[FIXED] Connecting jenkins agent: Malformed SSH byte string

Issue

I encontered this problem when was trying to connect jenkins agent trough the ssh. I've passed all the steps as described here: https://acloudguru.com/blog/engineering/adding-a-jenkins-agent-node, except one - instead yum install java-1.8.0-openjdk I've used apt install default-jre, which in the end has gave me openjdk 11.0.13 2021-10-19. After the all trying launch the agent I am getting error:

java.io.IOException: Malformed SSH byte string.
    at com.trilead.ssh2.packets.TypesReader.readByteString(TypesReader.java:132)
    at com.trilead.ssh2.packets.TypesReader.readMPINT(TypesReader.java:117)
    at com.trilead.ssh2.signature.RSAKeyAlgorithm.decodePublicKey(RSAKeyAlgorithm.java:113)
    at com.trilead.ssh2.signature.RSAKeyAlgorithm.decodePublicKey(RSAKeyAlgorithm.java:28)
    at com.trilead.ssh2.KnownHosts.addHostkey(KnownHosts.java:108)
    at com.trilead.ssh2.KnownHosts.initialize(KnownHosts.java:410)
    at com.trilead.ssh2.KnownHosts.initialize(KnownHosts.java:438)
    at com.trilead.ssh2.KnownHosts.<init>(KnownHosts.java:87)
    at hudson.plugins.sshslaves.verifiers.KnownHostsFileKeyVerificationStrategy.getPreferredKeyAlgorithms(KnownHostsFileKeyVerificationStrategy.java:108)
    at hudson.plugins.sshslaves.SSHLauncher$1.call(SSHLauncher.java:425)
    at hudson.plugins.sshslaves.SSHLauncher$1.call(SSHLauncher.java:421)
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at java.base/java.lang.Thread.run(Thread.java:829)
[01/04/22 15:30:12] Launch failed - cleaning up connection
[01/04/22 15:30:12] [SSH] Connection closed.

Please, help me to understand, what happens and how can it be resolved. I am newbuy with jenkins and not familiar with java. Googling of this problem did not supply some sensible result to me. Thank you.

Solution

This is most likely due to incorrect content of the KnownHosts file. Therefore, make sure that the KnownHosts file is correct with the content of id_rsa.pub(public file), if it is incorrect, you will need to add it after Host name or IP address. Such as;

HOSTNAME ssh-rsa AAAAB3NzaC1yc...

Answered By - Mesut
Answer Checked By - David Goodson (JavaFixing Volunteer)