opensense-docs/source/manual/how-tos/ipsec-s2s-conn-binat.rst

========================================
IPsec - NAT before IPsec
========================================


.. contents:: Index

--------------------------------
Network topology
--------------------------------

The schema below describes the situation we are implementing.
Two networks (A,B) to peer both firewalls, where the Ipsec policy includes :code:`10.1.0.0/24 <-> 192.168.1.0/24`,
but locally side A uses :code:`10.99.0.0/24`.

How to setup the tunnel itself is explained in the :doc:`ipsec-s2s-conn` document.

.. nwdiag::
  :scale: 100%

    nwdiag {

      span_width = 90;
      node_width = 180;
      network A {
          address = "[real] 10.99.0.0/24\n[policy] 10.1.0.0/24";
          pclana [label="PC Site A\n10.99.0.20",shape="cisco.pc"];
          fwa [shape = "cisco.firewall", address="10.99.0.1/24"];
      }
      network Ext {
          address = "10.10.1.0/24";
          label = "Ext";
          fwa [shape = "cisco.firewall", address="10.10.1.1/24"];
          fwb [shape = "cisco.firewall", address="10.10.1.2/24"];
      }
      network B {
          address = "192.168.1.0/24"
          fwb [shape = "cisco.firewall", address="192.168.1.20"];
          pclanb [label="PC Site B\n192.168.1.20",shape="cisco.pc"];
      }


    }


--------------------------------
Preparations
--------------------------------

Make sure the tunnel is up and running before trying out the NAT part, then edit the child entry and input a :code:`Reqid`
there which isn't used in any of the other tunnels. For this example we choose :code:`100` here.

.. Note::

    It's imperative to choose a static number here in order to be able to bind policies to the current tunnel.

--------------------------------
Add manual security policies
--------------------------------

In order for IPsec to trust the local network (:code:`10.99.0.0/24`) a manual policy needs to be added, go to
the "Manual" tab in :menuselection:`VPN->IPsec->Security Policy Database`. Next add a new entry containing the following items:

===========================================

======================= ===================
Property                site A
======================= ===================
Reqid                   **100**
Source network          **10.0.99.0/24**
======================= ===================

.. Tip::

    When the "Destination network" is left empty, the other end (in this case 192.168.1.0/24) will be received from the tunnel.
    In case multiple networks exist in the same child policy it's better to define which one this entry belongs too.

.. Tip::

    After changing manual security policies, make sure the tunnel is reconnected (restart or disconnect and connect)
    as the registration is being arranged using an `updown event <https://docs.strongswan.org/docs/5.9/plugins/updown.html>`__



--------------------------------
Configure NAT
--------------------------------

To map the networks, we will use a one to one rule created from the :menuselection:`Firewall->NAT->One-to-One` menu option.
The following settings apply here:

=================================================================================

======================= =================== =====================================
Property                site A              Notes
======================= =================== =====================================
Interface               **IPsec**
Type                    **BINAT**           Two way mapping
External network        **10.1.0.0**        As defined in the child connection
Source                  **10.99.0.0/24**    The local network
Destination             **192.168.1.0/24**  The remote network
======================= =================== =====================================


.. Note::

    When using BINAT all networks need to be equally sized (:code:`/24` in this case)