duplicheck Plugin


The duplicheck plugin for libcharon provides an advanced but very specialized peer identity duplicate checking that works independently from the unique connection feature.

The plugin is disabled by default and can be enabled with the ./configure option



The behavior of the duplicheck plugin is as follows:

  • While establishing a new IKE SA check if already one exists with the same peer identity

  • If yes:

    • Initiate an IKE_SA delete exchange on the old IKE SA to liveness check and simultaneously delete it

    • If no response is received after several retransmits to the delete, destroy the old IKE SA

    • If a response is received:

      • Also delete the newly established IKE SA

      • Send a notification over a UNIX socket to listening applications (if any)


The duplicheck plugin is configured using the following options in the charon.plugins.duplicheck section of strongswan.conf:

Key Default Description [Default]



Enable duplicheck functionality



Socket provided by the duplicheck plugin. [`unix://${piddir}/charon.dck]`


If two IKE SAs exist with the same peer identity and the old IKE SA confirmed the triggered delete message, a notification is sent to a listening application over a UNIX or TCP socket. An example application of a listener is provided with the duplicheck tool. It listens on the socket and receives the affected peer identity. To integrate notification listening to your application, see

You’ll have to start a dedicated thread to read from the socket or integrated the file descriptor to your applications main loop.