This documentation is for version v1.3.0 of NNG, but the latest released version is v1.8.0. see the documentation for v1.8.0 for the most up-to-date information.


#include <nng/transport/tls/tls.h>

int nng_tls_register(void);


The tls transport provides communication support between nng sockets across a TCP/IP network using TLS v1.2 on top of TCP. Both IPv4 and IPv6 are supported when the underlying platform also supports it.

The protocol details are documented in TLS Mapping for Scalability Protocols.


Depending upon how the library was built, it may be necessary to register the transport by calling nng_tls_register().


The tls transport depends on the use of an external library. As of this writing, mbedTLS version 2.0 or later is required.

Applications may need to add this library (or libraries) to their link line, particularly when using a statically built nng library.
The mbedTLS library uses different licensing terms than nng itself; as of this writing it is offered under either Apache License 2.0 or GNU GPL terms. You are responsible for understanding and adhering to the license terms of any libraries you make use of.

URI Format

This transport uses URIs using the scheme tls+tcp://, followed by an IP address or hostname, followed by a colon and finally a TCP port number. For example, to contact port 4433 on the localhost either of the following URIs could be used: tls+tcp:// or tls+tcp://localhost:4433.

A URI may be restricted to IPv6 using the scheme tls+tcp6://, and may be restricted to IPv4 using the scheme tls+tcp4://.

Specifying tls+tcp6:// may not prevent IPv4 hosts from being used with IPv4-in-IPv6 addresses, particularly when using a wildcard hostname with listeners. The details of this varies across operating systems.
Both tls+tcp6:// and tls+tcp4:// are nng extensions, and may not be understood by other implementations.
We recommend using either numeric IP addresses, or names that are specific to either IPv4 or IPv6 to prevent confusion and surprises.

When specifying IPv6 addresses, the address must be enclosed in square brackets ([]) to avoid confusion with the final colon separating the port.

For example, the same port 4433 on the IPv6 loopback address ('::1') would be specified as tls+tcp://[::1]:4433.

Certificate validation generally works when using names rather than IP addresses. This transport automatically uses the name supplied in the URL when validating the certificate supplied by the server.

The special value of 0 (INADDR_ANY) can be used for a listener to indicate that it should listen on all interfaces on the host. A short-hand for this form is to either omit the address, or specify the asterisk (*) character. For example, the following three URIs are all equivalent, and could be used to listen to port 9999 on the host:

  1. tls+tcp://

  2. tls+tcp://*:9999

  3. tls+tcp://:9999

The entire URI must be less than NNG_MAXADDRLEN bytes long.

Socket Address

When using an nng_sockaddr structure, the actual structure is either of type nng_sockaddr_in (for IPv4) or nng_sockaddr_in6 (for IPv6).

Transport Options

The following transport options are available. Note that setting these must be done before the transport is started.