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master cannot connect to slave
/var/log/secure on slave reports
sshd[1978]: fatal: no matching mac found: client hmac-sha1-96,hmac-sha1,hmac-md5-96,hmac-md5 server hmac-sha2-512,hmac-sha2-256,hmac-ripemd160
master reports
Key exchange was not finished, connection is closed.
ERROR: Unexpected error in launching a slave. This is probably a bug in Jenkins.
java.lang.IllegalStateException: Connection is not established!
at com.trilead.ssh2.Connection.getRemainingAuthMethods(Connection.java:1030)
at com.cloudbees.jenkins.plugins.sshcredentials.impl.TrileadSSHPublicKeyAuthenticator.getRemainingAuthMethods(TrileadSSHPublicKeyAuthenticator.java:88)
at com.cloudbees.jenkins.plugins.sshcredentials.impl.TrileadSSHPublicKeyAuthenticator.canAuthenticate(TrileadSSHPublicKeyAuthenticator.java:80)
at com.cloudbees.jenkins.plugins.sshcredentials.SSHAuthenticator.newInstance(SSHAuthenticator.java:207)
at com.cloudbees.jenkins.plugins.sshcredentials.SSHAuthenticator.newInstance(SSHAuthenticator.java:169)
at hudson.plugins.sshslaves.SSHLauncher.openConnection(SSHLauncher.java:1212)
at hudson.plugins.sshslaves.SSHLauncher$2.call(SSHLauncher.java:711)
at hudson.plugins.sshslaves.SSHLauncher$2.call(SSHLauncher.java:706)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)
This is related to JENKINS-26379 but the comments in that ticket only hint at the issue which, to writ, is:
https://github.com/jenkinsci/trilead-ssh2/blob/master/src/com/trilead/ssh2/crypto/digest/MAC.java
com.trilead.ssh2.crypto.digest/MAC.java
public final static String[] getMacList()
{
/* Higher Priority First */
return new String[] { "hmac-sha1-96", "hmac-sha1", "hmac-md5-96", "hmac-md5" };
}
See https://stribika.github.io/2015/01/04/secure-secure-shell.html
Here are the available MAC choices:
hmac-md5
hmac-md5-96
hmac-ripemd160
hmac-sha1
hmac-sha1-96
hmac-sha2-256
hmac-sha2-512
umac-64
umac-128
hmac-m...@openssh.com
hmac-md...@openssh.com
hmac-ripe...@openssh.com
hmac-s...@openssh.com
hmac-sha...@openssh.com
hmac-sha...@openssh.com
hmac-sha...@openssh.com
umac-...@openssh.com
umac-1...@openssh.com
The selection considerations:
Security of the hash algorithm: No MD5 and SHA1. Yes, I know that HMAC-SHA1 does not need collision resistance but why wait? Disable weak crypto today.
Encrypt-then-MAC: I am not aware of a security proof for CTR-and-HMAC but I also don't think CTR decryption can fail. Since there are no downgrade attacks, you can add them to the end of the list. You can also do this on a host by host basis so you know which ones are less safe.
Tag size: At least 128 bits. This eliminates umac-64-etm.
Key size: At least 128 bits. This doesn't eliminate anything at this point.
Recommended /etc/ssh/sshd_config snippet:
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