Stunnel is a proxy designed to add TLS encryption functionality to existing clients and servers without any changes in the programs' code. Its architecture is optimized for security, portability, and scalability (including load-balancing), making it suitable for large deployments.

Stunnel uses the OpenSSL library for cryptography, so it supports whatever cryptographic algorithms are compiled into the library. It can benefit from the FIPS 140-2 validation of the OpenSSL FIPS Provider, as long as the building process meets the OpenSSL FIPS 140-2 Security Policy. Our latest Windows installer includes the OpenSSL FIPS Provider.

Stunnel is a free software authored by Michał Trojnara. Although distributed under GNU GPL version 2 or later with OpenSSL exception, stunnel is not a community project. We retain the copyright of the source code. Please contact us for commercial support or non-GPL licenses. Free, community-based support is also available via stunnel-users mailing list.