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+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
