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MIT/X derivate License

https://curl.haxx.se/docs/copyright.html

Apache License 1.0 and four-clause BSD License

https://www.openssl.org/source/license.html

MIT license

http://www.chiark.greenend.org.uk/~sgtatham/putty/licence.html

GNU GPL version 2 or later with OpenSSL exception

https://www.stunnel.org/sdf_copying.html

Open Source Software

License

License Link

Standalone or linkage

Code modification?

cURL

MIT/X derivate License

License Link

Standalone

No

OpenSSL

Apache License 1.0 and four-clause BSD License

License Link

Standalone and used as DLLs from cURL

No

PuTTY

MIT license

License Link

Standalone

Yes; Keep-alive was added

see: plink building with keepalive feature

stunnel

GNU GPL version 2 or later with OpenSSL exception

License Link

Standalone

No