I am using cloud-init to set up a Raspberry Pi with Ubuntu Server 22.04.1. I would like to set the device's SSH keys so I can connect without being vulnerable to a man-in-the-middle attack.

My current user-data is:

#cloud-config

users:

- name: 'foo'

  groups: users,adm,dialout,audio,netdev,video,plugdev,cdrom,games,input,gpio,spi,i2c,render,sudo

  shell: /bin/bash

  lock_passwd: true

  ssh_authorized_keys:

    - 'ssh-ed25519 ...'

  ssh_keys:

    ed25519_private: |

      -----BEGIN OPENSSH PRIVATE KEY-----

      ...

      -----END OPENSSH PRIVATE KEY-----

    ed25519_public: ...

When I go to ssh into the device, its ECDSA fingerprint changes every time. (i.e. if I re-image the drive and let cloud-init run again, then go SSH in, the fingerprint is different. It never matches the fingerprint of the keys in ed25519_private/public from the user-data.) So I think it's generating new keys on boot, and I'm at a loss for how to stop that.

How can I set the device's SSH keys?

Update (per @muru):

The host key section of the cloud-init docs includes a few relevant entries:

• Theoretically if you specify ssh_keys then no new keys will be generated. I'm not sure what I'm doing wrong that this is not the case.


• I've tried putting ssh_deletekeys: false under ssh_keys, the user object, and at the global level. SSH'ing in still produces new key fingerprints each time.


• I've tried putting ssh_genkeytypes: [] under ssh_keys, the user object, and at the global level. Sometimes (I didn't record which) this results in me being unable to ssh in at all, presumably because no keys are present. In no case did I see the correct key fingerprint (as derived from ed25519_private).


• Some entries let me import public keys to my authorized keys, publish keys, suppress keygen output or enable/disable root login. I'm not interested in any of those things.


• ssh_keys is the only section that claims to let me set the private keys of the host


• No other entry in this section seems to let me control whether keys are generated, or specify private host keys to use. Maybe I'm misreading something?

Lastly, I just did an ssh-keyscan <host> | ssh-keygen -lf - and saw that, apparently, my device is generating an ECDSA key in addition to an ed25519 key:

$ ssh-keyscan <host> | ssh-keygen -lf -

# <host>:22 SSH-2.0-OpenSSH_8.9p1 Ubuntu-3

# <host>:22 SSH-2.0-OpenSSH_8.9p1 Ubuntu-3

# <host>:22 SSH-2.0-OpenSSH_8.9p1 Ubuntu-3

256 SHA256:T50YAWwIgIOsQNuIXGBpoz1xPFXJkzffafibruuABtQ <host> (ECDSA)

256 SHA256:DB00DS6xzx5v7ZdVBe+z4nLZhOGVrKuSdhwdenhAm4s <host> (ED25519)

(neither signature matches the signature of ed25519_private)