Stunnel is an open-source multi-platform computer program, used to provide universal TLS/SSL tunneling service.
Stunnel can be used to provide secure encrypted connections for clients or servers that do not speak TLS (Transport Layer Security) or SSL (Secure Socket Layer) natively. It runs on a variety of operating systems, including most Unix-like operating systems and Windows. Stunnel relies on a separate library, such as OpenSSL or SSLeay, to implement the underlying TLS or SSL protocol.
Stunnel uses public-key cryptography with X.509 digital certificates to secure the SSL connection. Clients can optionally be authenticated via a certificate too.
If linked against libwrap, it can be configured to act as a proxy-firewall service as well.
For example, one could use Stunnel to provide a secure SSL connection to an existing non-SSL-aware SMTP mail server. Assume the SMTP server expects TCP connections on port 25. One would configure Stunnel to map the SSL port 465 to non-SSL port 25. A mail client connects via SSL to port 465. Network traffic from the client initially passes over SSL to the Stunnel application, which transparently encrypts/decrypts traffic and forwards unsecured traffic to port 25 locally. The mail server sees a non-SSL mail client.
The Stunnel process could be running on the same or a different server from the unsecured mail application; however, both machines would typically be behind a firewall on a secure internal network (so that an intruder could not make its own unsecured connection directly to port 25).