openvpn3/README.rst

OpenVPN 3
=========

OpenVPN 3 is a C++ class library that implements the functionality
of an OpenVPN client, and is protocol-compatible with the OpenVPN
2.x branch.

OpenVPN 3 includes a minimal client wrapper (``cli``) that links in with
the library and provides basic command line functionality.

OpenVPN 3 is currently used in production as the core of the
OpenVPN Connect clients for iOS, Android, Linux, Windows, and Mac OS X.

NOTE: As of 2017, OpenVPN 3 is primarily of interest to developers,
as it does not yet replicate the full functionality of OpenVPN 2.x.
In particular, server functionality is not yet implemented.

.. contents:: Table of Contents

OpenVPN 3 Client API
--------------------

OpenVPN 3 is organized as a C++ class library, and the API is defined in
`<client/ovpncli.hpp>`_.

A simple command-line wrapper for the API is provided in
`<test/ovpncli/cli.cpp>`_.

Building the OpenVPN 3 client on Linux
--------------------------------------

These instructions were tested on Ubuntu 16.

Get prerequisites to allow for either mbedTLS or OpenSSL linkage::

  $ sudo apt-get install g++ make libmbedtls-dev libssl-dev liblz4-dev

Get Asio C++ library::

  $ cd ~
  $ git clone https://github.com/chriskohlhoff/asio.git

Set environmental variable used by OpenVPN 3 build scripts::

  $ export O3=~/ovpn3

Clone the OpenVPN 3 source repo::

  $ mkdir ~/ovpn3
  $ cd ~/ovpn3
  $ git clone https://github.com/OpenVPN/openvpn3.git core

Build the OpenVPN 3 client wrapper (cli) with mbedTLS crypto/ssl library
and LZ4 compression::

  $ cd $O3/core/test/ovpncli
  $ ECHO=1 PROF=linux ASIO_DIR=~/asio MTLS_SYS=1 LZ4_SYS=1 NOSSL=1 $O3/core/scripts/build cli

Or alternatively build with OpenSSL::

  $ cd $O3/core/test/ovpncli
  $ ECHO=1 PROF=linux ASIO_DIR=~/asio OPENSSL_SYS=1 LZ4_SYS=1 $O3/core/scripts/build cli

Run OpenVPN 3 client::

  $ sudo ./cli -a -c yes myprofile.ovpn route-nopull

Options used::

  -a             : use autologin sessions, if supported
  -c yes         : negotiate LZ4 compression
  myprofile.ovpn : OpenVPN config file (must have .ovpn extension)
  route-nopull   : if you are connected via ssh, prevent ssh session lockout


Building the OpenVPN 3 client on Mac OS X
-----------------------------------------

OpenVPN 3 should be built in a non-root Mac OS X account.
Make sure that Xcode is installed with optional command-line tools.
(These instructions have been tested with Xcode 5.1.1).

Create the directories ``~/src`` and ``~/src/mac``::

    $ mkdir -p ~/src/mac

Clone the OpenVPN 3 repo::

    $ cd ~/src
    $ mkdir ovpn3
    $ cd ovpn3
    $ git clone https://github.com/OpenVPN/openvpn3.git core

Export the shell variable ``O3`` to point to the OpenVPN 3 top level
directory::

    export O3=~/src/ovpn3

Download source tarballs (``.tar.gz`` or ``.tgz``) for these dependency
libraries into ``~/Downloads``

See the file ``$O3/core/deps/lib-versions`` for the expected
version numbers of each dependency.  If you want to use a different
version of the library than listed here, you can edit this file.

1. Asio — https://github.com/chriskohlhoff/asio
2. mbed TLS (2.3.0 or higher) — https://tls.mbed.org/
3. LZ4 — https://github.com/Cyan4973/lz4

For dependencies that are typically cloned from github vs.
provided as a .tar.gz file, tools are provided to convert
the github to a .tar.gz file.  See "snapshot" scripts under
``$O3/core/deps``

Note that while OpenSSL is listed in lib-versions, it is
not required for Mac builds.

Build the dependencies::

    $ DL=~/Downloads
    $ OSX_ONLY=1 $O3/core/scripts/mac/build-all

Now build the OpenVPN 3 client executable::

    $ cd $O3/core
    $ . vars/vars-osx64
    $ . vars/setpath
    $ cd test/ovpncli
    $ MTLS=1 LZ4=1 ASIO=1 build cli

This will build the OpenVPN 3 client library with a small client
wrapper (``cli``).  It will also statically link in all external
dependencies (Asio, mbedTLS, and LZ4), so ``cli`` may be distributed
to other Macs and will run as a standalone executable.

These build scripts will create a **x86_x64** Mac OS X executable,
with a minimum deployment target of 10.8.x.  The Mac OS X tuntap driver is not
required, as OpenVPN 3 can use the integrated utun interface if
available.

To view the client wrapper options::

    $ ./cli -h

To connect::

    $ ./cli client.ovpn


Building the OpenVPN 3 client on Windows
----------------------------------------

Prerequisites:

* Visual Studio 2019
* CMake
* vcpkg

Download and build dependencies::

  > git clone https://github.com/Microsoft/vcpkg.git
  > cd vcpkg
  > bootstrap-vcpkg.bat
  > vcpkg integrate install
  > vcpkg install openssl-windows:x64-windows asio:x64-windows tap-windows6:x64-windows lz4:x64-windows gtest:x64-windows

Download and build core test client::

  > git clone https://github.com/OpenVPN/openvpn3.git
  > cmake -DCMAKE_TOOLCHAIN_FILE=<path_to_vcpkg>\scripts\buildsystems\vcpkg.cmake -A x64 -B build openvpn3
  > cmake --build build --config Release --target ovpncli

Testing
-------

The OpenVPN 3 core includes a stress/performance test of
the OpenVPN protocol implementation.  The test basically
creates a virtualized lossy network between two OpenVPN
protocol objects, triggers TLS negotiations between them,
passes control/data channel messages, and measures the ability
of the OpenVPN protocol objects to perform and remain in
a valid state.

The OpenVPN protocol implementation that is being tested
is here: `<openvpn/ssl/proto.hpp>`_

The test code itself is here: `<test/ssl/proto.cpp>`_

Build the test::

  $ cd ovpn3/core/test/ssl
  $ ECHO=1 PROF=linux ASIO_DIR=~/asio MTLS_SYS=1 NOSSL=1 $O3/core/scripts/build proto

Run the test::

  $ time ./proto
  *** app bytes=72777936 net_bytes=122972447 data_bytes=415892854 prog=0000216599/0000216598 D=12700/600/12700/600 N=109/109 SH=17400/15300 HE=0/0

  real	0m15.813s
  user	0m15.800s
  sys	0m0.004s

The OpenVPN 3 core also includes unit tests, which are based on
Google Test framework. To run unit tests, you need to install
CMake and build Google Test.

Building Google Test on Linux::

  $ git clone https://github.com/google/googletest.git
  $ cd googletest
  $ cmake . && cmake --build .

Building Google Test on Windows::

  > git clone https://github.com/google/googletest.git
  > cd googletest
  > cmake -G "Visual Studio 14 2015 Win64" .
  > cmake --build .

After Google Test is built you are ready to build and run unit tests.

Build and run tests on Linux::

  $ cd ovpn3/core/test/unittests
  $ GTEST_DIR=~/googletest ECHO=1 PROF=linux ASIO_DIR=~/asio MTLS_SYS=1 LZ4_SYS=1 NOSSL=1 $O3/core/scripts/build test_log
  $ ./test_log

Build and run tests on Windows::

  $ cd ovpn3/core/win
  $ python build.py ../test/unittests/test_log.cpp unittest
  $ test_log.exe

Developer Guide
---------------

OpenVPN 3 is written in C++11 and developers who are moving
from C to C++ should take some time to familiarize themselves with
key C++ design patterns such as *RAII*:

https://en.wikipedia.org/wiki/Resource_acquisition_is_initialization

OpenVPN 3 Client Core
+++++++++++++++++++++

OpenVPN 3 is designed as a class library, with an API that
is essentially defined inside of namespace ``ClientAPI``
with headers and implementation in `<client>`_ and
header-only library files under `<openvpn>`_.

The consise definition of the client API is essentially ``class OpenVPNClient``
in `<client/ovpncli.hpp>`_ with several imporant extensions to
the API found in:

* **class TunBuilderBase** in `<openvpn/tun/builder/base.hpp>`_ —
  Provides an abstraction layer defining the *tun* interface,
  and is especially useful for interfacing with an OS-layer VPN API.

* **class ExternalPKIBase** in `<openvpn/pki/epkibase.hpp>`_ —
  Provides a callback for external private key operations, and
  is useful for interfacing with an OS-layer Keychain such as
  the Keychain on iOS, Mac OS X, and Android, and the Crypto API
  on Windows.

* **class LogReceiver** in `<client/ovpncli.hpp>`_ —
  Provides an abstraction layer for the delivery of logging messages.

OpenVPN 3 includes a command-line reference client (``cli``) for
testing the API.  See `<test/ovpncli/cli.cpp>`_.

The basic approach to building an OpenVPN 3 client is
to define a client class that derives from
``ClientAPI::OpenVPNClient``, then provide implementations
for callbacks including event and logging notifications:

.. code:: c++

  class Client : public ClientAPI::OpenVPNClient
  {
  public:
        virtual void event(const Event&) override {  // events delivered here
          ...
        }
        virtual void log(const LogInfo&) override {  // logging delivered here
          ...
        }

        ...
  };

To start the client, first create a ``ClientAPI::Config`` object
and initialize it with the OpenVPN config file and other options:

.. code:: c++

  ClientAPI::Config config;
  config.content = <config_file_content_as_multiline_string>;
  ...

Next, create a client object and evaluate the configuration:

.. code:: c++

  Client client;
  ClientAPI::EvalConfig eval = client.eval_config(config);
  if (eval.error)
    throw ...;

Finally, in a new worker thread, start the connection:

.. code:: c++

  ClientAPI::Status connect_status = client.connect();

Note that ``client.connect()`` will not return until
the session has terminated.

Top Layer
.........

The top layer of the OpenVPN 3 client is implemented
in `<test/ovpncli/cli.cpp>`_ and `<openvpn/client/cliopt.hpp>`_.
Most of what this code does is marshalling the configuration and
dispatching the higher-level objects that implement the OpenVPN
client session.

Connection
..........

``class ClientConnect`` in `<openvpn/client/cliconnect.hpp>`_
implements the top-level connection logic for an OpenVPN client
connection.  It is concerned with starting, stopping, pausing, and resuming
OpenVPN client connections.  It deals with retrying a connection and handles
the connection timeout.  It also deals with connection exceptions and understands
the difference between an exception that should halt any further reconnection
attempts (such as ``AUTH_FAILED``), and other exceptions such as network errors
that would justify a retry.

Some of the methods in the class
(such as ``stop``, ``pause``, and ``reconnect``) are often
called by another thread that is controlling the connection, therefore
thread-safe methods are provided where the thread-safe function posts a message
to the actual connection thread.

In an OpenVPN client connection, the following object stack would be used:

1. **class ClientConnect** in `<openvpn/client/cliconnect.hpp>`_ —
   The top-layer object in an OpenVPN client connection.
2. **class ClientProto::Session** in `<openvpn/client/cliproto.hpp>`_ —
   The OpenVPN client protocol object that subinstantiates the transport
   and tun layer objects.
3. **class ProtoContext** in `<openvpn/ssl/proto.hpp>`_ —
   The core OpenVPN protocol implementation that is common to both
   client and server.
4. **class ProtoStackBase<Packet>** in `<openvpn/ssl/protostack.hpp>`_ —
   The bottom-layer class that implements
   the basic functionality of tunneling a protocol over a reliable or
   unreliable transport layer, but isn't specific to OpenVPN per-se.

Transport Layer
...............

OpenVPN 3 defines abstract base classes for Transport layer
implementations in `<openvpn/transport/client/transbase.hpp>`_.

Currently, transport layer implementations are provided for:

* **UDP** — `<openvpn/transport/client/udpcli.hpp>`_
* **TCP** — `<openvpn/transport/client/tcpcli.hpp>`_
* **HTTP Proxy** — `<openvpn/transport/client/httpcli.hpp>`_

Tun Layer
.........

OpenVPN 3 defines abstract base classes for Tun layer
implementations in `<openvpn/tun/client/tunbase.hpp>`_.

There are two possible approaches to define a Tun
layer implementation:

1. Use a VPN API-centric model (such as for Android
   or iOS).  These models derive from **class TunBuilderBase**
   in `<openvpn/tun/builder/base.hpp>`_

2. Use an OS-specific model such as:

     * **Linux** — `<openvpn/tun/linux/client/tuncli.hpp>`_
     * **Windows** — `<openvpn/tun/win/client/tuncli.hpp>`_
     * **Mac OS X** — `<openvpn/tun/mac/client/tuncli.hpp>`_

Protocol Layer
..............

The OpenVPN protocol is implemented in **class ProtoContext**
in `<openvpn/ssl/proto.hpp>`_.

Options Processing
..................

The parsing and query of the OpenVPN config file
is implemented by ``class OptionList`` in
`<openvpn/common/options.hpp>`_.

Note that OpenVPN 3 always assumes an *inline* style of
configuration, where all certs, keys, etc. are
defined inline rather than through an external file
reference.

For config files that do use external file references,
``class ProfileMerge`` in `<openvpn/options/merge.hpp>`_
is provided to merge those external
file references into an inline form.

Calling the Client API from other languages
...........................................

The OpenVPN 3 client API, as defined by ``class OpenVPNClient``
in `<client/ovpncli.hpp>`_, can be wrapped by the
Swig_ tool to create bindings for other languages.

.. _Swig: http://www.swig.org/

For example, OpenVPN Connect for Android creates a Java
binding of the API using `<javacli/ovpncli.i>`_.

Security
++++++++

When developing security software in C++, it's very important to
take advantage of the language and OpenVPN library code
to insulate code from the kinds of
bugs that can introduce security vulnerabilities.

Here is a brief set of guidelines:

* When dealing with strings, use a ``std::string``
  rather than a ``char *``.

* When dealing with binary data or buffers, always try to use a ``Buffer``,
  ``ConstBuffer``, ``BufferAllocated``, or ``BufferPtr`` object to
  provide managed access to the buffer, to protect against security
  bugs that arise when using raw buffer pointers.
  See `<openvpn/buffer/buffer.hpp>`_ for the OpenVPN ``Buffer`` classes.

* When it's necessary to have a pointer to an object, use
  ``std::unique_ptr<>`` for non-shared objects and reference-counted
  smart pointers for shared objects.  For shared-pointers,
  OpenVPN code should use the smart pointer classes defined
  in `<openvpn/common/rc.hpp>`_.  Please see the comments in
  this file for documentation.

* Never use ``malloc`` or ``free``.  When allocating objects,
  use the C++ ``new`` operator and then immediately construct
  a smart pointer to reference the object:

  .. code:: c++

    std::unique_ptr<MyObject> ptr = new MyObject();
    ptr->method();

* When interfacing with C functions that deal with
  raw pointers, memory allocation, etc., consider wrapping
  the functionality in C++.  For an example, see ``enum_dir()``
  in `<openvpn/common/enumdir.hpp>`_,
  a function that returns a list of files in
  a directory (Unix only) via a high-level
  string vector, while internally calling
  the low level libc methods
  ``opendir``, ``readdir``, and ``closedir``.
  Notice how ``unique_ptr_del`` is used to wrap the
  ``DIR`` struct in a smart pointer with a custom
  deletion function.

* When grabbing random entropy that is to be used
  for cryptographic purposes (i.e. for keys, tokens, etc.),
  always ensure that the RNG is crypto-grade by calling
  ``assert_crypto()`` on the RNG.  This will throw
  an exception if the RNG is not crypto-grade:

  .. code:: c++

    void set_rng(RandomAPI::Ptr rng_arg) {
      rng_arg->assert_crypto();
      rng = std::move(rng_arg);
    }

* Any variable whose value is not expected to change should
  be declared ``const``.

* Don't use non-const global or static variables unless absolutely
  necessary.

* When formatting strings, don't use ``snprintf``.  Instead, use
  ``std::ostringstream`` or build the string using the '+' ``std::string``
  operator:

  .. code:: c++

    std::string format_reconnecting(const int n_seconds) {
      return "Reconnecting in " + openvpn::to_string(n_seconds) + " seconds.";
    }

  or:

  .. code:: c++

    std::string format_reconnecting(const int n_seconds) {
      std::ostringstream os;
      os << "Reconnecting in " << n_seconds << " seconds.";
      return os.str();
    }

* OpenVPN 3 is a "header-only" library, therefore all free functions
  outside of classes should have the ``inline`` attribute.

Conventions
+++++++++++

* Use the **Asio** library for I/O and timers.
  Don't deal with sockets directly.

* Never block.  If you need to wait for something, use **Asio** timers
  or sockets.

* Use the ``OPENVPN_LOG()`` macro to log stuff.  Don't use ``printf``.

* Don't call crypto/ssl libraries directly.  Instead use the abstraction
  layers (`<openvpn/crypto>`_ and `<openvpn/ssl>`_) that allow OpenVPN
  to link with different crypto/ssl libraries (such as **OpenSSL**
  or **mbed TLS**).

* Use ``RandomAPI`` as a wrapper for random number
  generators (`<openvpn/random/randapi.hpp>`_).

* If you need to deal with configuration file options,
  see ``class OptionList`` in `<openvpn/common/options.hpp>`_.

* If you need to deal with time or time durations, use the
  classes under `<openvpn/time>`_.

* If you need to deal with IP addresses, see the comprehensive classes
  under `<openvpn/addr>`_.

* In general, if you need a general-purpose library class or function,
  look under `<openvpn/common>`_.  Chances are good that it's already
  been implemented.

* The OpenVPN 3 approach to errors is to count them, rather than
  unconditionally log them.  If you need to add a new error
  counter, see `<openvpn/error/error.hpp>`_.

* If you need to create a new event type which can be transmitted
  as a notification back to the client API user, see
  `<openvpn/client/clievent.hpp>`_.

* Raw pointers or references can be okay when used by an object to
  point back to its parent (or container), if you can guarantee that
  the object will not outlive its parent.  Backreferences to a parent
  object is also a common use case for weak pointers.

* Use C++ exceptions for error handling and as an alternative
  to ``goto``.  See OpenVPN's general exception classes
  and macros in `<openvpn/common/exception.hpp>`_.

* Use C++ destructors for automatic object cleanup, and so
  that thrown exceptions will not leak objects.  Alternatively,
  use ``Cleanup`` in `<openvpn/common/cleanup.hpp>`_ when
  you need to specify a code block to execute prior to scope
  exit.  For example, ensure that the file ``pid_fn`` is
  deleted before scope exit:

  .. code:: c++

    auto clean = Cleanup([pid_fn]() {
      if (pid_fn)
        ::unlink(pid_fn);
    });

* When calling global methods (such as libc ``fork``),
  prepend "::" to the symbol name, e.g.:

  .. code:: c++

    struct dirent *e;
    while ((e = ::readdir(dir.get())) != nullptr) {
      ...
    }

* Use ``nullptr`` instead of ``NULL``.

Threading
+++++++++

The OpenVPN 3 client core is designed to run in a single thread, with
the UI or controller driving the OpenVPN API running in a different
thread.

It's almost never necessary to create additional threads within
the OpenVPN 3 client core.


Contributing
------------

See `<CONTRIBUTING.rst>`_.

License
-------

See `<LICENSE.rst>`_.