wiki:DeprecatedOptions

Deprecated Options in OpenVPN

OpenVPN is a software VPN product which has been around since May 2001. And it has mostly been backwards compatible on the most important features through all these years. But the world moves forward, security issues are discovered, and expectations of how a secure VPN should be configured have changed over the years.

As OpenVPN carries a lot of options (over 230), we need to do a clean-up here from time to time. The main goal is to be as backwards compatible in regards to the configuration files as possible. We do not recommend running any older OpenVPN releases than the latest v2.3 series. Whenever possible you should always upgrade to the latest available OpenVPN release. But some times we unfortunately need to remove old options as they impose a security risk to VPN configurations.

In this wiki page, we will try to keep an up-to-date list of all options we have deprecated, when they will be removed, the new alternative approach and the reasoning behind removing the option. This wiki page summarizes the "Deprecated features" section in the Changes.rst file which is distributed with the source code.

--key-method

Status Pending removal
Deprecated in: OpenVPN v2.4
To be removed in: OpenVPN v2.5
Affects: Client and server
Result if used: OpenVPN will ignore the option and provide a warning
Replaced by: Not replaced
Examples: (N/A)

OpenVPN have used --key-method 2 since OpenVPN v2.0 if it was not provided. Using the older --key-method 1 was primarily present to allow OpenVPN clients running older releases than v2.0 to connect to a v2.0 server. This older key-method is not recommended as the key negotiation method is not as strong as the current default.

--tls-remote

Status Removed in OpenVPN v2.4.0
Deprecated in: OpenVPN v2.3
To be removed in: OpenVPN v2.4
Affects: Client and server
Result if used: OpenVPN will not start due to unknown option
Replaced by: --verify-x509-name
Examples: --verify-x509-name 'C=KG, ST=NA, L=Bishkek, CN=Server-1'
--verify-x509-name Server-1 name
--verify-x509-name Server name-prefix

--compat-names

Status Pending removal
Deprecated in: OpenVPN v2.3
To be removed in: OpenVPN v2.5
Affects: Client and server
Result if used: OpenVPN will ignore the option and provide a warning
Replaced by: Not replaced
Examples: (N/A)

OpenVPN used the former OpenSSL formatting of X.509 Subject fields. They could look like this:

/C=US/L=Somewhere/CN=John Doe/emailAddress=john@example.com

As of OPenVPN v2.3, this format was changed to the more widely used X.509 formatting:

C=US, L=Somewhere, CN=John Doe, emailAddress=john@example.com

This option would in addition add remapping of characters and rendering most characters outside the typical a-z/A-Z/0-9 range to be replaced by an underscore (_) - unless the no-remapping flag was added. This behaviour would in many cases be required by older authentication plug-ins or scripts which was not able to process the newer format. As this behaviour is now considered bad, it is expected that authentication plug-ins and scripts will have had enough time to get an update to handle the new X.509 Subject formatting.

--no-name-remapping

Status Pending removal
Deprecated in: OpenVPN v2.3
To be removed in: OpenVPN v2.5
Affects: Client and server
Result if used: OpenVPN will not start due to unknown option
Replaced by: Not replaced
Examples: (N/A)

This is essentially just an alias for --compat-names no-remapping. This option would avoid the character remapping of characters being outside the typical a-z/A-Z/0-9 range in the X.509 Subject identifiers.

--no-iv

Status Removed in master branch
Deprecated in: OpenVPN v2.4
To be removed in: OpenVPN v2.5
Affects: Client and server
Result if used: OpenVPN will refuse the option and provide an error message that the option is no longer supported
Replaced by: Not replaced
Examples: (N/A)

This option will disable OpenVPN's use of the cipher initialization vector (IV). This is considered very harmful on today's ciphers and will severely reduce the security of VPN tunnels. As the use cases for --no-iv are few and mostly obscure, it was decided to remove this option to ensure the tunnels security can not be deliberately reduced.

--no-replay

Status Pending removal
Deprecated in: OpenVPN v2.4
To be removed in: OpenVPN v2.5
Affects: Client and server
Result if used: OpenVPN will refuse the option and provide an error message that the option is no longer supported
Replaced by: Not replaced
Examples: (N/A)

This option will disable OpenVPN's use of replay protection. This slightly reduces the overhead (8 bytes per packet for static keyed setups, 4 bytes for TLS with CBC mode, 0 bytes for TLS with GCM/OFB/CFB mode). The overhead reduction in CBC mode can better be achieved by switching to GCM mode. The remaining benefit for static key mode does not warrant the keeping this option around. The added code complexity adds attack surface and increases the chance of users reducing their security more than they realise.

Removal of insecure ciphers: Ciphers with cipher block-size less than 128 bits (most commonly BF, DES, CAST5, IDEA and RC2)

Status Pending removal
Deprecated in: OpenVPN v2.4
To be removed in: OpenVPN v2.6
Affects: Client and server
Result if used: OpenVPN will not start due to incorrect cipher being used
Replaced by: Replaced by stronger ciphers, most commonly AES-256-GCM
Examples: (N/A)

After the discovery of the SWEET32 Birthday attacks on 64-bit block ciphers any cipher using a cipher block length smaller than 128 bits is considered insecure and prune to be successfully attacked. The cipher block length is not an indication of the cipher key length.

Migrating away from deprecated ciphers

With the OpenVPN v2.4 release a new feature was introduced, Negotiable Crypto Parameters (NCP). This allows users to seamlessly migrate away from deprecated ciphers without much extra work. If both client and server runs OpenVPN v2.4 without NCP being disabled (--ncp-disable), the tunnel will automatically be upgraded to AES-256-GCM. If the environment also uses clients older than OpenVPN v2.4, the server can deploy:

--ncp-ciphers AES-256-GCM:AES-256-CBC:BF-CBC

This will allow older clients to add or change --cipher to use AES-256-CBC instead of the default BF-CBC or any other cipher enlisted. This can be done on client configuration files on a one-by-one approach. Unmodified clients will be able to connect as before. Once all clients have been updated to OpenVPN v2.4 or later (preferred) or have their configuration altered, the --ncp-ciphers list can be modified to remove BF-CBC.

WARNING: This migration approach will not work after the release of OpenVPN v2.6. As of that release, BF-CBC, CAST or RC2 ciphers will not be accepted any more.

NOTE: For Fedora 27, if the openvpn-server@.service unit file is used for server configurations, this migration path have already been enabled.

--keysize

Status Pending removal
Deprecated in: OpenVPN v2.4
To be removed in: OpenVPN v2.6
Affects: Client and server
Result if used: OpenVPN will ignore the option and provide a warning
Replaced by: Not replaced
Examples: (N/A)

The --keysize option was only useful to change the key length when using the BF, CAST6 or RC2 ciphers. For all other ciphers the key-size is fixed with the chosen cipher. As OpenVPN v2.6 will no longer support any of these variable length ciphers, this option will be removed as well to avoid confusion.

--comp-lzo

Status Planned for removal
Deprecated in: OpenVPN v2.4
To be removed in: (not decided)
Affects: Client and server
Result if used: OpenVPN will ignore the option and provide a warning
Replaced by: --compress
Examples: --compress
--compress lzo
--compress lz4

The --comp-lzo option would only enable the LZO compression algorithm. The --compress option allows also to use the improves LZ4 algorithm instead. Providing just --compress without an algorithm is the equivalent of --comp-lzo no which disables compression but enables the packet framing for compression. This will allow --compress to be pushed by the server on a per-client basis.

--ifconfig-pool-linear

Status Pending removal
Deprecated in: OpenVPN v2.1
To be removed in: OpenVPN v2.5
Affects: Client and server
Result if used: OpenVPN will not start due to unknown option
Replaced by: --topology
Examples: --topology p2p

This option will not work with Windows based clients. Since the --topology p2p mode is equivalent to --ifconfig-pool-linear and works with Windows, this option will be removed.

--client-cert-not-required

Status Pending removal
Deprecated in: OpenVPN v2.4
To be removed in: OpenVPN v2.5
Affects: Client and server
Result if used: OpenVPN will not start due to unknown option
Replaced by: --verify-client-cert
Examples: --verify-client-cert none
--verify-client-cert optional
--verify-client-cert require

The replacement option allows a far more fine grained control of authentication methods, and can allow a combination of only username/password authentication, only certificate based authentication or a combination. This would not be possible with the old --client-cert-not-required option.

--ns-cert-type

Status Pending removal
Deprecated in: OpenVPN v2.4 and v2.3.18
To be removed in: OpenVPN v2.5
Affects: Client and server
Result if used: OpenVPN will complain and remap to replacement option
Replaced by: --remote-cert-tls
Examples: --remote-cert-tls server
--remote-cert-tls client

As of OpenSSL v1.1, the nsCertType extension in X.509 certificates are no longer supported. This extension is old and have been deprecated for a long time. The replacement option, ---remote-cert-tls is a macro which sets the --remote-cert-ku and --remote-cert-eku to appropriate values, depending on it is wanted to check if the remote provided certificate is a server or client certificate. As the extended key usage extension is far more commonly used today, this is effectively the equivalent of --ns-cert-type. For the time being, if --ns-cert-type is used in OpenVPN v2.5 or later, it will currently be re-mapped to --remote-cert-tls and complain about a deprecated option being used.

--tun-ipv6

Status Removed in OpenVPN 2.4l
Deprecated in: OpenVPN v2.4
To be removed in: OpenVPN v2.4
Affects: Client and server
Result if used: OpenVPN will complain and remap to replacement option
Replaced by: --
Examples: --tun-ipv6

This option was useful when IPv6 tun support was non standard and was an internal/user specified flag that tracked the Ipv6 capability of the tun device.

Today, all supported OS support IPv6 and indicating explicit support is not needed anymore. Also tun-ipv6 is pushable by the remote so not putting tun-ipv6 does not forbid ipv6 addresses.

Last modified 5 weeks ago Last modified on 08/17/17 15:25:10