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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).
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 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 [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 (
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:

  • --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 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 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)


  • 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 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.)
  • 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.


  • 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


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


  • 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.