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    1: License Mixing
    2: ==============
    3: 
    4: libcurl can be built to use a fair amount of various third party libraries,
    5: libraries that are written and provided by other parties that are distributed
    6: using their own licenses. Even libcurl itself contains code that may cause
    7: problems to some. This document attempts to describe what licenses libcurl and
    8: the other libraries use and what possible dilemmas linking and mixing them all
    9: can lead to for end users.
   10: 
   11: I am not a lawyer and this is not legal advice!
   12: 
   13: One common dilemma is that [GPL](https://www.gnu.org/licenses/gpl.html)
   14: licensed 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
   16: announcement clause). You may still build your own copies that use them all,
   17: but distributing them as binaries would be to violate the GPL license - unless
   18: you accompany your license with an
   19: [exception](https://www.gnu.org/licenses/gpl-faq.html#GPLIncompatibleLibs). This
   20: particular problem was addressed when the [Modified BSD
   21: license](https://opensource.org/licenses/BSD-3-Clause) was created, which does
   22: not 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.