1License Mixing
2==============
3
4libcurl can be built to use a fair amount of various third party libraries,
5libraries that are written and provided by other parties that are distributed
6using their own licenses. Even libcurl itself contains code that may cause
7problems to some. This document attempts to describe what licenses libcurl and
8the other libraries use and what possible dilemmas linking and mixing them all
9can lead to for end users.
10
11I am not a lawyer and this is not legal advice!
12
13One common dilemma is that [GPL](https://www.gnu.org/licenses/gpl.html)
14licensed code is not allowed to be linked with code licensed under the
15[Original BSD license](https://spdx.org/licenses/BSD-4-Clause.html) (with the
16announcement clause). You may still build your own copies that use them all,
17but distributing them as binaries would be to violate the GPL license - unless
18you accompany your license with an
19[exception](https://www.gnu.org/licenses/gpl-faq.html#GPLIncompatibleLibs). This
20particular problem was addressed when the [Modified BSD
21license](https://opensource.org/licenses/BSD-3-Clause) was created, which does
22not have the announcement clause that collides with GPL.
23
24## libcurl
25
26 Uses an [MIT style license](https://curl.haxx.se/docs/copyright.html) that is
27 very liberal.
28
29## OpenSSL
30
31 (May be used for SSL/TLS support) Uses an Original BSD-style license with an
32 announcement clause that makes it "incompatible" with GPL. You are not
33 allowed to ship binaries that link with OpenSSL that includes GPL code
34 (unless that specific GPL code includes an exception for OpenSSL - a habit
35 that is growing more and more common). If OpenSSL's licensing is a problem
36 for you, consider using another TLS library.
37
38## GnuTLS
39
40 (May be used for SSL/TLS support) Uses the
41 [LGPL](https://www.gnu.org/licenses/lgpl.html) license. If this is a problem
42 for you, consider using another TLS library. Also note that GnuTLS itself
43 depends on and uses other libs (libgcrypt and libgpg-error) and they too are
44 LGPL- or GPL-licensed.
45
46## WolfSSL
47
48 (May be used for SSL/TLS support) Uses the GPL license or a proprietary
49 license. If this is a problem for you, consider using another TLS library.
50
51## NSS
52
53 (May be used for SSL/TLS support) Is covered by the
54 [MPL](https://www.mozilla.org/MPL/) license, the GPL license and the LGPL
55 license. You may choose to license the code under MPL terms, GPL terms, or
56 LGPL terms. These licenses grant you different permissions and impose
57 different obligations. You should select the license that best meets your
58 needs.
59
60## mbedTLS
61
62 (May be used for SSL/TLS support) Uses the [Apache 2.0
63 license](https://opensource.org/licenses/Apache-2.0) or the GPL license.
64 You may choose to license the code under Apache 2.0 terms or GPL terms.
65 These licenses grant you different permissions and impose different
66 obligations. You should select the license that best meets your needs.
67
68## BoringSSL
69
70 (May be used for SSL/TLS support) As an OpenSSL fork, it has the same
71 license as that.
72
73## libressl
74
75 (May be used for SSL/TLS support) As an OpenSSL fork, it has the same
76 license as that.
77
78## BearSSL
79
80 (May be used for SSL/TLS support) Uses an MIT license that is very liberal
81 and imposes no restrictions on any other library or part you may link with.
82
83## c-ares
84
85 (Used for asynchronous name resolves) Uses an MIT license that is very
86 liberal and imposes no restrictions on any other library or part you may link
87 with.
88
89## zlib
90
91 (Used for compressed Transfer-Encoding support) Uses an MIT-style license
92 that shouldn't collide with any other library.
93
94## MIT Kerberos
95
96 (May be used for GSS support) MIT licensed, that shouldn't collide with any
97 other parts.
98
99## Heimdal
100
101 (May be used for GSS support) Heimdal is Original BSD licensed with the
102 announcement clause.
103
104## GNU GSS
105
106 (May be used for GSS support) GNU GSS is GPL licensed. Note that you may not
107 distribute binary curl packages that uses this if you build curl to also link
108 and use any Original BSD licensed libraries!
109
110## libidn
111
112 (Used for IDNA support) Uses the GNU Lesser General Public License [3]. LGPL
113 is a variation of GPL with slightly less aggressive "copyleft". This license
114 requires more requirements to be met when distributing binaries, see the
115 license for details. Also note that if you distribute a binary that includes
116 this library, you must also include the full LGPL license text. Please
117 properly point out what parts of the distributed package that the license
118 addresses.
119
120## OpenLDAP
121
122 (Used for LDAP support) Uses a Modified BSD-style license. Since libcurl uses
123 OpenLDAP as a shared library only, I have not heard of anyone that ships
124 OpenLDAP linked with libcurl in an app.
125
126## libssh2
127
128 (Used for scp and sftp support) libssh2 uses a Modified BSD-style license.
129