Get that Linux feeling - on Windows
What are the licensing terms?
Most of the tools are covered by the GNU GPL, some are public domain, and others have a X11 style license. To cover the GNU GPL requirements, the basic rule is if you give out any binaries, you must also make the source available. For the full details, be sure to read the text of the GNU General Public License (GPL). For more information on the GPL see the GPL FAQ.
The Cygwin™ API library found in the winsup subdirectory of the source code is also covered by the GNU GPL (with exceptions; see below). By default, all executables link against this library (and in the process include GPL'd Cygwin™ glue code). This means that unless you modify the tools so that compiled executables do not make use of the Cygwin™ library, your compiled programs will also have to be free software distributed under the GPL with source code available to all.
The following text is a partial copy of the CYGWIN_LICENSE file from the Cygwin™ sources
Cygwin™ API Licensing Terms
Cygwin™ is free software. Red Hat, Inc. licenses Cygwin to you under the terms of the GNU General Public License as published by the Free Software Foundation; you can redistribute it and/or modify it under the terms of the GNU General Public License either version 3 of the license, or (at your option) any later version (GPLv3+), along with the additional permissions given below.
There is NO WARRANTY for this software, express or implied, including the implied warranties of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program. If not, see <http://gnu.org/licenses/>.
Cygwin™ Open Source Licensing Exception
As a special exception to GPLv3+, Red Hat grants you permission to link software whose sources are distributed under a license that satisfies the Open Source Definition with libcygwin.a, without libcygwin.a itself causing the resulting program to be covered by GPLv3+.
This means that you can port an Open Source application to Cygwin™, and distribute that executable as if it didn't include a copy of libcygwin.a linked into it. Note that this does not apply to the Cygwin™ DLL itself. If you distribute a (possibly modified) version of the Cygwin™ DLL, you must adhere to the terms of the GPLv3+, including the requirement to provide sources for the Cygwin™ DLL, unless you have obtained a special Cygwin™ license to distribute the Cygwin™ DLL in only its binary form (see below).
See http://opensource.org/docs/osd/ for the precise Open Source Definition referenced above.
Cygwin™ Alternative License
Red Hat sells a special Cygwin™ License for customers who are unable to provide their application in open source code form. For more information, please see: http://www.redhat.com/software/cygwin/, or call +1-866-2REDHAT ext. 45300 (toll-free in the US).
Outside the US call your regional Red Hat office.
Does Cygwin™ have an ECCN number?
No. Cygwin source and binary are made publicly available and free of charge to download so Cygwin is provided under TSU/TSPA exemption. As a result, Cygwin does not require an ECCN number.