= Introduction =

Official OpenVPN builds for Windows use the [https://github.com/OpenVPN/openvpn-build openvpn-build buildsystem], which uses MakeNSIS and mingw-w64 internally. Patched versions of both are required due to few issues:

* The ''openvpn.nsi'' script makes use of [http://nsis.sourceforge.net/Environmental_Variables:_append,_prepend,_and_remove_entries EnvVarUpdate.nsh] to append ''OpenVPN\bin'' to the system-wide PATH variable on Windows. When using default NSIS builds, PATH can't be longer than 1024 characters or OpenVPN install destroys it. To support longer PATHs you need rebuild ''makensis'' with larger ''NSIS_MAX_STRLEN''.
* The mingw-w64 packages have buggy header files which prevent building of recent OpenVPN versions (2.3.8+ or so).

= Preparations =

You need apt-src to build customized Debian packages:

{{{
$ apt-get install apt-src
}}}

Because apt-src is a bit messy, it's best to create a separate working directory:

{{{
$ mkdir ~/apt-src
}}}

= Rebuilding NSIS =

Here we rebuild ''nsis'' and ''nsis-common'':

{{{
$ cd ~/apt-src
$ apt-src install nsis
}}}

Next open ''nsis-2.46/debian/rules'' in a text editor and add ''NSIS_MAX_STRLEN=8192'' to the suitable place:

{{{
SCONSOPTS_NSIS   := $(SCONSOPTS) NSIS_MAX_STRLEN=8192 SKIPUTILS=$(IGNORE_UTILS_NSIS) SKIPDOC=COPYING
SCONSOPTS_COMMON := $(SCONSOPTS) NSIS_MAX_STRLEN=8192 SKIPUTILS=$(IGNORE_UTILS_COMMON) SKIPDOC=COPYING
}}}

Next add a changelog entry as described below, then build the packages:

{{{
$ apt-src build nsis
}}}

= Rebuilding mingw-w64 =

'''NOTE:''' On Ubuntu 16.04 and later you don't have to use a patched version of mingw-w64.

The process is about the same as for NSIS:

{{{
$ cd ~/apt-src
$ apt-src install mingw
}}}

Download the two patches (''fwptypes.h.patch'' and ''iketypes.h.patch''). Copy the patches to the patches directory and the series file:

{{{
$ cp *.patch ~/apt-src/mingw-w64-3.1.0/debian/patches
$ echo fwptypes.h.patch >> ~/apt-src/mingw-w64-3.1.0/debian/patches/series
$ echo iketypes.h.patch >> ~/apt-src/mingw-w64-3.1.0/debian/patches/series
}}}

Next add a changelog entry as described below, then build mingw-w64:

{{{
$ apt-src build mingw-w64
}}}

= Adding changelog entries =

Adding a changelog entry is important, because that determines the version and build number of the package. The changelog is in ''debian/changelog'', and an entry looks like this:

{{{
mingw-w64 (3.1.0-101) unstable; urgency=medium

  * Fixes to header files to allow OpenVPN builds

 -- John Doe <john@domain.com>  Wed, 13 Jan 2016 07:00:00 +0000
}}}

Note the last number (101) in the version: it prevents the package from getting updates (broken) by package updates from the stock Ubuntu repositories.