Stunnel is an

open-source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use and view the source code, design documents, or content of the product. The open source model is a decentrali ...

multi-platform application used to provide a universal TLS/SSL tunneling service. Stunnel is used to provide secure encrypted connections for clients or servers that do not speak TLS or SSL natively. It runs on a variety of operating systems, including most

Unix-like A Unix-like (sometimes referred to as UN*X, *nix or *NIX) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Uni ...

operating systems and

Windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...

. Stunnel relies on the

OpenSSL OpenSSL is a software library for applications that provide secure communications over computer networks against eavesdropping, and identify the party at the other end. It is widely used by Internet servers, including the majority of HTTPS web ...

library A library is a collection of Book, books, and possibly other Document, materials and Media (communication), media, that is accessible for use by its members and members of allied institutions. Libraries provide physical (hard copies) or electron ...

to implement the underlying TLS or SSL protocol. Stunnel uses

public-key cryptography Public-key cryptography, or asymmetric cryptography, is the field of cryptographic systems that use pairs of related keys. Each key pair consists of a public key and a corresponding private key. Key pairs are generated with cryptographic alg ...

with X.509

digital certificates In cryptography, a public key certificate, also known as a digital certificate or identity certificate, is an electronic document used to prove the validity of a Key authentication, public key. The certificate includes the public key and informati ...

to secure the SSL connection, and clients can optionally be authenticated via a certificate. If linked against libwrap, it can be configured to act as a proxy–

firewall Firewall may refer to: * Firewall (computing), a technological barrier designed to prevent unauthorized or unwanted communications between computer networks or hosts * Firewall (construction), a barrier inside a building, designed to limit the spre ...

service as well. Stunnel is maintained by Polish programmer Michał Trojnara and released under the terms of the

GNU General Public License The GNU General Public Licenses (GNU GPL or simply GPL) are a series of widely used free software licenses, or ''copyleft'' licenses, that guarantee end users the freedom to run, study, share, or modify the software. The GPL was the first ...

(GPL) with

exception.

Example

A stunnel can be used to provide a secure SSL connection to an existing non-SSL-aware

SMTP The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typi ...

mail server. Assuming the SMTP server expects TCP connections on

port A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Hamburg, Manch ...

25, the stunnel would be configured 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 and 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).

References

External links

* {{Portal bar, Free and open-source software Cryptographic software Free security software Unix network-related software Transport Layer Security implementation Tunneling protocols Network protocols

Example

See also

References

External links