new file mode 100644
@@ -0,0 +1,330 @@
+OpenVPN Interactive Service Notes
+=================================
+
+
+Introduction
+------------
+
+OpenVPN Interactive Service, also known as "iservice" or
+"OpenVPNServiceInteractive", is a Windows system service which allows
+unprivileged openvpn.exe process to do certain privileged operations, such as
+adding routes. This removes the need to always run OpenVPN as administrator,
+which was the case for a long time, and continues to be the case for OpenVPN
+2.3.x.
+
+The 2.4.x release and git "master" versions of OpenVPN contain the Interactive
+Service code and OpenVPN-GUI is setup to use it by default. Starting from
+version 2.4.0, OpenVPN-GUI is expected to be started as user (do not right-click
+and "run as administrator" or do not set the shortcut to run as administrator).
+This ensures that OpenVPN and the GUI run with limited privileges.
+
+
+How It Works
+------------
+
+Here is a brief explanation of how the Interactive Service works, based on
+`Gert's email`_ to openvpn-devel mailing list. The example user, *joe*, is not
+an administrator, and does not have any other extra privileges.
+
+- OpenVPN-GUI runs as user *joe*.
+
+- Interactive Service runs as a local Windows service with maximum privileges.
+
+- OpenVPN-GUI connects to the Interactive Service and asks it to "run
+ openvpn.exe with the given command line options".
+
+- Interactive Service starts openvpn.exe process as user *joe*, and keeps a
+ service pipe between Interactive Service and openvpn.exe.
+
+- When openvpn.exe wants to perform any operation that require elevation (e.g.
+ ipconfig, route, configure DNS), it sends a request over the service pipe to
+ the Interactive Service, which will then execute it (and clean up should
+ openvpn.exe crash).
+
+- ``--up`` scripts are run by openvpn.exe itself, which is running as user
+ *joe*, all privileges are nicely in place.
+
+- Scripts run by the GUI will run as user *joe*, so that automated tasks like
+ mapping of drives work as expected.
+
+This avoids the use of scripts for privilege escalation (as was possible by
+running an ``--up`` script from openvpn.exe which is run as administrator).
+
+
+Client-Service Communication
+----------------------------
+
+Connecting
+~~~~~~~~~~
+
+The client (OpenVPN GUI) and the Interactive Service communicate using a named
+message pipe. By default, the service provides the ``\\.\pipe\openvpn\service``
+named pipe.
+
+The client connects to the pipe for read/write and sets the pipe state to
+``PIPE_READMODE_MESSAGE``::
+
+ HANDLE pipe = CreateFile(_T("\\\\.\\pipe\\openvpn\\service"),
+ GENERIC_READ | GENERIC_WRITE,
+ 0,
+ NULL,
+ OPEN_EXISTING,
+ FILE_FLAG_OVERLAPPED,
+ NULL);
+
+ if (pipe == INVALID_HANDLE_VALUE)
+ {
+ // Error
+ }
+
+ DWORD dwMode = PIPE_READMODE_MESSAGE;
+ if (!SetNamedPipeHandleState(pipe, &dwMode, NULL, NULL)
+ {
+ // Error
+ }
+
+
+openvpn.exe Startup
+~~~~~~~~~~~~~~~~~~~
+
+After the client is connected to the service, the client must send a startup
+message to have the service start the openvpn.exe process. The startup message
+is comprised of three UTF-16 strings delimited by U0000 zero characters::
+
+ startupmsg = workingdir WZERO openvpnoptions WZERO stdin WZERO
+
+ workingdir = WSTRING
+ openvpnoptions = WSTRING
+ stdin = WSTRING
+
+ WSTRING = *WCHAR
+ WCHAR = %x0001-FFFF
+ WZERO = %x0000
+
+``workingdir``
+ Represents the folder openvpn.exe process should be started in.
+
+``openvpnoptions``
+ String contains ``--config`` and other OpenVPN command line options, without
+ the ``argv[0]`` executable name ("openvpn" or "openvpn.exe"). When there is
+ only one option specified, the ``--config`` option is assumed and the option
+ is the configuration filename.
+
+ Note that the interactive service validates the options. OpenVPN
+ configuration file must reside in the configuration folder defined by
+ ``config_dir`` registry value. The configuration file can also reside in any
+ subfolder of the configuration folder. For all other folders the invoking
+ user must be a member of local Administrators group, or a member of the group
+ defined by ``ovpn_admin_group`` registry value ("OpenVPN Administrators" by
+ default).
+
+``stdin``
+ The content of the ``stdin`` string is sent to the openvpn.exe process to its
+ stdin stream after it starts.
+
+ When a ``--management ... stdin`` option is present, the openvpn.exe process
+ will prompt for the management interface password on start. In this case, the
+ ``stdin`` must contain the password appended with an LF (U000A) to simulate
+ the [Enter] key after the password is "typed" in.
+
+ The openvpn.exe's stdout is redirected to ``NUL``. Should the client require
+ openvpn.exe's stdout, one should specify ``--log`` option.
+
+The message must be written in a single ``WriteFile()`` call.
+
+Example::
+
+ // Prepare the message.
+ size_t msg_len =
+ wcslen(workingdir) + 1 +
+ wcslen(options ) + 1 +
+ wcslen(manage_pwd) + 1;
+ wchar_t *msg_data = (wchar_t*)malloc(msg_len*sizeof(wchar_t));
+ _snwprintf(msg_data, msg_len, L"%s%c%s%c%s",
+ workingdir, L'\0',
+ options, L'\0',
+ manage_pwd)
+
+ // Send the message.
+ DWORD dwBytesWritten;
+ if (!WriteFile(pipe,
+ msg_data,
+ msg_len*sizeof(wchar_t),
+ &dwBytesWritten,
+ NULL))
+ {
+ // Error
+ }
+
+ // Sanitize memory, since the stdin component of the message
+ // contains the management interface password.
+ SecureZeroMemory(msg_data, msg_len*sizeof(wchar_t));
+ free(msg_data);
+
+
+openvpn.exe Process ID
+~~~~~~~~~~~~~~~~~~~~~~
+
+After receiving the startup message, the Interactive Service validates the user
+and specified options before launching the openvpn.exe process.
+
+The Interactive Service replies with a process ID message. The process ID
+message is comprised of three UTF-16 strings delimited by LFs (U000A)::
+
+ pidmsg = L"0x00000000" WLF L"0x" pid WLF L"Process ID"
+
+ pid = 8*8WHEXDIG
+
+ WHEXDIG = WDIGIT / L"A" / L"B" / L"C" / L"D" / L"E" / L"F"
+ WDIGIT = %x0030-0039
+ WLF = %x000a
+
+``pid``
+ A UTF-16 eight-character hexadecimal process ID of the openvpn.exe process
+ the Interactive Service launched on client's behalf.
+
+
+openvpn.exe Monitoring and Termination
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+After the openvpn.exe process is launched, the client can disconnect the pipe to
+the interactive service. However, it should monitor the openvpn.exe process
+itself. OpenVPN Management Interface is recommended for this.
+
+The client may choose to stay connected to the pipe. When the openvpn.exe
+process terminates, the service disconnects the pipe. Should the openvpn.exe
+process terminate with an error, the service sends an error message to the
+client before disconnecting the pipe.
+
+Note that Interactive Service terminates all child openvpn.exe processes when
+the service is stopped or restarted. This allows a graceful elevation-required
+clean-up (e.g. restore ipconfig, route, DNS).
+
+
+Error Messages
+~~~~~~~~~~~~~~
+
+In case of an error, the Interactive Service sends an error message to the
+client. Error messages are comprised of three UTF-16 strings delimited by LFs
+(U000A)::
+
+ errmsg = L"0x" errnum WLF func WLF msg
+
+ errnum = 8*8WHEXDIG
+ func = WSTRING
+ msg = WSTRING
+
+``errnum``
+ A UTF-16 eight-character hexadecimal error code. Typically, it is one of the
+ Win32 error codes returned by ``GetLastError()``.
+
+ However, it can be one of the Interactive Service specific error codes:
+
+ ===================== ==========
+ Error Code
+ ===================== ==========
+ ERROR_OPENVPN_STARTUP 0x20000000
+ ERROR_STARTUP_DATA 0x20000001
+ ERROR_MESSAGE_DATA 0x20000002
+ ERROR_MESSAGE_TYPE 0x20000003
+ ===================== ==========
+
+``func``
+ The name of the function call that failed or an error description.
+
+``msg``
+ The error description returned by a
+ ``FormatMessageW(FORMAT_MESSAGE_FROM_SYSTEM, 0, errnum, ...)`` call.
+
+
+Interactive Service Configuration
+---------------------------------
+
+The Interactive Service settings are read from the
+``HKEY_LOCAL_MACHINE\SOFTWARE\OpenVPN`` registry key by default.
+
+All the following registry values are of the ``REG_SZ`` type:
+
+*Default*
+ Installation folder (required, hereinafter ``install_dir``)
+
+``exe_path``
+ The absolute path to the openvpn.exe binary; defaults to
+ ``install_dir "\bin\openvpn.exe"``.
+
+``config_dir``
+ The path to the configuration folder; defaults to ``install_dir "\config"``.
+
+``priority``
+ openvpn.exe process priority; one of the following strings:
+
+ - ``"IDLE_PRIORITY_CLASS"``
+ - ``"BELOW_NORMAL_PRIORITY_CLASS"``
+ - ``"NORMAL_PRIORITY_CLASS"`` (default)
+ - ``"ABOVE_NORMAL_PRIORITY_CLASS"``
+ - ``"HIGH_PRIORITY_CLASS"``
+
+``ovpn_admin_group``
+ The name of the local group, whose members are authorized to use the
+ Interactive Service unrestricted; defaults to ``"OpenVPN Administrators"``
+
+
+Multiple Interactive Service Instances
+--------------------------------------
+
+OpenVPN 2.4.5 extended the Interactive Service to support multiple side-by-side
+running instances. This allows clients to use different Interactive Service
+versions with different settings and/or openvpn.exe binary version on the same
+computer.
+
+OpenVPN installs the default Interactive Service instance only. The default
+instance is used by OpenVPN GUI client and also provides backward compatibility.
+
+
+Installing a Non-default Interactive Service Instance
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+1. Choose a unique instance name. For example: "$v2.5-test". The instance name
+ is appended to the default registry path and service name. We choose to start
+ it with a dollar "$" sign analogous to Microsoft SQL Server instance naming
+ scheme. However, this is not imperative.
+
+ Appending the name to the registry path and service name also implies the
+ name cannot contain characters not allowed in Windows paths: "<", ">", double
+ quote etc.
+
+2. Create an ``HKEY_LOCAL_MACHINE\SOFTWARE\OpenVPN$v2.5-test`` registry key and
+ configure the Interactive Service instance configuration appropriately.
+
+ This allows using slightly or completely different settings from the default
+ instance.
+
+ See the `Interactive Service Configuration`_ section for the list of registry
+ values.
+
+3. Create and start the instance's Windows service from an elevated command
+ prompt::
+
+ sc create "OpenVPNServiceInteractive$v2.5-test" \
+ start= auto \
+ binPath= "<path to openvpnserv.exe> -instance interactive $v2.5-test" \
+ depend= tap0901/Dhcp \
+ DisplayName= "OpenVPN Interactive Service (v2.5-test)"
+
+ sc start "OpenVPNServiceInteractive$v2.5-test"
+
+ This allows using the same or a different version of openvpnserv.exe than the
+ default instance.
+
+ Note the space after "=" character in ``sc`` command line options.
+
+4. Set your OpenVPN client to connect to the
+ ``\\.\pipe\openvpn$v2.5-test\service``.
+
+ This allows the client to select a different installed Interactive Service
+ instance at run-time, thus allowing different OpenVPN settings and versions.
+
+ At the time writing, the OpenVPN GUI client supports connecting to the
+ default Interactive Service instance only.
+
+.. _`Gert's email`: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg00097.html