Jsch – UnknownHostKey exception

By mkyong | Updated: August 3, 2020

Viewed: 7,582 (+57 pv/w)

Tags: file transfer jsch ssh

This Java example tries to use Jsch to download a file from a remote server to the local system.

  <dependency>
     <groupId>com.jcraft</groupId>
     <artifactId>jsch</artifactId>
     <version>0.1.55</version>
  </dependency>

package com.mkyong.io.howto;

import com.jcraft.jsch.*;

public class SFTPFileTransfer {

    private static final String REMOTE_HOST = "1.1.1.1";
    private static final String USERNAME = "";
    private static final String PASSWORD = "";
    private static final int REMOTE_PORT = 22;

    public static void main(String[] args) {

        String localFile = "/home/mkyong/local/Test10.txt";
        String remoteFile = "/home/mkyong/remote/Test.txt";

        Session jschSession = null;

        try {

            JSch jsch = new JSch();

            // Why UnknownHostKey?
            jsch.setKnownHosts("/home/mkyong/.ssh/known_hosts");

            jschSession = jsch.getSession(USERNAME, REMOTE_HOST, REMOTE_PORT);

            jschSession.setPassword(PASSWORD);

            jschSession.connect();

            Channel sftp = jschSession.openChannel("sftp");

            sftp.connect();

            ChannelSftp channelSftp = (ChannelSftp) sftp;

            // download file from remote server to local
            channelSftp.get(remoteFile, localFile);

            channelSftp.exit();

        } catch (JSchException | SftpException e) {

            e.printStackTrace();

        } finally {
            if (jschSession != null) {
                jschSession.disconnect();
            }
        }

        System.out.println("Done");
    }

}

Run the above program, and it throws UnknownHostKey?

com.jcraft.jsch.JSchException: UnknownHostKey: 1.1.1.1. RSA key fingerprint is ::::
	at com.jcraft.jsch.Session.checkHost(Session.java:805)
	at com.jcraft.jsch.Session.connect(Session.java:345)
	at com.jcraft.jsch.Session.connect(Session.java:183)
	at com.mkyong.io.howto.SFTPFileTransfer.initJsch(SFTPFileTransfer.java:29)
	at com.mkyong.io.howto.SFTPFileTransfer.uploadFileUsingJsch(SFTPFileTransfer.java:35)
	at com.mkyong.io.howto.SFTPFileTransfer.main(SFTPFileTransfer.java:16)

Solution

To solve it, we can use ssh-keyscan to add the remote IP address or hostname into the ~/.ssh/known_hosts

ssh-keyscan -t rsa <HOST_NAME> >> ~/.ssh/known_hosts
ssh-keyscan -t rsa <IP_ADDRESS_OF_HOST_NAME> >> ~/.ssh/known_hosts

For example,

$ ssh-keyscan -t rsa 1.1.1.1 >> ~/.ssh/known_hosts

# 1.1.1.1:22 SSH-2.0-OpenSSH_7.4p1 Debian-10+deb9u7

References

• Jsch
• ssh-keyscan manual