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 immediatly 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).
 

 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 Files\\OpenVPN\\config) 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%\\OpenVPN\\config 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.

 Mac OS X Keychain management client

     Added contrib/keychain-mcd which allows to use Mac OS X keychain
     certificates with OpenVPN.

 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
 -------------------
 - ``--key-method 1`` is deprecated in 2.4 and will be removed in 2.5.  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.
 

 - 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.
 

 - ``--tls-remote`` is removed in 2.4, as indicated in the 2.3 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 2.3 and will

   be removed in 2.5.  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 2.4 and will be removed in 2.5.

 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 occurence 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 v2.4, 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 2.4.3
 =============

 Security
 --------
 - CVE-2017-7522: Fix --x509-track post-authentication remote DoS
   A client could crash a 2.4+ 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 eventuall 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.)

 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.

 Bugfixes
 --------
 - Fix fingerprint calculation in mbed TLS builds.  This means that mbed TLS users
   of OpenVPN 2.4.0, 2.4.1 and 2.4.2 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.
 
 

 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``.

 
 
 Version 2.4.2
 =============
 
 Bugfixes
 --------
 - Fix memory leak introduced in 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 2.4.1 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)