How to keep SSH connections alive using SSHJ?

Question

I am developing a library that uses SSHJ for SFTP transfer. Since requests are frequent, I have wondered whether I could just keep the connection open.

Obviously, this will achieve nothing if the server frequently times out the connection. Since I have no control of the server, I have to keep the connection alive: With a regular SSH client, I could specify a ServerAliveInterval and have the client do it for me.

I'd like to do the same with SSHJ, but I do not know what message to send.

The SSH manual just states that ServerAliveInterval

Sets a timeout interval in seconds after which if no data has been received from the server, ssh(1) will send a message through the encrypted channel to request a response from the server.

So I'm wondering: What message is sent? How could I reproduce this message through SSHJ?

Answer 1

Sorry this isn't documented, but you can set a heartbeat:

https://github.com/hierynomus/sshj/blob/master/examples/src/main/java/net/schmizz/sshj/examples/RemotePF.java#L51

Answer 2

Starting from version 0.11.0, you can use built-in KeepAliveProvider:

public class KeepAlive {

    public static void main(String... args)
            throws IOException, InterruptedException {
        DefaultConfig defaultConfig = new DefaultConfig();
        defaultConfig.setKeepAliveProvider(KeepAliveProvider.KEEP_ALIVE);
        final SSHClient ssh = new SSHClient(defaultConfig);
        try {
            ssh.addHostKeyVerifier(new PromiscuousVerifier());
            ssh.connect(args[0]);
            ssh.getConnection().getKeepAlive().setKeepAliveInterval(5); //every 60sec
            ssh.authPassword(args[1], args[2]);
            Session session = ssh.startSession();
            session.allocateDefaultPTY();
            new CountDownLatch(1).await();
            try {
                session.allocateDefaultPTY();
            } finally {
                session.close();
            }
        } finally {
            ssh.disconnect();
        }
    }
}

To send heartbeats, you may use KeepAliveProvider.HEARTBEAT.