Problems with creating an SSH CA, attempting to following the tutorial as a guide

  • Posted on February 12, 2015
  • beltorakAsked by beltorak

Hi; I am attempting to follow the tutorial posted here as a guide.

I’ve created three LXC machines to practice with: ssh-ca, ssh-server, and ssh-client.

On ssh-ca I self-signed the “ssh_host_ed25519_key” yielding “”.

I transferred the ssh-server “” to the ssh-ca machine as “”, signed it with the ssh-ca’s “ssh_host_ed25519_key” yielding “”, and transferred this back to ssh-server as “”. I edited ssh-server’s “sshd_config” to add HostCertificate /etc/ssh/ and restarted the ssh-server sshd daemon (service ssh restart).

I transferred the ssh-ca “” to ssh-client and stuck it in ~/.ssh/known_hosts file as (roughly)

@cert-authority * AAAAIHN ... QFRDQ==  root@trusty-builds

However, attempting to log in from ssh-client to ssh-server (abbreviated ssh -v output):

debug1: Server host key: ED25519-CERT 85:46:34:5f:93:4c:d0:f5:c3:a4:14:87:38:b9:41:7d
debug1: ssh_ed25519_verify: signature correct
debug1: No matching CA found. Retry with plain key
The authenticity of host ' (' can't be established.
ED25519 key fingerprint is 85:46:34:5f:93:4c:d0:f5:c3:a4:14:87:38:b9:41:7d.
Are you sure you want to continue connecting (yes/no)?

What am I doing wrong? Is * a valid hostname wildcard? Is it possible I messed up one of my signing commands? How could I verify that the ssh-server cert is signed by the ssh-ca key?


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Accepted Answer

The ssh client tries to validate the hostname given in the certificate the server sent. The hostname had to be given after the -n switch, when signing the server’s public key.

ssh-keygen -s server_ca -I host_host1 -h -n -V +52w /etc/ssh/

If you have forgotten the name, you can also see it in the debug messages when you try to connect to the server via ssh with -vv. (I had the same problem…)

debug1: Server host certificate: SHA256:...
debug2: Server host certificate hostname:
debug1: No matching CA found. Retry with plain key
The authenticity of host ' (' can't be established.

When trying to connect to the host via IP, one has to give the hostname in the options:

ssh -o root@

* is a valid hostname wildcard.

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