JSch multiple tunnels/jumphosts

Fangming :

I'm not sure if this is caused by using a private key instead of password for the port forwarding but here's what I'm trying to do

I need to forward local port 3308 all the way to the my SQL DB at 3306.

I can run things like this all together in terminal on my local

ssh -L 3308:localhost:3307 username@jumpbox "ssh -L 3307:mysqlDB:3306 username@server"

Or run the first part on my local and then the second part on the jumpbox. Both works fine and I can connect to my localhost:3308.

The problem comes when I start using JSch. Here is my code

JSch jsch = new JSch();
jsch.addIdentity("~/.ssh/id_rsa");

Session session = jsch.getSession("username", "jumpbox");
session.setConfig("StrictHostKeyChecking", "no");
session.connect();

int assinged_port = session.setPortForwardingL(3308, "localhost", 3307);
Session mysqlSession = jsch.getSession("username", "server", assinged_port);
mysqlSession.setConfig("StrictHostKeyChecking", "no");
mysqlSession.connect(); // Connection timed out here
mysqlSession.setPortForwardingL(3307, "mysqlDB", 3306);

The first connection is done but the second one timed out.

Exception in thread "main" com.jcraft.jsch.JSchException: java.net.ConnectException: Operation timed out (Connection timed out)

Am I doing something wrong here with JSch or port forwarding?

Martin Prikryl :

Your ssh command is making use of an SSH client (another ssh) running on "jump box".

When you want to implement the same using Java, you have two options:

1. Do the same in Java, i.e. use session to run ssh -L 3307:mysqlDB:3306 username@server on the "jump box".

   See Executing a command using JSch.

   Though, I do not think you should rely on ssh program for the second jump, for the same reason you use Java/JSch for the first jump (and not ssh program).

2. Avoid using a separate ssh tool, and instead open the other SSH session locally via yet another forwarded port. You can actually do the same using recent versions of ssh, with -J (jump) switch (supported since OpenSSH 7.3):

   ssh -L 3308:mysqlDB:3306 -J username@jumpbox username@server
   

   I prefer this approach.

---

To implement the latter approach:

• You have to forward some local port to server:22, so that you can open SSH connection to the server:

  JSch jsch = new JSch();
  jsch.addIdentity("~/.ssh/id_rsa");
  
  Session jumpboxSession = jsch.getSession("username", "jumpbox");
  jumpboxSession.connect();
  
  int serverSshPort = jumpboxSession.setPortForwardingL(0, "server", 22);
  Session serverSession = jsch.getSession("username", "localhost", serverSshPort);
  serverSession.connect();
  

• Then you forward another local port via server to MySQL port:

  int mysqlPort = serverSession.setPortForwardingL(0, "mysqlDB", 3306);
  

  Now you should be able to connect to localhost:mysqlPort using MySQL client.

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Obligatory warning: Do not use StrictHostKeyChecking=no to blindly accept all host keys. That is a security flaw. You lose a protection against MITM attacks.

For a correct (and secure) approach, see:
How to resolve Java UnknownHostKey, while using JSch SFTP library?