Cryptsetup SSH unlocker

Utility for unattended remote unlock of LUKS encrypted LVM/root disk partition using SSH and cryptsetup. Periodically polls defined servers for open SSH port, then tries to unlock the server using cryptsetup. Requires dropbear or any other SSH server which could be run from initial ramdisk.

Is it secure?

This utility should run from independent server (possibly VPS), thus separating all passphrases and SSH keys from the servers being unlocked. Server authentication is performed during the connection process against a known_hosts file.

Server will be unlocked only when SSH is available on the specified IP address and port and if the fingerprint in the known_hosts file matches. You should always use IP addresses in the host configuration rather than domain names to limit the attack possibility after compromising a DNS server.

Please also note that the server host keys are always stored on an unencrypted partition, so this script won't protect you against an attack with both physical access to the server and the IP address used for unlocking. To further limit the attack possibility, you should use monitoring and possibly disable SSH unlocker in the case of unexpected behavior.

Other possible security implications and solutions how to prevent them are beyond the scope of this documentation.

Requirements

Python 3.5 and higher is required for the installation, because the async/await syntax is used.

Installation and running

$ pip install ssh-unlocker

You should be able to run the utility by using ssh-unlocker. Run ssh-unlocker -h for the list of available options.

$ ssh-unlocker -h
usage: ssh-unlocker [-h] [-c CONFIG] [-v] [--logfile LOGFILE]

Cryptsetup SSH server unlocker. This utility is repeatedly polling configured
servers and tries to unlock the encrypted root partition using cryptsetup once
the SSH connection is available.

optional arguments:
  -h, --help            show this help message and exit
  -c CONFIG, --config CONFIG
                        Path to config file - defaults to config.ini
  -v, --verbose         Increase verbosity level to DEBUG
  --logfile LOGFILE     Path to log file. By default the log messages are
                        printed to stderr

Configuration

By default the configuration is read from config.ini configuration file, but this can be specified as a parameter. Server configuration should be specified using respective [server-identifier] sections. Unspecified parameters are inherited from the [DEFAULT] section.

Configuration attributes

• host – SSH server domain or IP address
• port – SSH server port. It's recommended to run the dropbear SSH daemon for unlocking on a port that is closed on a running (unlocked) server. This way the connection will be closed before attempting SSH authentication and you'll avoid spamming logs.
• username – username of a remote user
• ssh_private_key – private SSH key used for authentication (password authentication is not supported for security reasons and should be avoided)
• ssh_private_key_passphrase – passphrase for encrypted SSH private key (unused when blank)
• cryptsetup_passphrase – passphrase for unlocking encrypted disk using cryptsetup

• known_hosts – file with DSA host keys of SSH servers (used for server authentication and MITM attack prevention). This file cannot be empty - you should always provide server keys beforehand for security reasons
• connect_timeout – connection timeout for TCP handshake during port scanning
• ssh_connect_timeout – timeout for connection and SSH auth
• sleep_interval – time between port scanning checks

Example configuration file:

; [server-identifier]
; host = 127.0.0.1
; port = 22
; username = default
; cryptsetup_passphrase = securePassword13!

[DEFAULT]
connect_timeout = 5
ssh_connect_timeout = 5
sleep_interval = 2

port = 22
username = root
ssh_private_key = unlock_key.rsa
ssh_private_key_passphrase =
known_hosts = known_hosts

Starting automatically at system startup

In most scenarios ssh-unlocker should probably start automatically at system startup. You should also make sure, that the unlocker would be restarted in case of any unexpected exceptions. This can be done by a simple shell script or you can use one of more sophisticated solutions described below.

Systemd config

Systemd is already pre-installed in most modern distribution, so using it should be only a matter of adding new configuration file.

Example configuration:

[Unit]
Description=Utility for unattended remote unlock of LUKS encrypted LVM
After=network.target

[Service]
ExecStart=/root/ssh-unlocker/venv/bin/ssh-unlocker
WorkingDirectory=/root/ssh-unlocker/
Restart=always
TimeoutStopSec=60

# try to restart the service indefinitely (RestartSec * StartLimitBurst > StartLimitIntervalSec)
RestartSec=1
StartLimitBurst=20
StartLimitIntervalSec=1

[Install]
WantedBy=multi-user.target

Supervisor config

If you're used to Supervisor or you don't want to use Systemd for some reason, here is a sample configuration file:

[program:cryptsetup-ssh-unlocker]
command=/root/ssh-unlocker/venv/bin/ssh-unlocker --logfile /var/log/ssh-unlocker
directory=/root/ssh-unlocker/

autostart=true
autorestart=true
startsecs=5
stopwaitsecs=60

Server configuration

In order for this utility to work correctly, SSH daemon has to be installed into initial ramdisk. There are some tutorials, how to do that, such as this one for Ubuntu.

Minimal Ubuntu installation

Install dropbear and update configuration

$ apt update
$ apt install dropbear

Edit /etc/initramfs-tools/initramfs.conf and add or replace the following configuration:

DEVICE=eth0
IP=192.168.1.100::192.168.1.1:255.255.255.0::eth0:off
DROPBEAR_OPTIONS="-p 1022"

The format for IP is the following: [host ip]::[gateway ip]:[netmask]:[hostname]::[device]:[autoconf] (please notice double colons ::). SSH port is specified using DROPBEAR_OPTIONS. I strongly recommend using another port than 22.

Update /etc/default/dropbear and change NO_START=1 to NO_START=0

# disabled because OpenSSH is installed
# change to NO_START=0 to enable Dropbear
NO_START=0

DROPBEAR_PORT in the same configuration file is ignored in initial ramdisk phase and has to be specified in initramfs.conf.

Add SSH keys

Add all public keys you would like to use for authentication during the unlock phase to /etc/initramfs-tools/root/.ssh/authorized_keys. Create the file and/or folder(s) if they don't exist already.

Don't forget to add public key of the RSA key that will be used by this utility. You can easily generate new RSA keypair by running ssh-keygen -t rsa -b 4096.

[optional] Replace original host keys

$ dropbearkey -t rsa -f /etc/dropbear/dropbear_rsa_host_key
$ dropbearkey -t dss -f /etc/dropbear/dropbear_dss_host_key

Update ramdisk

$ sudo update-initramfs -u

License

This software is licensed under MIT license.