Protocol options ---------------- Options in this section affect features available in the OpenVPN wire protocol. Many of these options also define the encryption options of the data channel in the OpenVPN wire protocol. These options must be configured in a compatible way between both the local and remote side. --allow-compression mode As described in the ``--compress`` option, compression is a potentially dangerous option. This option allows controlling the behaviour of OpenVPN when compression is used and allowed. Valid syntaxes: :: allow-compression allow-compression mode The ``mode`` argument can be one of the following values: :code:`asym` OpenVPN will only *decompress downlink packets* but *not compress uplink packets*. This also allows migrating to disable compression when changing both server and client configurations to remove compression at the same time is not a feasible option. :code:`no` (default) OpenVPN will refuse any compression. If data-channel offloading is enabled, OpenVPN will additionally also refuse compression framing (stub). :code:`yes` OpenVPN will send and receive compressed packets. --auth alg Authenticate data channel packets and (if enabled) ``tls-auth`` control channel packets with HMAC using message digest algorithm ``alg``. (The default is ``SHA1`` ). HMAC is a commonly used message authentication algorithm (MAC) that uses a data string, a secure hash algorithm and a key to produce a digital signature. The OpenVPN data channel protocol uses encrypt-then-mac (i.e. first encrypt a packet then HMAC the resulting ciphertext), which prevents padding oracle attacks. If an AEAD cipher mode (e.g. GCM) is chosen then the specified ``--auth`` algorithm is ignored for the data channel and the authentication method of the AEAD cipher is used instead. Note that ``alg`` still specifies the digest used for ``tls-auth``. In static-key encryption mode, the HMAC key is included in the key file generated by ``--genkey``. In TLS mode, the HMAC key is dynamically generated and shared between peers via the TLS control channel. If OpenVPN receives a packet with a bad HMAC it will drop the packet. HMAC usually adds 16 or 20 bytes per packet. Set ``alg=none`` to disable authentication. For more information on HMAC see https://tools.ietf.org/html/rfc2104 --cipher alg This option should not be used any longer in TLS mode and still exists for two reasons: * compatibility with old configurations still carrying it around; * allow users connecting to OpenVPN peers older than 2.6.0 to have ``--cipher`` configured the same way as the remote counterpart. This can avoid MTU/frame size warnings. Before 2.4.0, this option was used to select the cipher to be configured on the data channel, however, later versions usually ignored this directive in favour of a negotiated cipher. Starting with 2.6.0, this option is always ignored in TLS mode when it comes to configuring the cipher and will only control the cipher for ``--secret`` pre-shared-key mode (note: this mode is deprecated and strictly not recommended). If you wish to specify the cipher to use on the data channel, please see ``--data-ciphers`` (for regular negotiation) and ``--data-ciphers-fallback`` (for a fallback option when the negotiation cannot take place because the other peer is old or has negotiation disabled). To see ciphers that are available with OpenVPN, use the ``--show-ciphers`` option. Set ``alg`` to :code:`none` to disable encryption. --compress algorithm **DEPRECATED** Enable a compression algorithm. Compression is generally not recommended. VPN tunnels which use compression are susceptible to the VORALCE attack vector. See also the :code:`migrate` parameter below. The ``algorithm`` parameter may be :code:`lzo`, :code:`lz4`, :code:`lz4-v2`, :code:`stub`, :code:`stub-v2`, :code:`migrate` or empty. LZO and LZ4 are different compression algorithms, with LZ4 generally offering the best performance with least CPU usage. The :code:`lz4-v2` and :code:`stub-v2` variants implement a better framing that does not add overhead when packets cannot be compressed. All other variants always add one extra framing byte compared to no compression framing. Especially :code:`stub-v2` is essentially identical to no compression and no compression framing as its header indicates IP version 5 in a tun setup and can (ab)used to complete disable compression to clients. (See the :code:`migrate` option below) If the ``algorithm`` parameter is :code:`stub`, :code:`stub-v2` or empty, compression will be turned off, but the packet framing for compression will still be enabled, allowing a different setting to be pushed later. Additionally, :code:`stub` and :code:`stub-v2` wil disable announcing ``lzo`` and ``lz4`` compression support via *IV_* variables to the server. Note: the :code:`stub` (or empty) option is NOT compatible with the older option ``--comp-lzo no``. Using :code:`migrate` as compression algorithm enables a special migration mode. It allows migration away from the ``--compress``/``--comp-lzo`` options to no compression. This option sets the server to no compression mode and the server behaves identical to a server without a compression option for all clients without a compression in their config. However, if a client is detected that indicates that compression is used (via OCC), the server will automatically add ``--push compress stub-v2`` to the client specific configuration if supported by the client and otherwise switch to ``comp-lzo no`` and add ``--push comp-lzo`` to the client specific configuration. ***Security Considerations*** Compression and encryption is a tricky combination. If an attacker knows or is able to control (parts of) the plain-text of packets that contain secrets, the attacker might be able to extract the secret if compression is enabled. See e.g. the *CRIME* and *BREACH* attacks on TLS and *VORACLE* on VPNs which also leverage to break encryption. If you are not entirely sure that the above does not apply to your traffic, you are advised to *not* enable compression. --comp-lzo mode **DEPRECATED** Enable LZO compression algorithm. Compression is generally not recommended. VPN tunnels which uses compression are suspectible to the VORALCE attack vector. Use LZO compression -- may add up to 1 byte per packet for incompressible data. ``mode`` may be :code:`yes`, :code:`no`, or :code:`adaptive` (default). In a server mode setup, it is possible to selectively turn compression on or off for individual clients. First, make sure the client-side config file enables selective compression by having at least one ``--comp-lzo`` directive, such as ``--comp-lzo no``. This will turn off compression by default, but allow a future directive push from the server to dynamically change the :code:`on`/:code:`off`/:code:`adaptive` setting. Next in a ``--client-config-dir`` file, specify the compression setting for the client, for example: :: comp-lzo yes push "comp-lzo yes" The first line sets the ``comp-lzo`` setting for the server side of the link, the second sets the client side. --comp-noadapt **DEPRECATED** When used in conjunction with ``--comp-lzo``, this option will disable OpenVPN's adaptive compression algorithm. Normally, adaptive compression is enabled with ``--comp-lzo``. Adaptive compression tries to optimize the case where you have compression enabled, but you are sending predominantly incompressible (or pre-compressed) packets over the tunnel, such as an FTP or rsync transfer of a large, compressed file. With adaptive compression, OpenVPN will periodically sample the compression process to measure its efficiency. If the data being sent over the tunnel is already compressed, the compression efficiency will be very low, triggering openvpn to disable compression for a period of time until the next re-sample test. --key-direction Alternative way of specifying the optional direction parameter for the ``--tls-auth`` and ``--secret`` options. Useful when using inline files (See section on inline files). --data-ciphers cipher-list Restrict the allowed ciphers to be negotiated to the ciphers in ``cipher-list``. ``cipher-list`` is a colon-separated list of ciphers, and defaults to :code:`AES-256-GCM:AES-128-GCM:CHACHA20-POLY1305` when Chacha20-Poly1305 is available and otherwise :code:`AES-256-GCM:AES-128-GCM`. For servers, the first cipher from ``cipher-list`` that is also supported by the client will be pushed to clients that support cipher negotiation. For more details see the chapter on `Data channel cipher negotiation`_. *Especially* if you need to support clients with OpenVPN versions older than 2.4! Starting with OpenVPN 2.6 a cipher can be prefixed with a :code:`?` to mark it as optional. This allows including ciphers in the list that may not be available on all platforms. E.g. :code:`AES-256-GCM:AES-128-GCM:?CHACHA20-POLY1305` would only enable Chacha20-Poly1305 if the underlying SSL library (and its configuration) supports it. Cipher negotiation is enabled in client-server mode only. I.e. if ``--mode`` is set to `server` (server-side, implied by setting ``--server`` ), or if ``--pull`` is specified (client-side, implied by setting ``--client``). If no common cipher is found during cipher negotiation, the connection is terminated. To support old clients/old servers that do not provide any cipher negotiation support see ``--data-ciphers-fallback``. If ``--compat-mode`` is set to a version older than 2.5.0 the cipher specified by ``--cipher`` will be appended to ``--data-ciphers`` if not already present. This list is restricted to be 127 chars long after conversion to OpenVPN ciphers. This option was called ``--ncp-ciphers`` in OpenVPN 2.4 but has been renamed to ``--data-ciphers`` in OpenVPN 2.5 to more accurately reflect its meaning. --data-ciphers-fallback alg Configure a cipher that is used to fall back to if we could not determine which cipher the peer is willing to use. This option should only be needed to connect to peers that are running OpenVPN 2.3 or older versions, and have been configured with ``--enable-small`` (typically used on routers or other embedded devices). --secret args **DEPRECATED** Enable Static Key encryption mode (non-TLS). Use pre-shared secret ``file`` which was generated with ``--genkey``. Valid syntaxes: :: secret file secret file direction The optional ``direction`` parameter enables the use of 4 distinct keys (HMAC-send, cipher-encrypt, HMAC-receive, cipher-decrypt), so that each data flow direction has a different set of HMAC and cipher keys. This has a number of desirable security properties including eliminating certain kinds of DoS and message replay attacks. When the ``direction`` parameter is omitted, 2 keys are used bidirectionally, one for HMAC and the other for encryption/decryption. The ``direction`` parameter should always be complementary on either side of the connection, i.e. one side should use :code:`0` and the other should use :code:`1`, or both sides should omit it altogether. The ``direction`` parameter requires that ``file`` contains a 2048 bit key. While pre-1.5 versions of OpenVPN generate 1024 bit key files, any version of OpenVPN which supports the ``direction`` parameter, will also support 2048 bit key file generation using the ``--genkey`` option. Static key encryption mode has certain advantages, the primary being ease of configuration. There are no certificates or certificate authorities or complicated negotiation handshakes and protocols. The only requirement is that you have a pre-existing secure channel with your peer (such as ``ssh``) to initially copy the key. This requirement, along with the fact that your key never changes unless you manually generate a new one, makes it somewhat less secure than TLS mode (see below). If an attacker manages to steal your key, everything that was ever encrypted with it is compromised. Contrast that to the perfect forward secrecy features of TLS mode (using Diffie Hellman key exchange), where even if an attacker was able to steal your private key, he would gain no information to help him decrypt past sessions. Another advantageous aspect of Static Key encryption mode is that it is a handshake-free protocol without any distinguishing signature or feature (such as a header or protocol handshake sequence) that would mark the ciphertext packets as being generated by OpenVPN. Anyone eavesdropping on the wire would see nothing but random-looking data. --tran-window n Transition window -- our old key can live this many seconds after a new a key renegotiation begins (default :code:`3600` seconds). This feature allows for a graceful transition from old to new key, and removes the key renegotiation sequence from the critical path of tunnel data forwarding. --force-tls-key-material-export This option is only available in --mode server and forces to use Keying Material Exporters (RFC 5705) for clients. This can be used to simulate an environment where the cryptographic library does not support the older method to generate data channel keys anymore. This option is intended to be a test option and might be removed in a future OpenVPN version without notice.