repost add SetOpenConnectionFunc in ClientOptions

client.go

@@ -392,8 +392,9 @@ func (c *client) attemptConnection() (net.Conn, byte, bool, error) {
 			DEBUG.Println(CLI, "using custom onConnectAttempt handler...")
 			tlsCfg = c.options.OnConnectAttempt(broker, c.options.TLSConfig)
 		}
+
 		// Start by opening the network connection (tcp, tls, ws) etc
-		conn, err = openConnection(broker, tlsCfg, c.options.ConnectTimeout, c.options.HTTPHeaders, c.options.WebsocketOptions, c.options.Dialer)
+		conn, err = c.Options.OpenConnFn(broker, tlsCfg, c.options.ConnectTimeout, c.options.HTTPHeaders, c.options.WebsocketOptions, c.options.Dialer)
 		if err != nil {
 			ERROR.Println(CLI, err.Error())
 			WARN.Println(CLI, "failed to connect to broker, trying next")

options.go

@@ -57,6 +57,10 @@ type ReconnectHandler func(Client, *ClientOptions)
 // ConnectionAttemptHandler is invoked prior to making the initial connection.
 type ConnectionAttemptHandler func(broker *url.URL, tlsCfg *tls.Config) *tls.Config
 
+// OpenConnectionFunc is invoked to establish the underlying network connection.
+// Does not carry out any MQTT specific handshakes.
+type OpenConnectionFunc func(uri *url.URL, tlsc *tls.Config, timeout time.Duration, headers http.Header, websocketOptions *WebsocketOptions, dialer *net.Dialer) (net.Conn, error)
+
 // ClientOptions contains configurable options for an Client. Note that these should be set using the
 // relevant methods (e.g. AddBroker) rather than directly. See those functions for information on usage.
 // WARNING: Create the below using NewClientOptions unless you have a compelling reason not to. It is easy
@@ -98,6 +102,7 @@ type ClientOptions struct {
 	WebsocketOptions        *WebsocketOptions
 	MaxResumePubInFlight    int // // 0 = no limit; otherwise this is the maximum simultaneous messages sent while resuming
 	Dialer                  *net.Dialer
+	OpenConnFn              OpenConnectionFunc
 }
 
 // NewClientOptions will create a new ClientClientOptions type with some
@@ -140,10 +145,23 @@ func NewClientOptions() *ClientOptions {
 		HTTPHeaders:             make(map[string][]string),
 		WebsocketOptions:        &WebsocketOptions{},
 		Dialer:                  &net.Dialer{Timeout: 30 * time.Second},
+		OpenConnFn:              openConnection,
 	}
 	return o
 }
 
+
+// SetOpenConnectionFunc replaces the inbuilt function that establishes a connection with a custom function.
+// The passed in function should return an open `net.Conn` or an error (see the existing openConnection function for an example)
+func (o *ClientOptions) SetOpenConnectionFunc(fn OpenConnectionFunc) *ClientOptions {
+	o.OpenConnFn = fn
+	if o.OpenConnFn == nil {
+	   o.OpenConnFn = openConnection //use the default one.
+	}
+	return o
+}
+
+
 // AddBroker adds a broker URI to the list of brokers to be used. The format should be
 // scheme://host:port
 // Where "scheme" is one of "tcp", "ssl", or "ws", "host" is the ip-address (or hostname)