java - JSch: UnknownHostKey exception even when the hostkey fingerprint is present in the known_hosts file

Question

There are two questions about this exception already:

• JSchException: UnknownHostKey and
• com.jcraft.jsch.JSchException: UnknownHostKey

I am using a Windows machine and trying to connect to a VM created with Vagrant running Ubuntu. Here is my code:

public static void main(String[] args) {
    String host = "localhost";
    String username = "vagrant";
    int port = 2200;
    String privateKey = "C:\keys\openSSH_pair1\open_ssh_private";
    JSch js = new JSch();
    try {
        js.addIdentity(privateKey, "pass");
        js.setKnownHosts("C:\Users\user\.ssh\known_hosts");
        Session session = js.getSession(username, host, port);
        session.connect();
        System.out.println("Connected");
    } catch (JSchException e) {
        e.printStackTrace();
    }
}

@Pascal suggests setting strictHostKeyChecking to no, which works for me, but this is not the preferred solution. His preferred solution is to SSH from the command line so that the host will be added to the known_hosts file. I have Git installed and executed ssh -i openSSH_pair1open_ssh_private vagrant@localhost -p 2200 and received this output before being prompted for the pass phrase and establishing a connection

The authenticity of host '[localhost]:2200 ([127.0.0.1]:2200)' can't be established. ECDSA key fingerprint is 11:5d:55:29:8a:77:d8:08:b4:00:9b:a3:61:93:fe:e5. Are you sure you want to continue connecting (yes/no)? yes Warning: Permanently added '[localhost]:2200' (ECDSA) to the list of known hosts.

So now my known_hosts file in git_home.ssh contains an entry for localhost:2200, I also placed the known_hosts file into user_home.ssh. I also put my private key on the VM I'm trying to ssh into and ran this to generate a public key and add it to the authorized_keys

ssh-keygen -y -f open_ssh_private > open_ssh_gen.pub
cat open_ssh_gen.pub >> ~/.ssh/authorized_keys

However I still get this exception

com.jcraft.jsch.JSchException: UnknownHostKey: localhost. RSA key fingerprint is 50:db:75:ba:11:2f:43:c9:ab:14:40:6d:7f:a1:ee:e3
    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 connect.Main.main(Main.java:24)

The answer to the other question suggests adding the below which doesn't work for me either

js.setKnownHosts("C:\Users\user\.ssh\known_hosts");

Answer 1

The problem is that you have added ECDSA host key to the known_hosts, as the ssh prefers that key type:

ECDSA key fingerprint is 11:5d:55:29:8a:77:d8:08:b4:00:9b:a3:61:93:fe:e5.

But JSch prefers RSA key, which it won't find in the known_hosts:

RSA key fingerprint is 50:db:75:ba:11:2f:43:c9:ab:14:40:6d:7f:a1:ee:e3

---

You probably need JCE to enable ECDSA In JSch.

See JSch Algorithm negotiation fail.

---

Or make ssh use RSA key with -o HostKeyAlgorithms=ssh-rsa.
See How can I force SSH to give an RSA key instead of ECDSA?

You can also use ssh-keyscan:

ssh-keyscan -t rsa example.com