From Wikipedia, the free encyclopedia
Jump to: navigation, search
Developer(s) Michał Trojnara
Stable release 5.01 [1] / April 8, 2014; 10 days ago (2014-04-08)
Operating system Multi-platform
Type Proxy, Encryption
License GNU General Public License

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 or SSL natively.[2] It runs on a variety of operating systems,[3] 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.[4]

If linked against libwrap, it can be configured to act as a proxy-firewall service as well.

Stunnel is maintained by Michał Trojnara. Released under the terms of the GNU General Public License (GPL) with OpenSSL exception.

Example scenario[edit]

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).


  1. ^ "stunnel: ChangeLog". Retrieved 2014-04-14. 
  2. ^ O'Donovan, Barry Secure Communication with Stunnel, Linux Gazette, Issue 107, October 2004
  3. ^ Stunnel Ports
  4. ^ stunnel(8) manual

External links[edit]