Bindings to OpenSSL-compatible libraries for TLS support in SwiftNIO
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Lukasa and weissi Prevent crashes when closing twice. (#52)

If a user called close on a channel without attaching a promise, and
subsequently called close again with a promise, before the TLS shutdown
completes, this would lead to a crash due to a force-unwrap. It has
been my experience that crashes are less than ideal, especially when
users have done nothing wrong.


Replaced the force-unwrap with code that correctly persists the promise.
Added a test.


Less likely to crash.
Latest commit 085af4c Nov 7, 2018


SwiftNIO SSL is a Swift package that contains an implementation of TLS based on OpenSSL-compatible libraries (that is, any library that ships a libssl that is compatible with OpenSSL's). This package allows users of SwiftNIO to write protocol clients and servers that use TLS to secure data in flight.

The name is inspired primarily by the names of the libraries this package supports (e.g. OpenSSL, LibreSSL, and friends), and not because we don't know the name of the protocol. We know the protocol is TLS!

Using SwiftNIO SSL

SwiftNIO SSL provides two ChannelHandlers to use to secure a data stream: the OpenSSLClientHandler and the OpenSSLServerHandler. Each of these can be added to a Channel to secure the communications on that channel.

Additionally, we provide a number of low-level primitives for configuring your TLS connections. These will be shown below.

To secure a server connection, you will need a X.509 certificate chain in a file (either PEM or DER, but PEM is far easier), and the associated private key for the leaf certificate. These objects can then be wrapped up in a TLSConfiguration object that is used to initialize the ChannelHandler.

For example:

let configuration = TLSConfiguration.forServer(certificateChain: [.file("cert.pem")], privateKey: .file("key.pem")) 
let sslContext = try SSLContext(configuration: configuration)
let handler = try OpenSSLServerHandler(context: sslContext)
// Add the created handler to the pipeline.

For clients, it is a bit simpler as there is no need to have a certificate chain or private key (though clients may have these things). Setup for clients may be done like this:

let configuration = TLSConfiguration.forClient()
let sslContext = try SSLContext(configuration: configuration)
let handler = try OpenSSLClientHandler(context: sslContext)
// Add the created handler to the pipeline.


This binding can cause numerous issues during the build process on different systems, depending on the environment you're in. These will usually manifest as build errors, either during the compilation stage (due to missing development headers) or during the linker stage (due to an inability to find a library to link).

If you encounter any of these errors, here are your options.

Darwin (iOS, macOS, tvOS, ...)

If you target iOS/tv 12+ or macOS 10.14+ we recommend using swift-nio-transport-services which is SwiftNIO on top of Network.framework which supports TLS out of the box.

If you need to target older versions on macOS we recommend installing libressl from Homebrew (brew install libressl) as there is no easily-available copy of libssl.dylib with accompanying development headers.


On Linux distributions it is almost always possible to get development headers for the system copy of libssl (e.g. via apt-get install libssl-dev). If you encounter problems during the compile phase, try running this command.

In some unusual situations you may encounter problems during the link phase. This is usually the result of having an extremely locked down system that does not grant you sufficient permissions to the on the system.