How to Solve the 'UnknownHostKey' Error in Java SFTP (JSch)?

Understanding the UnknownHostKey Error in Java SFTP (JSch)

You have encountered an "UnknownHostKey" error while utilizing the Java SFTP library JSch. This error indicates that JSch is unable to verify the authenticity of the remote host's public key. To resolve this problem, it's essential to understand its underlying cause.

Cause:

When JSch connects to a remote host, it compares the host's public key with a list of known host keys. If the remote host's key is not found in the known host list, it prompts for user confirmation and asks whether to add the key to the list. By default, JSch's "StrictHostKeyChecking" is enabled, which prevents it from automatically connecting to hosts with unknown or unverified keys.

Solution:

1. Ignore Host Key Verification (Not Recommended):

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

This approach allows JSch to connect to the remote host without checking the host key, which is not a secure practice and should only be used in trusted environments. It does not resolve the root cause of the issue.

2. Configure Host Key Verification:

Option 1: Use a Known Hosts File

JSch.setKnownHosts("/path/to/known_hosts");

This file contains a list of known host keys and their corresponding fingerprints. When connecting to a remote host, JSch compares the host's public key with the keys in the known hosts file. If a match is found, the connection proceeds without user confirmation.

Option 2: Manually Add Host Key

HostKey knownHostKey = new HostKey("127.0.0.1", 22, "rsa", "A2:39:3F:44:88:E9:1F:D7:D1:71:F4:85:98:FB:90:DC");
session.getHostKeyRepository().add(knownHostKey, session);

This method allows you to manually provide the expected host key for the remote host, allowing JSch to verify the connection without any user prompts.

By implementing either of these methods, you can resolve the "UnknownHostKey" error and establish a secure SFTP connection to the remote host.

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