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+Data channel cipher negotiation
+===============================
+
+OpenVPN 2.4 and higher have the capability to negotiate the data cipher that
+is used to encrypt data packets. This section describes the mechanism in more detail and the
+different backwards compatibility mechanism with older server and clients.
+
+OpenVPN 2.5 and higher behaviour
+--------------------------------
+When both client and server are at least running OpenVPN 2.5, that the order of
+the ciphers of the server's ``--data-ciphers`` is used to pick the the data cipher.
+That means that the first cipher in that list that is also in the client's
+``--data-ciphers`` list is chosen. If no common cipher is found the client is rejected
+with a AUTH_FAILED message (as seen in client log):
+
+    AUTH: Received control message: AUTH_FAILED,Data channel cipher negotiation failed (no shared cipher)
+
+OpenVPN 2.5 will only allow the ciphers specified in ``--data-ciphers``. To ensure
+backwards compatibility also if a cipher is specified using the ``--cipher`` option
+it is automatically added to this list. If both options are unset the default is
+:code:`AES-256-GCM:AES-128-GCM`.
+
+OpenVPN 2.4 clients
+-------------------
+The negotiation support in OpenVPN 2.4 was the first iteration of the implementation
+and still had some quirks. Its main goal was "upgrade to AES-256-GCM when possible".
+An OpenVPN 2.4 client that is built against a crypto library that supports AES in GCM
+mode and does not have ``--ncp-disable`` will always announce support for
+`AES-256-GCM` and `AES-128-GCM` to a server by sending :code:`IV_NCP=2`.
+
+This only causes a problem if ``--ncp-ciphers`` option has been changed from the
+default of :code:`AES-256-GCM:AES-128-GCM` to a value that does not include
+these two ciphers. When a OpenVPN servers try to use `AES-256-GCM` or
+`AES-128-GCM` the connection will then fail. It is therefore recommended to
+always have the `AES-256-GCM` and `AES-128-GCM` ciphers to the ``--ncp-ciphers``
+options to avoid this behaviour.
+
+OpenVPN 3 clients
+-----------------
+Clients based on the OpenVPN 3.x library (https://github.com/openvpn/openvpn3/)
+do not have a configurable ``--ncp-ciphers`` or ``--data-cipher`` option. Instead
+these clients will announce support for all their supported AEAD ciphers
+(`AES-256-GCM`, `AES-128-GCM` and in newer versions also `Chacha20-Poly1305`).
+
+To support OpenVPN 3.x based clients at least one of these ciphers needs to be
+included in the server's ``--data-ciphers`` option.
+
+
+OpenVPN 2.3 and older clients (and clients with ``--ncp-disable``)
+------------------------------------------------------------------
+When a client without cipher negotiation support connects to a server the
+cipher specified with the ``--cipher`` option in the client configuration
+must be included in the ``--data-ciphers`` option of the server to allow
+the client to connect. Otherwise the client will be sent the ``AUTH_FAILED``
+message that indicates no shared cipher.
+
+If the client is 2.3 or older and has been configured with the
+``--enable-small``  :code:`./configure` argument, using
+``data-ciphers-fallback cipher`` in the server config file with the explicit
+cipher used by the client is necessary.
+
+OpenVPN 2.4 server
+------------------
+When a client indicates support for `AES-128-GCM` and `AES-256-GCM`
+(with ``IV_NCP=2``) an OpenVPN 2.4 server will send the first
+cipher of the ``--ncp-ciphers`` to the OpenVPN client regardless of what
+the cipher is. To emulate the behaviour of an OpenVPN 2.4 client as close
+as possible and have compatibility to a setup that depends on this quirk,
+adding  `AES-128-GCM` and `AES-256-GCM` to the client's ``--data-ciphers``
+option is required. OpenVPN 2.5+ will only announce the ``IV_NCP=2`` flag if
+those ciphers are present.
+
+OpenVPN 2.3 and older servers (and servers with ``--ncp-disable``)
+------------------------------------------------------------------
+The cipher used by the server must be included in ``--data-ciphers`` to
+allow the client connecting to a server without cipher negotiation
+support.
+(For compatibility OpenVPN 2.5 will also accept the cipher set with
+``--cipher``)
+
+If the server is 2.3 or older and  has been configured with the
+``--enable-small`` :code:`./configure` argument, adding
+``data-ciphers-fallback cipher`` to the client config with the explicit
+cipher used by the server is necessary.
+
+Blowfish in CBC mode (BF-CBC) deprecation
+------------------------------------------
+The ``--cipher`` option defaulted to ``BF-CBC`` in OpenVPN 2.4 and older
+version. The default was never changed to ensure backwards compatibility.
+In OpenVPN 2.5 this behaviour has now been changed so that if the ``--cipher``
+is not explicitly set it does not allow the weak ``BF-CBC`` cipher any more
+and needs to explicitly added as ``--cipher BFC-CBC`` or added to
+``-data-ciphers``.
+
+We strongly recommend to switching away from BF-CBC to a
+more secure cipher as soon as possible instead.