com.jcraft.jsch.JSchException: UnknownHostKey

I would either:

1. Try to ssh from the command line and accept the public key (the host will be added to ~/.ssh/known_hosts and everything should then work fine from Jsch) -OR-

2. Configure JSch to not use "StrictHostKeyChecking" (this introduces insecurities and should only be used for testing purposes), using the following code:

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

Option #1 (adding the host to the ~/.ssh/known_hosts file) has my preference.

While the question has been answered in general, I've found myself that there's a case when even existing known_hosts entry doesn't help. This happens when an SSH server sends ECDSA fingerprint and as a result, you'll have an entry like this:

|1|+HASH=|HASH= ecdsa-sha2-nistp256 FINGERPRINT=

The problem is that JSch prefers SHA_RSA and while connecting it will try to compare SHA-RSA fingerprint, which will result with error about "unknown host".

To fix this simply run:

$ ssh-keyscan -H -t rsa example.org >> known_hosts

or complain to Jcraft about prefering SHA_RSA instead of using the local HostKeyAlgorithms setting, although they don't seem to be too eager to fix their bugs.

It is a security risk to avoid host key checking.

JSch uses HostKeyRepository interface and its default implementation KnownHosts class to manage this. You can provide an alternate implementation that allows specific keys by implementing HostKeyRepository. Or you could keep the keys that you want to allow in a file in the known_hosts format and call

jsch.setKnownHosts(knownHostsFileName);

Or with a public key String as below.

String knownHostPublicKey = "mysite.com ecdsa-sha2-nistp256 AAAAE............/3vplY";
jsch.setKnownHosts(new ByteArrayInputStream(knownHostPublicKey.getBytes()));

see Javadoc for more details.

This would be a more secure solution.

Jsch is open source and you can download the source from here. In the examples folder, look for KnownHosts.java to know more details.