4.3 KiB
SSH Pubkey Authentication
To start with, you’ll want to secure SSH a little more than normal.
I highly recommend this article, which we’ll be following in this process.
First, create a file: <basedir>/overlay/etc/ssh/sshd_config using the following. Comments and blank lines have been stripped out for brevity.
PermitRootLogin prohibit-password HostKey /etc/ssh/ssh_host_ed25519_key HostKey /etc/ssh/ssh_host_rsa_key AuthorizedKeysFile .ssh/authorized_keys PasswordAuthentication no PermitEmptyPasswords no ChallengeResponseAuthentication no UsePAM yes PrintMotd no # pam does that Subsystem sftp /usr/lib/ssh/sftp-server KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256 Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com
We’ll also want to implement a more secure ssh_config file to avoid possible leaks. The following is <basedir>/overlay/etc/ssh/ssh_config:
Host *
KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256
PasswordAuthentication no
ChallengeResponseAuthentication no
PubkeyAuthentication yes
HostKeyAlgorithms ssh-ed25519-cert-v01@openssh.com,ssh-rsa-cert-v01@openssh.com,ssh-ed25519,ssh-rsa
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com
We’ll want to create our own moduli. This can take a long time, but only needs to be done once — it doesn’t need to be done for every build. The following commands should be run in <basedir>/overlay/etc/ssh/:
ssh-keygen -G moduli.all -b 4096 ssh-keygen -T moduli.safe -f moduli.all mv moduli.safe moduli rm moduli.all
Then we generate hostkeys. This isn’t strictly necessary as the live media will create them automatically when starting SSH if they’re missing, but this does provide some verification that the host you’re SSHing to is, in fact, running the BDisk instance that you yourself built. The following commands should be run in <basedir>/overlay/etc/ssh/:
ssh-keygen -t ed25519 -f ssh_host_ed25519_key -N "" < /dev/null ssh-keygen -t rsa -b 4096 -f ssh_host_rsa_key -N "" < /dev/null
Make sure you have keys on your host workstation generated so you can SSH into BDisk. If you don’t have any ED25519 or RSA SSH keys, this will create them for you. The following should be run as the host (build machine, or what have you) user you want to be able to SSH into BDisk as:
ssh-keygen -t ed25519 -o -a 100 ssh-keygen -t rsa -b 4096 -o -a 100
The defaults are fine. Adding a password to your private key is not necessary, but recommended (though note that doing so will inhibit automated SSHing). You should now have in ~/.ssh/ the following files (assuming you kept the defaults above):
id_ed25519 id_ed25519.pub id_rsa id_rsa.pub
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Warning
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The files ending in .pub are public — they can be published anywhere. However, the ones that are not appended with .pub are your private keys and should not be shared with anyone, whether they’re password-protected or not! |
Now you’ll want to get the public key of your SSH keys so you can add them to your BDisk build. The following commands should be run in <basedir>/overlay/:
mkdir -p root/.ssh
chmod 700 root/.ssh
touch root/.ssh/authorized_keys
chmod 600 root/.ssh/authorized_keys
cat ~/.ssh/id_{ed25519,rsa}.pub > root/.ssh/authorized_keys
If you decided to enable a regular non-root user in your build, you’ll want to perform the same steps above for the regular user as well (or forego the above and just enable SSH for the user you create). Remember to replace root/ with home/<username>/!
Lastly, we need to enable SSH to start on boot. Run the following command in <basedir>/overlay/etc/systemd/system/multi-user.target.wants/:
ln -s /usr/lib/systemd/system/sshd.service sshd.service
You should now have SSH automatically start once the instance boots.