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). 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
announcement 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
(May be used for SSL/TLS support) Uses an Original BSD-style license
with an announcement 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 or yassl
instead.
GnuTLS http://www.gnutls.org/
(May be used for SSL/TLS support) 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.
yassl http://www.yassl.com/
(May be used for SSL/TLS support) Uses the GPL[1] license. If this is
a problem for you, consider using OpenSSL or GnuTLS instead.
NSS http://www.mozilla.org/projects/security/pki/nss/
(May be used for SSL/TLS support) Is covered by the MPL[4] license,
the GPL[1] license and the LGPL[3] license. You may choose to license
the code under MPL terms, GPL terms, or LGPL terms. These licenses
grant you different permissions and impose different obligations. You
should select the license that best meets your needs.
axTLS http://axtls.sourceforge.net/
(May be used for SSL/TLS support) Uses a Modified BSD-style license.
c-ares http://daniel.haxx.se/projects/c-ares/license.html
(Used for asynchronous name resolves) 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
(Used for compressed Transfer-Encoding support) 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 partly Original BSD-licensed with the announcement
clause. Some of the code in libcurl that is written to deal with
Kerberos4 is Modified BSD-licensed.
MIT Kerberos http://web.mit.edu/kerberos/www/dist/
(May be used for GSS support) MIT licensed, that shouldn't collide
with any other parts.
Heimdal http://www.pdc.kth.se/heimdal/
(May be used for GSS support) Heimdal is Original BSD licensed with
the announcement clause.
GNU GSS http://www.gnu.org/software/gss/
(May be used for GSS support) GNU GSS is GPL licensed. Note that you
may not distribute binary curl packages that uses this if you build
curl to also link and use any Original BSD licensed libraries!
fbopenssl
(Used for SPNEGO support) 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://josefsson.org/libidn/
(Used for IDNA support) Uses the GNU Lesser General Public
License [3]. 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
(Used for LDAP support) 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.
libssh2 http://www.libssh2.org/
(Used for scp and sftp support) libssh2 uses a Modified BSD-style
license.
[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
[4] = MPL - Mozilla Public License:
http://www.mozilla.org/MPL/