp = no          # Use UDP encapsulation even without NAT
  }
}
kernel-libipsec still requires a routing plugin. On Linux, load it alongside kernel-netlink (which handles routes only, not SAs).

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Choosing a kernel plugin

strongswan.conf
charon {
  # kernel-netlink handles both SAs and routing
  load = ... kernel-netlink ...
}

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