License Mixing with apps, libcurl and Third Party Libraries
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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[1]-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 - unless you accompany your
license with an exception[2]. 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). If OpenSSL's licensing is a problem for you,
        consider using GnuTLS instead.

GnuTLS  http://www.gnutls.org/

        Uses the LGPL[3] license. If this is a problem for you, consider using
        OpenSSL instead. Also note that GnuTLS itself depends on and uses
        other libs (libgcrypt and libgpg-error) and they too are LGPL- or
        GPL-licensed.

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.


[1] = GPL - GNU General Public License: http://www.gnu.org/licenses/gpl.html
[2] = http://www.fsf.org/licenses/gpl-faq.html#GPLIncompatibleLibs details on
      how to write such an exception to the GPL
[3] = LGPL - GNU Lesser General Public License:
      http://www.gnu.org/licenses/lgpl.html