Remote Access to ZFSBootMenu

Having SSH access to ZFSBootMenu can be critical because it allows some measure of recovery over a remote connection. If your boot environments reside in encrypted filesystems, SSH access is necessary if you ever intend to reboot a machine when you are not physically present. Because ZFSBootMenu supports Dracut and mkinitcpio, any mechanism that can provide remote access to a Dracut or mkinitcpio initramfs will work.

Enabling Network Access

Dracutmkinitcpio

The dracut-crypt-ssh module provides a straightforward approach to configuring and launching an SSH server in Dracut images. This module is packaged on several distributions, but if you run a distribution that does not package dracut-crypt-ssh, you will need to track down its dependencies: the dracut-network module for network access and dropbear for the SSH server; other prerequisites are probably already installed on your system.

Note

The dracut-crypt-ssh module comes with a few helper utilities in the module/60crypt-ssh/helper directory that are designed to simplify providing passwords and snooping console output so that you can interact with unlock processes that are already running in the initramfs. These components are not required for ZFSBootMenu and do not provide a lot of value. If you have no problems installing the module as intended, it is okay to leave the helpers installed. If your distribution has trouble compiling the helpers, just copy the contents of the 60crypt-ssh directory, except for the helper directory and Makefile, to the modules directory for Dracut. This will most likely be /usr/lib/dracut/modules.d/60crypt-ssh.

If you do not install the contents of helper, you may wish to edit the module-setup.sh script provided by the package to remove references to installing the helper. At the time of writing, these references consist of the last four lines (five, if you count the harmless comment) of the install() functioned. Removing these lines should not be critical, as Dracut should happily continue the initramfs creation process even if those installation commands fail.

If you use Dracut to produce the initramfs images in your boot environment, you may wish to disable the crypt-ssh module in those images. Just add:

omit_dracutmodules+=" crypt-ssh "

to a configuration file in /etc/dracut.conf.d. The configuration file must have a .conf extension to be recognized; see dracut.conf(5) for more information.

To inform dracut-network that it must bring up a network interface, pass the kernel command-line parameters ip=dhcp and rd.neednet=1 to your ZFSBootMenu image. If you use another boot loader to start ZFSBootMenu, e.g. rEFInd or syslinux, this can be accomplished by configuring that loader. However, it may be more convenient to add these parameters directly to the ZFSBootMenu image:

mkdir -p /etc/cmdline.d
echo "ip=dhcp rd.neednet=1" > /etc/cmdline.d/dracut-network.conf

It is possible to specify a static IP configuration by replacing dhcp with a properly formatted configuration string. Consult the dracut documentation for details about static IP configuration.

There are methods besides writing to /etc/cmdline.d or configuring another boot loader to specify kernel command-line arguments that will configure networking in Dracut. However, Dracut uses the /etc/cmdline.d directory to store "fake" arguments, which it processes directly rather than handing to the kernel. Using other methods (like adding these arguments to the kernel_cmdline Dracut option for a UEFI bundle) can cause the ip=dhcp argument to appear more than once on the kernel command-line, which may cause dracut-network to fail catastrophically and refuse to boot. Writing a configuration file in /etc/cmdline.d is a reliable way to ensure that ip=dhcp appears exactly once to dracut-network.

Unless you've taken steps not described here, the network-enabled ZFSBootMenu image will not advertise itself via dynamic DNS or mDNS. You will need to know the IP address of the ZFSBootMenu host to connect. Thus, you should either configure a static IP address or configure your DHCP server to reserve a known address for the MAC address of the network interface you configured.

Configuring Dropbear

First, install dropbear, if not already installed.

By default, dropbear will generate random host keys for your ZFSBootMenu initramfs. This is undesirable because SSH will complain about unknown keys every time you reboot. If you wish, you can configure it to copy your regular host keys into the image. However, there are two problems with this:

1. The ZFSBootMenu image will generally be installed on a filesystem with no access permissions, allowing anybody to read your private host keys; and

2. The dropbearconvert program may be incapable of converting modern OpenSSH host keys into the required dropbear format.

To create dedicated host keys in the proper format, decide on a location, for example /etc/dropbear, and create the new keys:

mkdir -p /etc/dropbear
for keytype in rsa ecdsa ed25519; do
    dropbearkey -t "${keytype}" -f "/etc/dropbear/dropbear_${keytype}_host_key"
done

Note

The dracut module expects to install RSA and ECDSA keys, so at minimum those keys should be created. The mkinitcpio module supports RSA, ECDSA, and ED25519 keys.

Not all versions of dropbear support ED25519 keys, so it is fine if the ED25519 key fails to generate.

The Dracut and mkinitcpio dropbear modules do not allow for password authentication over SSH; instead key-based authentication is forced. The authorized keys for dropbear can be configured by putting an authorized_keys file at /etc/dropbear/root_key. On a single-user machine, this can be realized by symlinking your user's authorized_keys file:

ln -s "${HOME}/.ssh/authorized_keys" /etc/dropbear/root_key

Dracutmkinitcpio

With critical pieces in place, ZFSBootMenu can be configured to bundle dracut-crypt-ssh in its images. Create the Dracut configuration file /etc/zfsbootmenu/dracut.conf.d/dropbear.conf with the following contents:

# Enable dropbear ssh server and pull in network configuration args
add_dracutmodules+=" crypt-ssh "
install_optional_items+=" /etc/cmdline.d/dracut-network.conf "
# Copy system keys for consistent access
dropbear_rsa_key=/etc/dropbear/ssh_host_rsa_key
dropbear_ecdsa_key=/etc/dropbear/ssh_host_ecdsa_key
dropbear_acl=/etc/dropbear/root_key

Note

The default configuration will start dropbear on TCP port 222. This can be overridden with the dropbear_port configuration option. Generally, you do not want the server listening on the default port 22. Clients that expect to find your normal host keys when connecting to an SSH server on port 22 will refuse to connect when they find different keys provided by dropbear.

Final Steps

With the above configuration complete, running generate-zbm should produce a ZFSBootMenu image that contains the necessary components to enable an SSH server in your bootloader. This can be verified with the lsinitrd tool provided by dracut or the lsinitcpio tool provided by mkinitcpio. (The lsinitcpio tool is not able to inspect UEFI bundles, but lsinitrd can.) In the file listing, you should see keys in /etc/dropbear, the dropbear and ip executables, and the file /root/.ssh/authorized_keys.

After rebooting, ZFSBootMenu should configure the network interface, launch an SSH server and accept connections on TCP port 222 (for Dracut) or TCP port 22 (for mkinitcpio) by default, unless otherwise configured. If your SSH client complains because it finds ZFSBootMenu keys when it expects to find your normal host keys, you may wish to reconfigure dropbear to listen on a non-standard port and re-run generate-zbm.

Accessing ZFSBootMenu Remotely

When you connect to ZFSBootMenu via SSH, you will be presented a simple shell prompt. Launch zfsbootmenu to start the menu interface over the remote connection:

zfsbootmenu

You may then use the menu as if you were connected locally.

Note

Recent versions of ZFSBootMenu automatically set the TERM environment variable to linux. If you are running an older version, your SSH client may have provided a more specific terminal definition that will not be recognized by the restricted environment provided by ZFSBootMenu. Under these circumstances, you may need to run:

export TERM=linux

from the login shell to ensure that basic terminal functionality works as expected.

If you followed the Void Linux ZFSBootMenu install guide and configured rEFInd to launch ZFSBootMenu, you may need to remove the zbm.skip argument from the default menu entry if you would like remote access and you have no encrypted boot environments. Otherwise, rEFInd will attempt to bypass the ZFSBootMenu countdown and your default boot environment will be started immediately if possible. In this case, either set zbm.timeout to a suitably long delay (e.g., 60 sec) to give yourself time to connect and launch ZFSBootMenu remotely before the automatic boot can proceed, or use zbm.show by default to prevent automatic boot and force the local instance to show the interactive menu immediately.

Note

To provide some safety against multi-user conflicts, only one ZFSBootMenu instance is allowed to run at any given time. If you have encrypted boot environments, this will generally not present an issue, because the local instance will always block awaiting passphrase entry before launching the menu instance. Otherwise, the later instance of ZFSBootMenu will wait patiently for the earlier instance to terminate before continuing. If you are certain that the currently running instance is not being actively used, you can interrupt the wait loop by pressing [ESC] and then run:

rm /zfsbootmenu/active

to eliminate the indicator of the other running instance. You may then run zfsbootmenu again to launch the menu.