[Openvpn-devel,v2,2/2] Document different behaviour of dynamic cipher negotiation

Message ID 20200810090032.4220-1-arne@rfc2549.org
State Accepted
Headers show
Series
  • Untitled series #865
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Commit Message

Arne Schwabe Aug. 10, 2020, 9 a.m.
This adds a section in the man page that details the various behaviour
of older client/servers when using OpenVPN 2.5.

Patch V2: Include grammar/spelling fixes from
          Richard Bonhomme <tincanteksup@gmail.com>

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
---
 Changes.rst                             | 23 +++++++
 doc/man-sections/cipher-negotiation.rst | 92 +++++++++++++++++++++++++
 doc/openvpn.8.rst                       |  1 +
 3 files changed, 116 insertions(+)
 create mode 100644 doc/man-sections/cipher-negotiation.rst

Comments

Gert Doering Aug. 10, 2020, 12:49 p.m. | #1
Acked-by: Gert Doering <gert@greenie.muc.de>

Thanks, this is necessary (Changes.rst) and helpful (cipher-negotiation.rst).

I still think we might need a matrix in the wiki (because users of older 
OpenVPN versions might not find the new man page).  But this can be done
independently.

Fixed an extra B, reworded Changes.rst slightly.

Your patch has been applied to the master branch.

commit bf42466dd00c56b8b61f5cd0c7f451882e5bc861
Author: Arne Schwabe
Date:   Mon Aug 10 11:00:32 2020 +0200

     Document different behaviour of dynamic cipher negotiation

     Signed-off-by: Arne Schwabe <arne@rfc2549.org>
     Acked-by: Gert Doering <gert@greenie.muc.de>
     Message-Id: <20200810090032.4220-1-arne@rfc2549.org>
     URL: doc/man-sections/cipher-negotiation.rst
     Signed-off-by: Gert Doering <gert@greenie.muc.de>


--
kind regards,

Gert Doering

Patch

diff --git a/Changes.rst b/Changes.rst
index 37792342..8c6cafb6 100644
--- a/Changes.rst
+++ b/Changes.rst
@@ -29,6 +29,29 @@  Improved Data channel cipher negotiation
     ``data-ciphers ChaCha20-Poly1305:AES-256-GCM`` on the server that
     prefers ChaCha20-Poly1305 but uses it only if the client supports it.
 
+    See the data channel negotiation section in the manual for more details.
+
+Removal of BF-CBC support in default configuration:
+    By default OpenVPN 2.5 will only accept AES-256-GCM and AES-128-GCM as
+    data ciphers. OpenVPN 2.4 allows AES-256-GCM,AES-128-GCM and BF-CBC when
+    no --cipher and --ncp-cipher options are present. Accepting BF-CBC can be
+    enabled by adding
+
+        data-ciphers AES-256-GCM:AES-128-GCM:BF-CBC
+
+    and when you need to support very old peers also
+
+        data-ciphers-fallback BF-CBC
+
+    to offer backwards compatibility with older config an *explicit*
+
+        cipher BF-CBC
+
+    in the configuration will be automatically translated in adding BF-CBC
+    to the data-ciphers option and setting data-ciphers-fallback to BF-CBC
+    commands above. We strongly recommend to switching away from BF-CBC to a
+    more secure cipher.
+
 Asynchronous (deferred) authentication support for auth-pam plugin.
     See src/plugins/auth-pam/README.auth-pam for details.
 
diff --git a/doc/man-sections/cipher-negotiation.rst b/doc/man-sections/cipher-negotiation.rst
new file mode 100644
index 00000000..1bd57258
--- /dev/null
+++ b/doc/man-sections/cipher-negotiation.rst
@@ -0,0 +1,92 @@ 
+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 a first 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` even if the ``--ncp-ciphers`` option does not include
+those two ciphers. It is therefore recommended to add `AES-256-GCM` and `AES-128-GCM`
+to the ``--ncp-ciphers`` options to workaround this bug.
+
+
+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 clients and older (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 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 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 (BFB-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.
\ No newline at end of file
diff --git a/doc/openvpn.8.rst b/doc/openvpn.8.rst
index 4a7ca3aa..db81274f 100644
--- a/doc/openvpn.8.rst
+++ b/doc/openvpn.8.rst
@@ -75,6 +75,7 @@  placed in a configuration file.
 .. include:: man-sections/client-options.rst
 .. include:: man-sections/server-options.rst
 .. include:: man-sections/encryption-options.rst
+.. include:: man-sections/cipher-negotiation.rst
 .. include:: man-sections/network-config.rst
 .. include:: man-sections/script-options.rst
 .. include:: man-sections/management-options.rst