reTurn is a STUN server (RFC 5389, with RFC 3489 compatibility for binding requests), a TURN relay server (RFC 5766), and a client library for both.

It is built on asio and rutil, and needs OpenSSL for message integrity and TLS.

Client Library

The client library lives in reTurn/client, in the reTurn namespace.

reTurn::TurnAsyncSocket : The asynchronous client, with reTurn::TurnAsyncUdpSocket, reTurn::TurnAsyncTcpSocket and reTurn::TurnAsyncTlsSocket for each transport. Its functions, such as bindRequest(), createAllocation(), refreshAllocation(), setActiveDestination() and send(), return at once; results and received data are delivered to a reTurn::TurnAsyncSocketHandler. It runs on an asio::io_context supplied by the application, and is what reflow uses.
reTurn::TurnSocket : A simpler, blocking client, with reTurn::TurnUdpSocket, reTurn::TurnTcpSocket and reTurn::TurnTlsSocket.

A typical TURN client sets credentials with setUsernameAndPassword(), calls createAllocation(), adds its peer with setActiveDestination(), and then exchanges data with send() and the handler's onReceiveSuccess() callback, calling refreshAllocation() before the allocation expires.

Server

The reTurnServer application (reTurn/reTurnServer.cxx) listens for STUN and TURN over UDP, TCP and TLS, and relays allocations over UDP, for IPv4 and IPv6 (RFC 6156). It is configured with reTurnServer.config (see reTurn::ReTurnConfig), and authenticates TURN users with the long-term credential mechanism against a user database file, which can hold plain or hashed passwords.

The main server classes are reTurn::TurnManager, which allocates relay ports and enforces the per-user allocation and per-address connection limits; reTurn::TurnAllocationManager, which tracks allocations; reTurn::RequestHandler, which processes STUN and TURN requests; reTurn::TurnAllocation, a single relayed allocation and its permissions and channels; and reTurn::UdpServer, reTurn::TcpServer and reTurn::TlsServer, the listening sockets.

reTurn::StunMessage encodes and decodes STUN and TURN messages, and is shared by the client and the server.