ssh - How to resolve Java UnknownHostKey, while using JSch SFTP library?

Question

I'm running a java program where I transfer a file from one folder to another, using Java SFTP. The problem I'm having is that I'm getting the following error in my Java SFTP (using JSch) :

C:OracleMiddlewareOracle_Homeoracle_commonjdkinjavaw.exe -server -classpath C:JDevelopermyworkJava_Hello_World.adf;C:JDevelopermyworkJava_Hello_WorldClientclasses;C:UsersADMINDownloadsjsch-0.1.53.jar -Djavax.net.ssl.trustStore=C:UsersIBM_AD~1AppDataLocalTemprustStore5840796204189742395.jks FileTransfer com.jcraft.jsch.JSchException: UnknownHostKey: 127.0.0.1. RSA key fingerprint is a2:39:3f:44:88:e9:1f:d7:d1:71:f4:85:98:fb:90:dc at com.jcraft.jsch.Session.checkHost(Session.java:797) at com.jcraft.jsch.Session.connect(Session.java:342) at com.jcraft.jsch.Session.connect(Session.java:183) at FileTransfer.main(FileTransfer.java:33) Process exited with exit code 0.

The following is my code so far:

FileTransfer fileTransfer = new FileTransfer();              

JSch jsch = new JSch();

try {

    String host = "127.0.0.1";
    int port = 22;

    String user = "user";
    Session session = jsch.getSession(user, host, port);      
    session = jsch.getSession("username", "127.0.0.1", 22);
    session.connect();  // bug here , java.net.ConnectException

    ChannelSftp sftp = null;
    sftp = (ChannelSftp)session.openChannel("sftp") ; //channel;

    //extra config code
    java.util.Properties config = new java.util.Properties(); 
    config.put("StrictHostKeyChecking", "no");
    session.setConfig(config);
    // end extra config code

    sftp.rename("C:\Users\ADMIN\Desktop\Work\ConnectOne_Bancorp\Java_Work\SFTP_1\house.bmp", "C:\Users\ADMIN\Desktop\Work\ConnectOne_Bancorp\Java_Work\SFTP_2\house.bmp");  
    session.disconnect();

} catch (JSchException e) {
    e.printStackTrace();  
} catch (SftpException e) {
    e.printStackTrace();
} //end-catch

My Cygwin is set up, and I checked (with netstat -a -b ) that it's running.

Answer 1

You are trying to skip a host key checking by setting StrictHostKeyChecking to no.

But you have to do that before the checking, i.e. before the session.connect().

---

Anyway, you should never do this, unless you do not care about security. The host key checking is there to protect you from man-in-the-middle attacks.

Instead, set up an expected host key to let JSch verify it.

For example:

• Call JSch.setKnownHosts providing a path to a .ssh/known_hosts-like file.

  To generate the .ssh/known_hosts-like file, you can use an ssh-keyscan command from OpenSSH. If you are connecting from a *nix server, you should have the command available, just run

  ssh-keyscan example.com > known_hosts
  

  It will have a format like:

  example.com ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEA0hVqZOvZ7yWgie9OHdTORJVI5fJJoH1yEGamAd5G3werH0z7e9ybtq1mGUeRkJtea7bzru0ISR0EZ9HIONoGYrDmI7S+BiwpDBUKjva4mAsvzzvsy6Ogy/apkxm6Kbcml8u4wjxaOw3NKzKqeBvR3pc+nQVA+SJUZq8D2XBRd4EDUFXeLzwqwen9G7gSLGB1hJkSuRtGRfOHbLUuCKNR8RV82i3JvlSnAwb3MwN0m3WGdlJA8J+5YAg4e6JgSKrsCObZK7W1R6iuyuH1zA+dtAHyDyYVHB4FnYZPL0hgz2PSb9c+iDEiFcT/lT4/dQ+kRW6DYn66lS8peS8zCJ9CSQ==
  

  And reference the generated known_hosts file in your JSch code.

  If you are on Windows, you can get a Windows build of ssh-keyscan from Win32-OpenSSH project or Git for Windows.

• Call JSch.getHostKeyRepository().add() to provide the expected host key (e.g. hard-coded, as your other credentials).

  See Creating JSch HostKey instance from a public key in .pub format.