Java Swing app MySQL connection through SSH tunnel is failing with "local port 127.0.0.1:... cannot be bound ... Address already in use: JVM_Bind"

Question

I want that a Java Swing Desktop application to connect to a MySQL database which is running in a Linux Server.

I connect to this database with MySQL Workbench, using a SSH connection, no problem here, but from the code... impossible so far.

I have tried the following:

private static void doSshTunnel( String strSshUser, String strSshPassword, String strSshHost, 
        int nSshPort, String strRemoteHost, int nLocalPort, int nRemotePort ) throws JSchException {
    
    final JSch jsch = new JSch();
    Session session = jsch.getSession( strSshUser, strSshHost, 22 );
    session.setPassword( strSshPassword );
     
    final Properties config = new Properties();
    config.put( "StrictHostKeyChecking", "no" );
    session.setConfig( config );
     
    session.connect();
    session.setPortForwardingL(nLocalPort, strRemoteHost, nRemotePort);
}

public static final Connection conexion() throws JSchException, SQLException {
    
    Connection con = null;
    
    try {
        
        String strSshUser = "xxxx";                  // SSH loging username
        String strSshPassword = "xxx";                   // SSH login password
        String strSshHost = "12.345.678.901";          // hostname or ip or SSH server
        int nSshPort = 22;                                    // remote SSH host port number
        String strRemoteHost = "127.0.0.1";  // hostname or ip of your database server
        int nLocalPort = 1234;                                // local port number use to bind SSH tunnel
        int nRemotePort = 3306;                               // remote port number of your database 
        String strDbUser = "dbuser";                    // database loging username
        String strDbPassword = "dbpw";                    // database login password
           
        doSshTunnel(strSshUser, strSshPassword, strSshHost, nSshPort, strRemoteHost, nLocalPort, nRemotePort);
           
        Class.forName("com.mysql.cj.jdbc.Driver");
        con = DriverManager.getConnection("jdbc:mysql://localhost:"+nLocalPort+"/dbname", strDbUser, strDbPassword);

    } catch (ClassNotFoundException e) {            
        e.printStackTrace();
    } catch (JSchException e) {
        e.printStackTrace();
    } catch (SQLException e) {
        e.printStackTrace();
    }
    
    
    return con;
}

But I get the error:

com.jcraft.jsch.JSchException: PortForwardingL: local port 127.0.0.1:1234 cannot be bound.

...

Caused by: java.net.BindException: Address already in use: JVM_Bind

I've seen this issue in some other posts... trying to change the port 1234 does not work... I've tried with many ports but I always get the same result...

Martin Prikryl · Accepted Answer · 2021-09-08 12:57:30Z

1

In general, do not bind specific port. No matter which you choose, you will eventually clash with some other software. Make the system auto-assign any free local port:

int nLocalPort = session.setPortForwardingL(0, strRemoteHost, nRemotePort);

Though in your case, it turned out, that you run your code snippet many times, so the problem was that you clash with yourself.

You have to close the SSH session to release the forwarded port, before you can reuse it again.

Or keep just one session opened throughout lifetime of your application/service.

answered Sep 8, 2021 at 12:57

Martin Prikryl

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