[[TOC(inline, depth=1)]]

= Introduction =

The traditional way to build OpenVPN for Windows is to cross-compile it on *NIX. This works, but does not allow signing the TUN/TAP driver, which is required for Windows Vista/7 and later. Due to this a new, relatively simple Python-based build system was written. This new build system allows building OpenVPN on Windows more easily, but requires the use of a commercial version of Visual Studio development environment. However, the new build system is "external" from Visual Studio's viewpoint, so meddling with VS's graphical user interface is not necessary.

= Installing prequisites =

== Visual Studio 2008 Professional ==

Visual Studio 2008 Professional is required to build OpenVPN on Windows. Note that the free Express edition

== Windows Software Development Kit ==

[http://msdn.microsoft.com/en-us/windows/bb980924.aspx Windows Software Development Kit] ([http://en.wikipedia.org/wiki/Microsoft_Windows_SDK Wikipedia page]) ''may'' be necessary to build OpenVPN (verify this).

== Windows Driver Kit ==

[http://www.microsoft.com/whdc/devtools/WDK/default.mspx Windows Driver Kit] ([http://en.wikipedia.org/wiki/Windows_Driver_Kit Wikipedia page]) is required to build the TUN/TAP driver. 

== Python ==

The new Windows build system is written in [http://www.python.org Python].

== WinRAR ==

[http://www.rarlab.com WinRAR] or some other tool capable of extracting .tar.gz and .tar.bz2 archives is necessary to extract the LZO and OpenSSL release archives.

== !ActivePerl ==

[http://www.activestate.com/activeperl ActivePerl] is required to build OpenSSL, which in turn is required to build OpenVPN. Look [http://ehsanakhgari.org/blog/2008-05-04/compiling-openvpn-windows here] for details. Probably a [http://www.perl.org/get.html standard Perl installation] would also do the trick.

= Installing optional Git support =

Git support is needed if you want to conveniently build the latest development code. You don't need Git support if you fetch the sources from another computer and copy them over, or use source code from release tar.gz or zip files.

== Git for Windows ==

[http://code.google.com/p/msysgit/ Git for Windows] or ''msysgit'' is required to work with Git repositories on Windows.

== !GitExtensions ==

[http://code.google.com/p/gitextensions/ GitExtensions] makes working with Git easier on Windows. It includes Visual Studio 2008 plugin, Windows explorer support and a GUI to configure and use Git repositories.

== KDiff3 ==

[http://kdiff3.sourceforge.net/ KDiff3] is used in handling merge conflicts. It's required by ''!GitExtensions''.

== Git Source Control Provider ==

[http://gitscc.codeplex.com/ Git Source Control Provider] is a Visual Studio 2008/2010 plugin. It can also be used to launch ''Git for Windows'' and ''!GitExtensions'' from within Visual Studio.

= Building OpenSSL =

First download OpenSSL from [http://www.openssl.org/source here] and extract it somewhere. Using the latest one ensures there are no (known) security holes in OpenSSL. For the most part you can then follow the instructions in ''INSTALL.W32'' and ''INSTALL.W64'' files. Before you start, though, launch the ''Visual Studio 2008 Command Prompt'', which can be found from the ''Start menu''. Unlike the standard command prompt it has all the paths to VC binaries set correctly.

From within this command prompt you'll first configure OpenSSL using the provided Perl script:

{{{
C:\openssl-1.0.0> perl Configure VC-WIN32 --prefix=c:/<openssl-install-directory>
}}}

Some of the crypto routines are written in assembler to increase performance, so you need to/should use an assembler in the next step. If you're building OpenSSL 0.9.8x you can choose between [http://en.wikipedia.org/wiki/Microsoft_Macro_Assembler Microsoft Macro Assembler] and [http://www.nasm.us/ NASM assembler]. On OpenSSL-1.0.0 you need to use NASM as MASM is not supported anymore. Note that you need to add ''nasm.exe'' to the PATH. There are good generic instructions [http://vlaurie.com/computers2/Articles/environment.htm here].

If you're using MASM, run

{{{
C:\openssl-1.0.0> ms\do_masm
}}}

Or if you're using NASM, run

{{{
C:\openssl-1.0.0> ms\do_nasm
}}}

Next compile OpenSSL using the generated makefile:

{{{
C:\openssl-1.0.0> '''nmake -f ms\ntdll.mak'''
C:\openssl-1.0.0> nmake -f ms\ntdll.mak test
C:\openssl-1.0.0> nmake -f ms\ntdll.mak install 
}}}

= Building LZO =

The [http://www.oberhumer.com/opensource/lzo/ LZO library] is required to build OpenVPN. Once you've unpacked the source package, open the ''B/00README.txt'' file to get an overview of the Windows build process. If all goes well, you'll only need to run one .bat file:

{{{
C:\lzo-2.0.4> B\win32\vc_dll.bat
}}}

= Preparing the source tree for building =

If you want to build the latest development code fetch it using ''Git for Windows'' (Git shell) or ''!GitExtensions'' (GUI). Check [wiki:TesterDocumentation these instructions] to see which Git URI to use. Once you have fetched OpenVPN sources you need to copy ''openssl'' and ''lzo'' these header file directories the root build directory:

 * <lzo-directory>/include/lzo
 * <openssl-directory>/include/openssl

After this the OpenVPN build directory should look like this (files omitted):

{{{
|-- contrib
|-- debug
|-- easy-rsa
|-- images
|-- install-win32
|-- lzo
|-- management
|-- msvc
|-- openssl
|-- plugin
|-- sample-config-files
|-- sample-keys
|-- sample-scripts
|-- service-win32
|-- suse
|-- tap-win32
|-- win
}}}

= Building OpenVPN =

Fire up a console/MS DOS prompt and go to ''<openvpn-root-build-directory>/win''. Then just start the build:

{{{
C:\Python27\python.exe build.py
}}}

To clean up before or after the build, issue

{{{
C:\Python27\python.exe build.py clean
}}}


= Integrating Git with Visual Studio (optional) =

If you wish to use Windows for OpenVPN development integrating Git with Visual Studio (2008) may make sense.
 
= Troubleshooting =

== Compiler warnings during OpenSSL build ==

When building OpenSSL you're most likely encounter issues with trivial compiler warnings stopping the entire build. To circumvent this remove the ''/WX'' flag in the makefile as suggested in ''INSTALL.W32'' file. This is harder than it seems for two reasons:

 * The correct makefile is ''ms\ntdll.mak'' (not the ''Makefile'' in build root)
 * You need to use ''Wordpad'' or other UNIX linefeed-aware editor to open and save the makefile

== VS 2008 cross-tools issues during OpenSSL build == 

OpenSSL build is likely to break at several places if you're running  ''Visual Studio 2008 x64 Cross-Tools Command Prompt'' instead of plain ''Visual Studio 2008 Command prompt''. Consider reinstalling Visual Studio without cross-tools if you encounter either of these errors:

 * ''fatal error LNK1112: module machine type X86 conflicts with target machine type x64'' (in the middle of the build)
 * ''Linker can't find link.obj'' (at the end of the build)

= External links =

 * [http://ehsanakhgari.org/blog/2008-05-04/compiling-openvpn-windows Compiling OpenVPN on Windows]
 * [http://rfc2324.org/howto/openvpn4win Build OpenVPN for Windows using Mingw32]
 * [http://article.gmane.org/gmane.network.openvpn.user/22590 Compiling 2.1rc7 on Windows with VC] (email thread)
 * [http://msdn.microsoft.com/en-us/library/ff547649.aspx Installing Test-Signed Driver Packages]
 * [http://en.wikipedia.org/wiki/Visual_studio Visual Studio article on Wikipedia]
 * [http://editor.html.it/guide/leggi/139/guida-visual-studio-2008/ Visual Studio Guide] (in Italian)
 * [http://www.mrwebmaster.it/editor/guide/guida-visual-studio-2008/ Another Visual Studio Guide] (in Italian)
 * [http://blog.kowalczyk.info/article/clexe-cmd-line-args.html Visual Studio C compiler command-line arguments]
 * [http://www.gtopala.com/ System Information for Windows]: a useful tool for debugging processor architecture issues