Tls error unsupported protocol openvpn

Overview of changes in 2.6

Project changes

We want to deprecate our old Trac bug tracking system.
Please report any issues with this release in GitHub
instead: https://github.com/OpenVPN/openvpn/issues

New features

Support unlimited number of connection entries and remote entries

New management commands to enumerate and list remote entries

Use remote-entry-count and remote-entry-get
commands from the management interface to get the number of
remote entries and the entries themselves.

Keying Material Exporters (RFC 5705) based key generation

As part of the cipher negotiation OpenVPN will automatically prefer
the RFC5705 based key material generation to the current custom
OpenVPN PRF. This feature requires OpenSSL or mbed TLS 2.18+.

Compatibility with OpenSSL in FIPS mode

OpenVPN will now work with OpenSSL in FIPS mode. Note, no effort
has been made to check or implement all the
requirements/recommendation of FIPS 140-2. This just allows OpenVPN
to be run on a system that be configured OpenSSL in FIPS mode.

mlock will now check if enough memlock-able memory has been reserved,

and if less than 100MB RAM are available, use setrlimit() to upgrade
the limit. See Trac #1390. Not available on OpenSolaris.

Certificate pinning/verify peer fingerprint

The --peer-fingerprint option has been introduced to give users an
easy to use alternative to the tls-verify for matching the
fingerprint of the peer. The option takes use a number of allowed
SHA256 certificate fingerprints.

See the man page section «Small OpenVPN setup with peer-fingerprint»
for a tutorial on how to use this feature. This is also available online
under https://github.com/openvpn/openvpn/blob/master/doc/man-sections/example-fingerprint.rst

TLS mode with self-signed certificates

When --peer-fingerprint is used, the --ca and --capath option
become optional. This allows for small OpenVPN setups without setting up
a PKI with Easy-RSA or similar software.

Deferred auth support for scripts

The --auth-user-pass-verify script supports now deferred authentication.

Pending auth support for plugins and scripts

Both auth plugin and script can now signal pending authentication to
the client when using deferred authentication. The new client-crresponse
script option and OPENVPN_PLUGIN_CLIENT_CRRESPONSE plugin function can
be used to parse a client response to a CR_TEXT two factor challenge.

See sample/sample-scripts/totpauth.py for an example.

Compatibility mode (--compat-mode)

The modernisation of defaults can impact the compatibility of OpenVPN 2.6.0
with older peers. The options --compat-mode allows UIs to provide users
with an easy way to still connect to older servers.

OpenSSL 3.0 support

OpenSSL 3.0 has been added. Most of OpenSSL 3.0 changes are not user visible but
improve general compatibility with OpenSSL 3.0. --tls-cert-profile insecure
has been added to allow selecting the lowest OpenSSL security level (not
recommended, use only if you must). OpenSSL 3.0 no longer supports the Blowfish
(and other deprecated) algorithm by default and the new option --providers
allows loading the legacy provider to renable these algorithms.

Optional ciphers in --data-ciphers

Ciphers in --data-ciphers can now be prefixed with a ? to mark
those as optional and only use them if the SSL library supports them.

Improved --mssfix and --fragment calculation

The --mssfix and --fragment options now allow an optional mtu
parameter to specify that different overhead for IPv4/IPv6 should taken into
account and the resulting size is specified as the total size of the VPN packets
including IP and UDP headers.

Cookie based handshake for UDP server

Instead of allocating a connection for each client on the initial packet
OpenVPN server will now use an HMAC based cookie as its session id. This
way the server can verify it on completing the handshake without keeping
state. This eliminates the amplification and resource exhaustion attacks.
For tls-crypt-v2 clients, this requires OpenVPN 2.6 clients or later
because the client needs to resend its client key on completing the hand
shake. The tls-crypt-v2 option allows controlling if older clients are
accepted.

By default the rate of initial packet responses is limited to 100 per 10s
interval to avoid OpenVPN servers being abused in reflection attacks
(see --connect-freq-initial).

Data channel offloading with ovpn-dco

2.6.0+ implements support for data-channel offloading where the data packets
are directly processed and forwarded in kernel space thanks to the ovpn-dco
kernel module. The userspace openvpn program acts purely as a control plane
application. Note that DCO will use DATA_V2 packets in P2P mode, therefore,
this implies that peers must be running 2.6.0+ in order to have P2P-NCP
which brings DATA_V2 packet support.

Session timeout

It is now possible to terminate a session (or all) after a specified amount
of seconds has passed session commencement. This behaviour can be configured
using --session-timeout. This option can be configured on the server, on
the client or can also be pushed.

Inline auth username and password

Username and password can now be specified inline in the configuration file
within the <auth-user-pass></auth-user-pass> tags. If the password is
missing OpenVPN will prompt for input via stdin. This applies to inline’d
http-proxy-user-pass too.

Tun MTU can be pushed

The client can now also dynamically configure its MTU and the server
will try to push the client MTU when the client supports it. The
directive --tun-mtu-max has been introduced to increase the maximum
pushable MTU size (defaults to 1600).

Improved control channel packet size control (max-packet-size)

The size of control channel is no longer tied to
--link-mtu/--tun-mtu and can be set using --max-packet-size.
Sending large control channel frames is also optimised by allowing 6
outstanding packets instead of just 4. max-packet-size will also set
mssfix to try to limit data-channel packets as well.

Deprecated features

inetd has been removed

This was a very limited and not-well-tested way to run OpenVPN, on TCP
and TAP mode only.

verify-hash has been deprecated

This option has very limited usefulness and should be replaced by either
a better --ca configuration or with a --tls-verify script.

secret has been deprecated

static key mode (non-TLS) is no longer considered «good and secure enough»
for today’s requirements. Use TLS mode instead. If deploying a PKI CA
is considered «too complicated», using --peer-fingerprint makes
TLS mode about as easy as using --secret.

ncp-disable has been removed

This option mainly served a role as debug option when NCP was first
introduced. It should now no longer be necessary.

TLS 1.0 and 1.1 are deprecated

tls-version-min is set to 1.2 by default. OpenVPN 2.6.0 defaults
to a minimum TLS version of 1.2 as TLS 1.0 and 1.1 should be generally
avoided. Note that OpenVPN versions older than 2.3.7 use TLS 1.0 only.

--cipher argument is no longer appended to --data-ciphers

by default. Data cipher negotiation has been introduced in 2.4.0
and been significantly improved in 2.5.0. The implicit fallback
to the cipher specified in --cipher has been removed.
Effectively, --cipher is a no-op in TLS mode now, and will
only have an effect in pre-shared-key mode (--secret).
From now on --cipher should not be used in new configurations
for TLS mode.
Should backwards compatibility with older OpenVPN peers be
required, please see the --compat-mode instead.

--prng has beeen removed

OpenVPN used to implement its own PRNG based on a hash. However implementing
a PRNG is better left to a crypto library. So we use the PRNG
mbed TLS or OpenSSL now.

--keysize has been removed

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 no longer supports any of these variable length
ciphers, this option was removed as well to avoid confusion.

Compression no longer enabled by default

Unless an explicit compression option is specified in the configuration,
--allow-compression defaults to no in OpeNVPN 2.6.0.
By default, OpenVPN 2.5 still allowed a server to enable compression by
pushing compression related options.

PF (Packet Filtering) support has been removed

The built-in PF functionality has been removed from the code base. This
feature wasn’t really easy to use and was long unmaintained.
This implies that also --management-client-pf and any other compile
time or run time related option do not exist any longer.

Option conflict checking is being deprecated and phased out

The static option checking (OCC) is no longer useful in typical setups
that negotiate most connection parameters. The --opt-verify and
--occ-disable options are deprecated, and the configure option
--enable-strict-options has been removed. Logging of mismatched
options has been moved to debug logging (verb 7).

User-visible Changes

• CHACHA20-POLY1305 is included in the default of --data-ciphers when available.
• Option --prng is ignored as we rely on the SSL library random number generator.
• Option --nobind is default when --client or --pull is used in the configuration
• link_mtu parameter is removed from environment or replaced with 0 when scripts are
  called with parameters. This parameter is unreliable and no longer internally calculated.
• control channel packet maximum size is no longer influenced by
  --link-mtu/--tun-mtu and must be set by --max-packet-size now.
  The default is 1250 for the control channel size.
• In point-to-point OpenVPN setups (no --server), using
  --explict-exit-notiy on one end would terminate the other side at
  session end. This is considered a no longer useful default and has
  been changed to «restart on reception of explicit-exit-notify message».
  If the old behaviour is still desired, --remap-usr1 SIGTERM can be used.
• FreeBSD tun interfaces with --topology subnet are now put into real
  subnet mode (IFF_BROADCAST instead of IFF_POINTOPOINT) — this might upset
  software that enumerates interfaces, looking for «broadcast capable?» and
  expecting certain results. Normal uses should not see any difference.
• The default configurations will no longer allow connections to OpenVPN 2.3.x
  peer or earlier, use the new --compat-mode option if you need
  compatibility with older versions. See the manual page on the
  --compat-mode for details.

Common errors with OpenSSL 3.0 and OpenVPN 2.6

Both OpenVPN 2.6 and OpenSSL 3.0 tighten the security considerable, so some
configuration will no longer work. This section will cover the most common
causes and error message we have seen and explain their reason and temporary
workarounds. You should fix the underlying problems as soon as possible since
these workaround are not secure and will eventually stop working in a future
update.

• weak SHA1 or MD5 signature on certificates

  This will happen on either loading of certificates or on connection
  to a server:

  OpenSSL: error:0A00018E:SSL routines::ca md too weak
  Cannot load certificate file cert.crt
  Exiting due to fatal error
  

  OpenSSL 3.0 no longer allows weak signatures on certificates. You can
  downgrade your security to allow them by using --tls-cert-profile insecure
  but should replace/regenerate these certificates as soon as possible.

• 1024 bit RSA certificates, 1024 bit DH parameters, other weak keys

  This happens if you use private keys or other cryptographic material that
  does not meet today’s cryptographic standards anymore. Messages are similar
  to:

  OpenSSL: error:0A00018F:SSL routines::ee key too small
  OpenSSL: error:1408518A:SSL routines:ssl3_ctx_ctrl:dh key too small
  

  DH parameters (--dh) can be regenerated with openssl dhparam 2048.
  For other cryptographic keys, these keys and certificates need to be
  regenerated. TLS Security level can be temporarily lowered with
  --tls-cert-profile legacy or even --tls-cert-profile insecure.

• Connecting to a OpenVPN 2.3.x server or allowing OpenVPN 2.3.x or earlier
  clients

  This will normally result in messages like:

  OPTIONS ERROR: failed to negotiate cipher with server.  Add the server's cipher ('AES-128-CBC') to --data-ciphers (currently 'AES-256-GCM:AES-128-GCM:CHACHA20-POLY1305') if you want to connect to this server.
  
  or
  
  client/127.0.0.1:49954 SENT CONTROL [client]: 'AUTH_FAILED,Data channel cipher negotiation failed (no shared cipher)' (status=1)
  

  You can manually add the missing cipher to the --data-ciphers. The
  standard ciphers should be included as well, e.g.
  --data-ciphers AES-256-GCM:AES-128-GCM:?Chacha20-Poly1305:?AES-128-CBC.
  You can also use the --compat-mode option. Note that these message may
  also indicate other cipher configuration problems. See the data channel
  cipher negotiation manual section for more details. (Available online under
  https://github.com/OpenVPN/openvpn/blob/master/doc/man-sections/cipher-negotiation.rst)

• Use of a legacy or deprecated cipher (e.g. 64bit block ciphers)

  OpenSSL 3.0 no longer supports a number of insecure and outdated ciphers in
  its default configuration. Some of these ciphers are known to be vulnerable (SWEET32 attack).

  This will typically manifest itself in messages like:

  OpenSSL: error:0308010C:digital envelope routines::unsupported
  Cipher algorithm 'BF-CBC' not found
  Unsupported cipher in --data-ciphers: BF-CBC
  

  If your OpenSSL distribution comes with the legacy provider (see
  also man OSSL_PROVIDER-legacy), you can load it with
  --providers legacy default. This will re-enable the old algorithms.

• OpenVPN version not supporting TLS 1.2 or later

  The default in OpenVPN 2.6 and also in many distributions is now TLS 1.2 or
  later. Connecting to a peer that does not support this will results in
  messages like:

  TLS error: Unsupported protocol. This typically indicates that client and
  server have no common TLS version enabled. This can be caused by mismatched
  tls-version-min and tls-version-max options on client and server. If your
  OpenVPN client is between v2.3.6 and v2.3.2 try adding tls-version-min 1.0
  to the client configuration to use TLS 1.0+ instead of TLS 1.0 only
  OpenSSL: error:0A000102:SSL routines::unsupported protocol
  

  This can be an OpenVPN 2.3.6 or earlier version. compat-version 2.3.0 will
  enable TLS 1.0 support if supported by the OpenSSL distribution. Note that
  on some Linux distributions enabling TLS 1.1 or 1.0 is not possible.

Overview of changes in 2.5

New features

Client-specific tls-crypt keys (--tls-crypt-v2)

tls-crypt-v2 adds the ability to supply each client with a unique
tls-crypt key. This allows large organisations and VPN providers to profit
from the same DoS and TLS stack protection that small deployments can
already achieve using tls-auth or tls-crypt.

ChaCha20-Poly1305 cipher support

Added support for using the ChaCha20-Poly1305 cipher in the OpenVPN data
channel.

Improved Data channel cipher negotiation

The option ncp-ciphers has been renamed to data-ciphers.
The old name is still accepted. The change in name signals that
data-ciphers is the preferred way to configure data channel
ciphers and the data prefix is chosen to avoid the ambiguity that
exists with --cipher for the data cipher and tls-cipher
for the TLS ciphers.

OpenVPN clients will now signal all supported ciphers from the
data-ciphers option to the server via IV_CIPHERS. OpenVPN
servers will select the first common cipher from the data-ciphers
list instead of blindly pushing the first cipher of the list. This
allows to use a configuration like
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-ciphers 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 configs an explicit

cipher BF-CBC

in the configuration will be automatically translated into adding BF-CBC
to the data-ciphers option and setting data-ciphers-fallback to BF-CBC
(as in the example 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.

Deferred client-connect

The --client-connect option and the connect plugin API allow
asynchronous/deferred return of the configuration file in the same way
as the auth-plugin.

Faster connection setup

A client will signal in the IV_PROTO variable that it is in pull
mode. This allows the server to push the configuration options to
the client without waiting for a PULL_REQUEST message. The feature
is automatically enabled if both client and server support it and
significantly reduces the connection setup time by avoiding one
extra packet round-trip and 1s of internal event delays.

Netlink support

On Linux, if configured without --enable-iproute2, configuring IP
addresses and adding/removing routes is now done via the netlink(3)
kernel interface. This is much faster than calling ifconfig or
route and also enables OpenVPN to run with less privileges.

If configured with —enable-iproute2, the ip command is used
(as in 2.4). Support for ifconfig and route is gone.

Wintun support

On Windows, OpenVPN can now use wintun devices. They are faster
than the traditional tap9 tun/tap devices, but do not provide
--dev tap mode — so the official installers contain both. To use
a wintun device, add --windows-driver wintun to your config
(and use of the interactive service is required as wintun needs
SYSTEM privileges to enable access).

IPv6-only operation

It is now possible to have only IPv6 addresses inside the VPN tunnel,
and IPv6-only address pools (2.4 always required IPv4 config/pools
and IPv6 was the «optional extra»).

Improved Windows 10 detection

Correctly log OS on Windows 10 now.

Linux VRF support

Using the new --bind-dev option, the OpenVPN outside socket can
now be put into a Linux VRF. See the «Virtual Routing and Forwarding»
documentation in the man page.

TLS 1.3 support

TLS 1.3 support has been added to OpenVPN. Currently, this requires
OpenSSL 1.1.1+.
The options --tls-ciphersuites and --tls-groups have been
added to fine tune TLS protocol options. Most of the improvements
were also backported to OpenVPN 2.4 as part of the maintainance
releases.

Support setting DHCP search domain

A new option --dhcp-option DOMAIN-SEARCH my.example.com has been
defined, and Windows support for it is implemented (tun/tap only, no
wintun support yet). Other platforms need to support this via --up
script (Linux) or GUI (OSX/Tunnelblick).

per-client changing of --data-ciphers or data-ciphers-fallback

from client-connect script/dir (NOTE: this only changes preference of
ciphers for NCP, but can not override what the client announces as
«willing to accept»)

Handle setting of tun/tap interface MTU on Windows

If IPv6 is in use, MTU must be >= 1280 (Windows enforces IETF requirements)

Add support for OpenSSL engines to access private key material (like TPM).

HMAC based auth-token support

The --auth-gen-token support has been improved and now generates HMAC
based user token. If the optional --auth-gen-token-secret option is
used clients will be able to seamlessly reconnect to a different server
using the same secret file or to the same server after a server restart.

Improved support for pending authentication

The protocol has been enhanced to be able to signal that
the authentication should use a secondary authentication
via web (like SAML) or a two factor authentication without
disconnecting the OpenVPN session with AUTH_FAILED. The
session will instead be stay in a authenticated state and
wait for the second factor authentication to complete.

This feature currently requires usage of the managent interface
on both client and server side. See the management-notes.txt
client-pending-auth and cr-response commands for more
details.

VLAN support

OpenVPN servers in TAP mode can now use 802.1q tagged VLANs
on the TAP interface to separate clients into different groups
that can then be handled differently (different subnets / DHCP,
firewall zones, …) further down the network. See the new
options --vlan-tagging, --vlan-accept, --vlan-pvid.

802.1q tagging on the client side TAP interface is not handled
today (= tags are just forwarded transparently to the server).

Support building of .msi installers for Windows

Allow unicode search string in --cryptoapicert option (Windows)

Support IPv4 configs with /31 netmasks now

(By no longer trying to configure «broadcast x.x.x.x» in
ifconfig calls, /31 support «just works»)

New option --block-ipv6 to reject all IPv6 packets (ICMPv6)

this is useful if the VPN service has no IPv6, but the clients
might have (LAN), to avoid client connections to IPv6-enabled
servers leaking «around» the IPv4-only VPN.

--ifconfig-ipv6 and --ifconfig-ipv6-push will now accept

hostnames and do a DNS lookup to get the IPv6 address to use

Deprecated features

For an up-to-date list of all deprecated options, see this wiki page:
https://community.openvpn.net/openvpn/wiki/DeprecatedOptions

• ncp-disable has been deprecated

  With the improved and matured data channel cipher negotiation, the use
  of ncp-disable should not be necessary anymore.

• inetd has been deprecated
  This is a very limited and not-well-tested way to run OpenVPN, on TCP
  and TAP mode only, which complicates the code quite a bit for little gain.
  To be removed in OpenVPN 2.6 (unless users protest).
• no-iv has been removed
  This option was made into a NOOP option with OpenVPN 2.4. This has now
  been completely removed.
• --client-cert-not-required has been removed
  This option will now cause server configurations to not start. Use
  --verify-client-cert none instead.
• --ifconfig-pool-linear has been removed
  This option is removed. Use --topology p2p or --topology subnet
  instead.
• --compress xxx is considered risky and is warned against, see below.
• --key-method 1 has been removed

User-visible Changes

• If multiple connect handlers are used (client-connect, ccd, connect
  plugin) and one of the handler succeeds but a subsequent fails, the
  client-disconnect-script is now called immediately. Previously it
  was called, when the VPN session was terminated.
• Support for building with OpenSSL 1.0.1 has been removed. The minimum
  supported OpenSSL version is now 1.0.2.
• The GET_CONFIG management state is omitted if the server pushes
  the client configuration almost immediately as result of the
  faster connection setup feature.
• --compress is nowadays considered risky, because attacks exist
  leveraging compression-inside-crypto to reveal plaintext (VORACLE). So
  by default, --compress xxx will now accept incoming compressed
  packets (for compatibility with peers that have not been upgraded yet),
  but will not use compression outgoing packets. This can be controlled with
  the new option --allow-compression yes|no|asym.
• Stop changing --txlen aways from OS defaults unless explicitly specified
  in config file. OS defaults nowadays are actually larger then what we used
  to configure, so our defaults sometimes caused packet drops = bad performance.
• remove --writepid pid file on exit now
• plugin-auth-pam now logs via OpenVPN logging method, no longer to stderr
  (this means you’ll have log messages in syslog or openvpn log file now)
• use ISO 8601 time format for file based logging now (YYYY-MM-DD hh:mm:dd)
  (syslog is not affected, nor is --machine-readable-output)
• --clr-verify now loads all CRLs if more than one CRL is in the same
  file (OpenSSL backend only, mbedTLS always did that)
• when --auth-user-pass file has no password, and the management interface
  is active, query management interface (instead of trying console query,
  which does not work on windows)
• skip expired certificates in Windows certificate store (--cryptoapicert)
• --socks-proxy + --proto udp* will now allways use IPv4, even if
  IPv6 is requested and available. Our SOCKS code does not handle IPv6+UDP,
  and before that change it would just fail in non-obvious ways.
• TCP listen() backlog queue is now set to 32 — this helps TCP servers that
  receive lots of «invalid» connects by TCP port scanners
• do no longer print OCC warnings («option mismatch») about key-method,
  keydir, tls-auth and cipher — these are either gone now, or
  negotiated, and the warnings do not serve a useful purpose.
• dhcp-option DNS and dhcp-option DNS6 are now treated identically
  (= both accept an IPv4 or IPv6 address for the nameserver)

Maintainer-visible changes

• the man page is now in maintained in .rst format, so building the openvpn.8
  manpage from a git checkout now requires python-docutils (if this is missing,
  the manpage will not be built — which is not considered an error generally,
  but for package builders or make distcheck it is). Release tarballs
  contain the openvpn.8 file, so unless some .rst is changed, doc-utils are
  not needed for building.
• OCC support can no longer be disabled
• AEAD support is now required in the crypto library
• --disable-server has been removed from configure (so it is no longer
  possible to build a client-/p2p-only OpenVPN binary) — the saving in code
  size no longer outweighs the extra maintenance effort.
• --enable-iproute2 will disable netlink(3) support, so maybe remove
  that from package building configs (see above)
• support building with MSVC 2019
• cmocka based unit tests are now only run if cmocka is installed externally
  (2.4 used to ship a local git submodule which was painful to maintain)
• --disable-crypto configure option has been removed. OpenVPN is now always
  built with crypto support, which makes the code much easier to maintain.
  This does not affect --cipher none to do a tunnel without encryption.
• --disable-multi configure option has been removed

Overview of changes in 2.4

New features

Seamless client IP/port floating

Added new packet format P_DATA_V2, which includes peer-id. If both the
server and client support it, the client sends all data packets in
the new format. When a data packet arrives, the server identifies peer
by peer-id. If peer’s ip/port has changed, server assumes that
client has floated, verifies HMAC and updates ip/port in internal structs.
This allows the connection to be immediately restored, instead of requiring
a TLS handshake before the server accepts packets from the new client
ip/port.

Data channel cipher negotiation

Data channel ciphers (--cipher) are now by default negotiated. If a
client advertises support for Negotiable Crypto Parameters (NCP), the
server will choose a cipher (by default AES-256-GCM) for the data channel,
and tell the client to use that cipher. Data channel cipher negotiation
can be controlled using --ncp-ciphers and --ncp-disable.

A more limited version also works in client-to-server and server-to-client
scenarios where one of the end points uses a v2.4 client or server and the
other side uses an older version. In such scenarios the v2.4 side will
change to the --cipher set by the remote side, if permitted by by
--ncp-ciphers. For example, a v2.4 client with --cipher BF-CBC
and ncp-ciphers AES-256-GCM:AES-256-CBC can connect to both a v2.3
server with cipher BF-CBC as well as a server with
cipher AES-256-CBC in its config. The other way around, a v2.3 client
with either cipher BF-CBC or cipher AES-256-CBC can connect to a
v2.4 server with e.g. cipher BF-CBC and
ncp-ciphers AES-256-GCM:AES-256-CBC in its config. For this to work
it requires that OpenVPN was built without disabling OCC support.

AEAD (GCM) data channel cipher support

The data channel now supports AEAD ciphers (currently only GCM). The AEAD
packet format has a smaller crypto overhead than the CBC packet format,
(e.g. 20 bytes per packet for AES-128-GCM instead of 36 bytes per packet
for AES-128-CBC + HMAC-SHA1).

ECDH key exchange

The TLS control channel now supports for elliptic curve diffie-hellmann
key exchange (ECDH).

Improved Certificate Revocation List (CRL) processing

CRLs are now handled by the crypto library (OpenSSL or mbed TLS), instead
of inside OpenVPN itself. The crypto library implementations are more
strict than the OpenVPN implementation was. This might reject peer
certificates that would previously be accepted. If this occurs, OpenVPN
will log the crypto library’s error description.

Dualstack round-robin DNS client connect

Instead of only using the first address of each --remote OpenVPN
will now try all addresses (IPv6 and IPv4) of a --remote entry.

Support for providing IPv6 DNS servers

A new DHCP sub-option DNS6 is added alongside with the already existing
DNS sub-option. This is used to provide DNS resolvers available over
IPv6. This may be pushed to clients where « —up« scripts and --plugin
can act upon it through the foreign_option_<n> environment variables.

Support for the Windows client picking up this new sub-option is added,
however IPv6 DNS resolvers need to be configured via netsh which requires
administrator privileges unless the new interactive services on Windows is
being used. If the interactive service is used, this service will execute
netsh in the background with the proper privileges.

New improved Windows Background service

The new OpenVPNService is based on openvpnserv2, a complete rewrite of the OpenVPN
service wrapper. It is intended for launching OpenVPN instances that should be
up at all times, instead of being manually launched by a user. OpenVPNService is
able to restart individual OpenVPN processes if they crash, and it also works
properly on recent Windows versions. OpenVPNServiceLegacy tends to work poorly,
if at all, on newer Windows versions (8+) and its use is not recommended.

New interactive Windows service

The installer starts OpenVPNServiceInteractive automatically and configures
it to start at system startup.

The interactive Windows service allows unprivileged users to start
OpenVPN connections in the global config directory (usually
C:Program FilesOpenVPNconfig) using OpenVPN GUI without any
extra configuration.

Users who belong to the built-in Administrator group or to the
local «OpenVPN Administrator» group can also store configuration
files under %USERPROFILE%OpenVPNconfig for use with the
interactive service.

redirect-gateway ipv6

OpenVPN has now feature parity between IPv4 and IPv6 for redirect
gateway including the handling of overlapping IPv6 routes with
IPv6 remote VPN server address.

LZ4 Compression and pushable compression

Additionally to LZO compression OpenVPN now also supports LZ4 compression.
Compression options are now pushable from the server.

Filter pulled options client-side: pull-filter

New option to explicitly allow or reject options pushed by the server.
May be used multiple times and is applied in the order specified.

Per-client remove push options: push-remove

New option to remove options on a per-client basis from the «push» list
(more fine-grained than --push-reset).

Http proxy password inside config file

Http proxy passwords can be specified with the inline file option
<http-proxy-user-pass> .. </http-proxy-user-pass>

Windows version detection

Windows version is detected, logged and possibly signalled to server
(IV_PLAT_VER=<nn> if --push-peer-info is set on client).

Authentication tokens

In situations where it is not suitable to save user passwords on the client,
OpenVPN has support for pushing a —auth-token since v2.3. This option is
pushed from the server to the client with a token value to be used instead
of the users password. For this to work, the authentication plug-in would
need to implement this support as well. In OpenVPN 2.4 —auth-gen-token
is introduced, which will allow the OpenVPN server to generate a random
token and push it to the client without any changes to the authentication
modules. When the clients need to re-authenticate the OpenVPN server will
do the authentication internally, instead of sending the re-authentication
request to the authentication module . This feature is especially
useful in configurations which use One Time Password (OTP) authentication
schemes, as this allows the tunnel keys to be renegotiated regularly without
any need to supply new OTP codes.

keying-material-exporter

Keying Material Exporter [RFC-5705] allow additional keying material to be
derived from existing TLS channel.

Android platform support

Support for running on Android using Android’s VPNService API has been added.
See doc/android.txt for more details. This support is primarily used in
the OpenVPN for Android app (https://github.com/schwabe/ics-openvpn)

AIX platform support

AIX platform support has been added. The support only includes tap
devices since AIX does not provide tun interface.

Control channel encryption (--tls-crypt)

Use a pre-shared static key (like the --tls-auth key) to encrypt control
channel packets. Provides more privacy, some obfuscation and poor-man’s
post-quantum security.

Asynchronous push reply

Plug-ins providing support for deferred authentication can benefit from a more
responsive authentication where the server sends PUSH_REPLY immediately once
the authentication result is ready, instead of waiting for the the client to
to send PUSH_REQUEST once more. This requires OpenVPN to be built with
./configure --enable-async-push. This is a compile-time only switch.

Deprecated features

For an up-to-date list of all deprecated options, see this wiki page:
https://community.openvpn.net/openvpn/wiki/DeprecatedOptions

• --key-method 1 is deprecated in OpenVPN 2.4 and will be removed in v2.5.
  Migrate away from --key-method 1 as soon as possible. The recommended
  approach is to remove the --key-method option from the configuration
  files, OpenVPN will then use --key-method 2 by default. Note that this
  requires changing the option in both the client and server side configs.
• --tls-remote is removed in OpenVPN 2.4, as indicated in the v2.3
  man-pages. Similar functionality is provided via --verify-x509-name,
  which does the same job in a better way.
• --compat-names and --no-name-remapping were deprecated in OpenVPN 2.3
  and will be removed in v2.5. All scripts and plug-ins depending on the old
  non-standard X.509 subject formatting must be updated to the standardized
  formatting. See the man page for more information.
• --no-iv is deprecated in OpenVPN 2.4 and will be removed in v2.5.
• --keysize is deprecated in OpenVPN 2.4 and will be removed in v2.6
  together with the support of ciphers with cipher block size less than
  128-bits.
• --comp-lzo is deprecated in OpenVPN 2.4. Use --compress instead.
• --ifconfig-pool-linear has been deprecated since OpenVPN 2.1 and will be
  removed in v2.5. Use --topology p2p instead.
• --client-cert-not-required is deprecated in OpenVPN 2.4 and will be removed
  in v2.5. Use --verify-client-cert none for a functional equivalent.
• --ns-cert-type is deprecated in OpenVPN 2.3.18 and v2.4. It will be removed
  in v2.5. Use the far better --remote-cert-tls option which replaces this
  feature.

User-visible Changes

• When using ciphers with cipher blocks less than 128-bits,
  OpenVPN will complain loudly if the configuration uses ciphers considered
  weak, such as the SWEET32 attack vector. In such scenarios, OpenVPN will by
  default renegotiate for each 64MB of transported data (--reneg-bytes).
  This renegotiation can be disabled, but is HIGHLY DISCOURAGED.

• For certificate DNs with duplicate fields, e.g. «OU=one,OU=two», both fields
  are now exported to the environment, where each second and later occurrence
  of a field get _$N appended to it’s field name, starting at N=1. For the
  example above, that would result in e.g. X509_0_OU=one, X509_0_OU_1=two.
  Note that this breaks setups that rely on the fact that OpenVPN would
  previously (incorrectly) only export the last occurrence of a field.

• proto udp and proto tcp now use both IPv4 and IPv6. The new
  options proto udp4 and proto tcp4 use IPv4 only.

• --sndbuf and --recvbuf default now to OS defaults instead of 64k

• OpenVPN exits with an error if an option has extra parameters;
  previously they were silently ignored

• --tls-auth always requires OpenVPN static key files and will no
  longer work with free form files

• --proto udp6/tcp6 in server mode will now try to always listen to
  both IPv4 and IPv6 on platforms that allow it. Use --bind ipv6only
  to explicitly listen only on IPv6.

• Removed --enable-password-save from configure. This option is now
  always enabled.

• Stricter default TLS cipher list (override with --tls-cipher), that now
  also disables:

  • Non-ephemeral key exchange using static (EC)DH keys
  • DSS private keys

• mbed TLS builds: changed the tls_digest_N values exported to the script
  environment to be equal to the ones exported by OpenSSL builds, namely
  the certificate fingerprint (was the hash of the ‘to be signed’ data).

• mbed TLS builds: minimum RSA key size is now 2048 bits. Shorter keys will
  not be accepted, both local and from the peer.

• --connect-timeout now specifies the timeout until the first TLS packet
  is received (identical to --server-poll-timeout) and this timeout now
  includes the removed socks proxy timeout and http proxy timeout.

  In --static mode connect-timeout specifies the timeout for TCP and
  proxy connection establishment

• --connect-retry-max now specifies the maximum number of unsuccessful
  attempts of each remote/connection entry before exiting.

• --http-proxy-timeout and the static non-changeable socks timeout (5s)
  have been folded into a «unified» --connect-timeout which covers all
  steps needed to connect to the server, up to the start of the TLS exchange.
  The default value has been raised to 120s, to handle slow http/socks
  proxies graciously. The old «fail TCP fast» behaviour can be achieved by
  adding «--connect-timeout 10» to the client config.

• --http-proxy-retry and --sock-proxy-retry have been removed. Proxy connections
  will now behave like regular connection entries and generate a USR1 on failure.

• --connect-retry gets an optional second argument that specifies the maximum
  time in seconds to wait between reconnection attempts when an exponential
  backoff is triggered due to repeated retries. Default = 300 seconds.

• Data channel cipher negotiation (see New features section) can override
  ciphers configured in the config file. Use --ncp-disable if you do not want
  this behavior.

• All tun devices on all platforms are always considered to be IPv6
  capable. The --tun-ipv6 option is ignored (behaves like it is always
  on).

• On the client side recursively routed packets, which have the same destination
  as the VPN server, are dropped. This can be disabled with
  —allow-recursive-routing option.

• On Windows, when the --register-dns option is set, OpenVPN no longer
  restarts the dnscache service — this had unwanted side effects, and
  seems to be no longer necessary with currently supported Windows versions.

• If no flags are given, and the interactive Windows service is used, «def1»
  is implicitly set (because «delete and later reinstall the existing
  default route» does not work well here). If not using the service,
  the old behaviour is kept.

• OpenVPN now reloads a CRL only if the modication time or file size has
  changed, instead of for each new connection. This reduces the connection
  setup time, in particular when using large CRLs.

• OpenVPN now ships with more up-to-date systemd unit files which take advantage
  of the improved service management as well as some hardening steps. The
  configuration files are picked up from the /etc/openvpn/server/ and
  /etc/openvpn/client/ directories (depending on unit file). This also avoids
  these new unit files and how they work to collide with older pre-existing
  unit files.

• Using --no-iv (which is generally not a recommended setup) will
  require explicitly disabling NCP with --disable-ncp. This is
  intentional because NCP will by default use AES-GCM, which requires
  an IV — so we want users of that option to consciously reconsider.

Maintainer-visible changes

• OpenVPN no longer supports building with crypto support, but without TLS
  support. As a consequence, OPENSSL_CRYPTO_{CFLAGS,LIBS} and
  OPENSSL_SSL_{CFLAGS,LIBS} have been merged into OPENSSL_{CFLAGS,LIBS}. This
  is particularly relevant for maintainers who build their own OpenSSL library,
  e.g. when cross-compiling.
• Linux distributions using systemd is highly encouraged to ship these new unit
  files instead of older ones, to provide a unified behaviour across systemd
  based Linux distributions.
• With OpenVPN 2.4, the project has moved over to depend on and actively use
  the official C99 standard (-std=c99). This may fail on some older compiler/libc
  header combinations. In most of these situations it is recommended to
  use -std=gnu99 in CFLAGS. This is known to be needed when doing
  i386/i686 builds on RHEL5.

Version 2.4.5

New features

• The new option --tls-cert-profile can be used to restrict the set of
  allowed crypto algorithms in TLS certificates in mbed TLS builds. The
  default profile is ‘legacy’ for now, which allows SHA1+, RSA-1024+ and any
  elliptic curve certificates. The default will be changed to the ‘preferred’
  profile in the future, which requires SHA2+, RSA-2048+ and any curve.

Version 2.4.3

New features

• Support building with OpenSSL 1.1 now (in addition to older versions)
• On Win10, set low interface metric for TAP adapter when block-outside-dns
  is in use, to make Windows prefer the TAP adapter for DNS queries
  (avoiding large delays)

Security

• CVE-2017-7522: Fix --x509-track post-authentication remote DoS
  A client could crash a v2.4+ mbedtls server, if that server uses the
  --x509-track option and the client has a correct, signed and unrevoked
  certificate that contains an embedded NUL in the certificate subject.
  Discovered and reported to the OpenVPN security team by Guido Vranken.
• CVE-2017-7521: Fix post-authentication remote-triggerable memory leaks
  A client could cause a server to leak a few bytes each time it connects to the
  server. That can eventually cause the server to run out of memory, and thereby
  causing the server process to terminate. Discovered and reported to the
  OpenVPN security team by Guido Vranken. (OpenSSL builds only.)
• CVE-2017-7521: Fix a potential post-authentication remote code execution
  attack on servers that use the --x509-username-field option with an X.509
  extension field (option argument prefixed with ext:). A client that can
  cause a server to run out-of-memory (see above) might be able to cause the
  server to double free, which in turn might lead to remote code execution.
  Discovered and reported to the OpenVPN security team by Guido Vranken.
  (OpenSSL builds only.)
• CVE-2017-7520: Pre-authentication remote crash/information disclosure for
  clients. If clients use a HTTP proxy with NTLM authentication (i.e.
  --http-proxy <server> <port> [<authfile>|'auto'|'auto-nct'] ntlm2),
  a man-in-the-middle attacker between the client and the proxy can cause
  the client to crash or disclose at most 96 bytes of stack memory. The
  disclosed stack memory is likely to contain the proxy password. If the
  proxy password is not reused, this is unlikely to compromise the security
  of the OpenVPN tunnel itself. Clients who do not use the --http-proxy
  option with ntlm2 authentication are not affected.
• CVE-2017-7508: Fix remotely-triggerable ASSERT() on malformed IPv6 packet.
  This can be used to remotely shutdown an openvpn server or client, if
  IPv6 and --mssfix are enabled and the IPv6 networks used inside the VPN
  are known.
• Fix null-pointer dereference when talking to a malicious http proxy
  that returns a malformed Proxy-Authenticate: headers for digest auth.
• Fix overflow check for long --tls-cipher option
• Windows: Pass correct buffer size to GetModuleFileNameW()
  (OSTIF/Quarkslabs audit, finding 5.6)

User-visible Changes

• --verify-hash can now take an optional flag which changes the hashing
  algorithm. It can be either SHA1 or SHA256. The default if not provided is
  SHA1 to preserve backwards compatibility with existing configurations.
• Restrict the supported --x509-username-field extension fields to subjectAltName
  and issuerAltName. Other extensions probably didn’t work anyway, and would
  cause OpenVPN to crash when a client connects.

Bugfixes

• Fix fingerprint calculation in mbed TLS builds. This means that mbed TLS users
  of OpenVPN 2.4.0, v2.4.1 and v2.4.2 that rely on the values of the
  tls_digest_* env vars, or that use --verify-hash will have to change
  the fingerprint values they check against. The security impact of the
  incorrect calculation is very minimal; the last few bytes (max 4, typically
  4) are not verified by the fingerprint. We expect no real-world impact,
  because users that used this feature before will notice that it has suddenly
  stopped working, and users that didn’t will notice that connection setup
  fails if they specify correct fingerprints.
• Fix edge case with NCP when the server sends an empty PUSH_REPLY message
  back, and the client would not initialize it’s data channel crypto layer
  properly (trac #903)
• Fix SIGSEGV on unaligned buffer access on OpenBSD/Sparc64
• Fix TCP_NODELAY on OpenBSD
• Remove erroneous limitation on max number of args for --plugin
• Fix NCP behaviour on TLS reconnect (Server would not send a proper
  «cipher …» message back to the client, leading to client and server
  using different ciphers) (trac #887)

Version 2.4.2

Bugfixes

• Fix memory leak introduced in OpenVPN 2.4.1: if --remote-cert-tls is
  used, we leaked some memory on each TLS (re)negotiation.

Security

• Fix a pre-authentication denial-of-service attack on both clients and
  servers. By sending a too-large control packet, OpenVPN 2.4.0 or v2.4.1 can
  be forced to hit an ASSERT() and stop the process. If --tls-auth or
  --tls-crypt is used, only attackers that have the --tls-auth or
  --tls-crypt key can mount an attack.
  (OSTIF/Quarkslab audit finding 5.1, CVE-2017-7478)
• Fix an authenticated remote DoS vulnerability that could be triggered by
  causing a packet id roll over. An attack is rather inefficient; a peer
  would need to get us to send at least about 196 GB of data.
  (OSTIF/Quarkslab audit finding 5.2, CVE-2017-7479)

Version 2.4.1

• --remote-cert-ku now only requires the certificate to have at least the
  bits set of one of the values in the supplied list, instead of requiring an
  exact match to one of the values in the list.
• --remote-cert-tls now only requires that a keyUsage is present in the
  certificate, and leaves the verification of the value up to the crypto
  library, which has more information (i.e. the key exchange method in use)
  to verify that the keyUsage is correct.
• --ns-cert-type is deprecated. Use --remote-cert-tls instead.
  The nsCertType x509 extension is very old, and barely used.
  --remote-cert-tls uses the far more common keyUsage and extendedKeyUsage
  extension instead. Make sure your certificates carry these to be able to
  use --remote-cert-tls.