SSH connection closed by <HOST> error

[Cubic PPA]

When trying to ssh to a remote computer whose OS was installed using a Cubic generated ISO, the remote computer does not allow SSH connections.

The user receives the error:
    SSH connection closed by TEST error

(Where TEST is the remote computer).

Additionally, running `systemctl status sshd` on the remote computer gives the following...

$ systemctl status sshd
● ssh.service - OpenBSD Secure Shell server
   Loaded: loaded (/lib/systemd/system/ssh.service; enabled; vendor preset: enabled)
   Active: active (running) since Fri 2019-04-19 14:33:48 EDT; 13min ago
     Docs: man:sshd(8)
           man:sshd_config(5)
  Process: 840 ExecStartPre=/usr/sbin/sshd -t (code=exited, status=0/SUCCESS)
 Main PID: 857 (sshd)
    Tasks: 1 (limit: 4915)
   Memory: 4.2M
   CGroup: /system.slice/ssh.service
           └─857 /usr/sbin/sshd -D

Apr 19 14:33:48 TEST systemd[1]: Starting OpenBSD Secure Shell server...
Apr 19 14:33:48 TEST systemd[1]: Started OpenBSD Secure Shell server.
Apr 19 14:34:13 TEST sshd[2243]: error: Could not load host key: /etc/ssh/ssh_host_rsa_key
Apr 19 14:34:13 TEST sshd[2243]: error: Could not load host key: /etc/ssh/ssh_host_ecdsa_key
Apr 19 14:34:13 TEST sshd[2243]: error: Could not load host key: /etc/ssh/ssh_host_ed25519_key
Apr 19 14:34:13 TEST sshd[2243]: fatal: No supported key exchange algorithms [preauth]
Apr 19 14:38:02 TEST sshd[3939]: error: Could not load host key: /etc/ssh/ssh_host_rsa_key
Apr 19 14:38:02 TEST sshd[3939]: error: Could not load host key: /etc/ssh/ssh_host_ecdsa_key
Apr 19 14:38:02 TEST sshd[3939]: error: Could not load host key: /etc/ssh/ssh_host_ed25519_key
Apr 19 14:38:02 TEST sshd[3939]: fatal: No supported key exchange algorithms [preauth]

[Cubic PPA (cubic-wizard)]

The solution is to run

    $ sudo ssh-keygen -A

on the computer where the OS was installed using a Cubic generate ISO.

[Cubic PPA (cubic-wizard)]

Per the fix for Bug #1824715, Cubic no longr copies the ssh_host keys in /etc/ssh to the customized linux file system. These keys are needed by remote computers to make ssh connections. When openssh-server is installed (in Cubic' chroot environment) using `apt openssh-server` I think new keys are generated, and these new keys should be copied to the customized Linux file system.

[Cubic PPA (cubic-wizard)]

Fixed in release revision 53.
Fixed in trunk revision
Fixed by reverting the change for Bug #1824715.

When openssh-server is installed in the chroot environment, the ssh_host keys that are generated use the name of the machine Cubic is running on.

These new keys get copied to the remastered ISO. This can be a security risk, because every machine that the ISO is installed on, will have the same ssh_host keys.

However, without these keys on the ISO, it is not possible to ssh into a computer running a remastered Live ISO, without first generating the ssh_host keys using `sudo ssh-keygen -A`. Also, it is not possible to ssh into a computer that had the OS installed a remastered Live ISO, without first generating the ssh_host keys using `sudo ssh-keygen -A`.

Both of these situations can be addressed by generating the missing keys (`sudo ssh-keygen -A`). The issue arises when the Live ISO or computer with a new OS installed from a remastered Live ISO does not allow a user to log in or enter a recovery mode. In this case, the only option to fix the uissue would be to ssh into the computer from a remote machine. Unfortunately, SSH will not work. As a result the computer can not be repaired.

In order to overcome this dilemma, Bug #1824715 has been reverted.

To address the security concern, the user should generate new ssh_host keys using `sudo ssh-keygen -A` after installing the OS from a remastered Live ISO on each new machine. If the user is not installing openssh-server, then the user should manually delete the host keys (in /etc/ssh) in Cubic's chroot environment during the customization process.