License Mixing with apps, libcurl and Third Party Libraries =========================================================== libcurl can be built to use a fair amount of various third party libraries, libraries that are written and provided by other parties that are distributed using their own licenses. Even libcurl itself contains code that may cause problems to some. This document attempts to describe what licenses libcurl and the other libraries use and what possible dilemmas linking and mixing them all can lead to for end users. I am not a lawyer and this is not legal advice! One common dilemma is that GPL[*]-licensed code is not allowed to be linked with code licensed under the Original BSD license (with the announcement clause, unless there's a specified exception in the GPL-licensed module). You may still build your own copies that use them all, but distributing them as binaries would be to violate the GPL license. This particular problem was addressed when the Modified BSD license was created, which does not have the annoncement clause that collides with GPL. libcurl http://curl.haxx.se/docs/copyright.html Uses an MIT (or Modified BSD)-style license that is as liberal as possible. Some of the source files that deal with KRB4 have Original BSD-style announce-clause licenses. You may not distribute binaries with krb4-enabled libcurl that also link with GPL-licensed code! OpenSSL http://www.openssl.org/source/license.html Uses an Original BSD-style license with an announement clause that makes it "incompatible" with GPL. You are not allowed to ship binaries that link with OpenSSL that includes GPL code (unless that specific GPL code includes an exception for OpenSSL - a habit that is growing more and more common). c-ares http://daniel.haxx.se/projects/c-ares/license.html Uses an MIT license that is very liberal and imposes no restrictions on any other library or part you may link with. zlib http://www.gzip.org/zlib/zlib_license.html Uses an MIT-style license that shouldn't collide with any other library. krb4 While nothing in particular says that a Kerberos4 library must use any particular license, the one I've tried and used successfully so far (kth-krb4) is Original BSD-licensed with the announcement clause. Some of the code in libcurl that is written to deal with Kerberos4 likewise have such a license. GSSAPI While nothing in particular says that a GSS/Kerberos5 library must use any particular license, the one I've used (Heimdal) is Original BSD- licensed with the announcement clause. fbopenssl Unclear license. Based on its name, I assume that it uses the OpenSSL license and thus shares the same issues as described for OpenSSL above. libidn http://www.gnu.org/licenses/lgpl.html Uses the GNU Lesser General Public License. LGPL is a variation of GPL with slightly less aggressive "copyleft". This license requires more requirements to be met when distributing binaries, see the license for details. Also note that if you distribute a binary that includes this library, you must also include the full LGPL license text. Please properly point out what parts of the distributed package that the license addresses. OpenLDAP http://www.openldap.org/software/release/license.html Uses a Modified BSD-style license. Since libcurl uses OpenLDAP as a shared library only, I have not heard of anyone that ships OpenLDAP linked with libcurl in an app. [*] = GPL - GNU General Public License: http://www.gnu.org/licenses/gpl.html