Could not load host key: /etc/ssh/ssh_host_ed25519_key in /var/log/auth.log

Question

I have an Ubuntu 14.04 instance running on AWS.

I was browsing the auth logs (/var/log/auth.log*), and noticed the following line appearing:

Could not load host key: /etc/ssh/ssh_host_ed25519_key

This line appears between 1k-10k times a day, presumably due to failed hack attempts.

In researching this problem, I came up with a number of questions.

What is the effect of the absence of this key? I can still log in fine with the key provided by AWS. Is it forcing the security of my SSH connections to be downgraded? Has it been disabled on purpose because of security concerns?

I've seen some people suggest that the key wasn't generated correctly during installation, and that I need to regenerate the key with dpkg-reconfigure or ssh-keygen.

Alternatively, I've also seen people recommend just commenting out the line: HostKey /etc/ssh/ssh_host_ed25519_key in /etc/ssh/sshd_config

Answer 1

You can generate the missing hostkey via:

ssh-keygen -A

ed25519 is a bit faster and more secure. It's not necessary and, aside from logspam, this doesn't have that much effect. You could, indeed, comment out HostKey /etc/ssh/ssh_host_ed25519_key, without much effect. But, at the same time, `ed25519 is a good host-key format to support.