How to Resolve the 'UnknownHostKey' Error in Java SFTP Using JSch?

Resolving UnknownHostKey Error in Java SFTP Using JSch

The "UnknownHostKey" error occurs when JSch encounters an unknown host key while establishing an SFTP connection. To resolve this issue, you need to provide JSch with the expected host key for authentication.

Setting StrictHostKeyChecking

Initially, you attempted to disable host key checking by setting "StrictHostKeyChecking" to "no" before connecting to the SFTP server. This is a security risk and should not be done unless absolutely necessary.

Proper Approach: Setting Expected Host Key

Instead, you should set up an expected host key for JSch to verify against. There are two primary methods for achieving this:

Using known_hosts File

• Generate a ".ssh/known_hosts-like" file using "ssh-keyscan" command.
• Set the path to this file using "JSch.setKnownHosts()" before establishing the SFTP session.

ssh-keyscan example.com > known_hosts
JSch.setKnownHosts("path/to/known_hosts");

Using HostKeyRepository

• Manually create an instance of HostKey using the expected public key.
• Add the HostKey to the HostKeyRepository using "JSch.getHostKeyRepository().add()".

// Create HostKey from public key
HostKey key = new HostKey("example.com", "rsa", ...);

// Add HostKey to repository
JSch.getHostKeyRepository().add(key);

Note: Both approaches ensure that JSch can verify the authenticity of the SFTP server using the expected host key, thereby resolving the "UnknownHostKey" error.

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