The crypt-ssh dracut module allows remote unlocking of systems with full disk encryption via ssh.
There are a number of reasons why you would want to do this:
- It provides a way of entering encryption keys for a number of servers without console switching
- It allows booting of remote or co-located encrypted servers without console access
Users are strictly authenticated using their SSH public keys. These can be either:
/root/.ssh/authorized_keys or a custom file (dropbear_acl option). Depending
on your environment, it may make sense to make the preboot authorized_keys file
different from the normal one.
Plain text password authentication and port forwarding are disabled.
When possible, installation via distribution packages is a convenient way to
install dracut-crypt-ssh. Please contact us via
GitHub issues
if you are able to provide packages for other distributions and would like
brief instructions for your distribution included here.
-
Void Linux provides an official package:
xbps-install dracut-crypt-ssh
-
Gentoo provides a package in Portage:
emerge sys-kernel/dracut-crypt-ssh
-
Arch Linux provides packages in the AUR for both tagged releases and the git HEAD.
-
An unofficial COPR repository provides packages for Fedora 36+:
dnf copr enable uriesk/dracut-crypt-ssh dnf install dracut-crypt-ssh
Manual installation of dracut-crypt-ssh requires the following packages at
run time:
- Dropbear SSH
- OpenSSH
- Dracut
and its
dracut-networkmodule
When building, the following additional packages are required:
- util-linux and
its
libblkidcomponent, including headers (e.g.,libblkid-devel) - which
- A C compiler, probably GCC
Retrieve a copy the source, for example via git with
git clone https://github.com/dracut-crypt-ssh/dracut-crypt-ssh.gitWithin the source directory, configure and install the package
./configure
make
make installThe make install command probably needs to be run as root.
After the first installation and every time you update the dracut-crypt-ssh configuration,
it is required to rebuild the initramfs:
# dracut --force
You will need to adjust your boot loader to configure network access for your
initramfs. The kernel and initramfs should be booted with the kernel
command-line arguments rd.neednet=1 and an appropriate ip= argument for
your network. For DHCP configuration,
rd.neednet=1 ip=dhcp
should be sufficient. For static configuration, use something like
rd.neednet=1 ip=192.168.0.100::192.168.0.1:255.255.255.0::eth0:off
Refer to the network documentation of dracut
for more options (man dracut.cmdline).
For GRUB users, the kernel command-line often can be set in /etc/default/grub
by appending the necessary arguments to the end of the GRUB_CMDLINE_LINUX
variable:
/etc/default/grub:
...
GRUB_CMDLINE_LINUX="... rd.neednet=1 ip=dhcp"
...
Afterwards, regenerate your GRUB config:
# grub2-mkconfig --output /etc/grub2.cfg
When rebooting the system, dropbear sshd is started by the initramfs. You should be able to login and unlock the volumes:
% ssh -p 222 root@192.168.0.100
You can use the console_peek command to see what's currently showing on the
console and the console_auth command to input a passphrase that will be sent
to the console.
If unlocking the device succeeded, the initramfs will clean up itself and dropbear terminates itself and your connection.
The unlock binary reads a passphrase from stdin, parses /etc/crypttab
and attempts to call cryptsetup luksOpen on all luks-encrypted drives that
don't have a keyfile, passing the passphrase that unlock got in stdin to luksOpen.
What this means in practice is you can do:
% ssh root@remote.server -p 222 unlock < passwordFile
or:
% gpg -d password.gpg | ssh root@remote.server -p 222 unlock
If you want to only unlock specific drives / LUKS volumes, you can provide wildcards on the command line, e.g.
% ssh root@remote.server -p 222 unlock luks-3467c luks-34c13
unlock will search the crypttab for mapper names (first column in
/etc/crypttab) that start with the listed names. Volumes that match via this
method may have a keyfile listed in /etc/crypttab, it will be assumed that you
want to unlock the volume/s with an alternative key.
Note that the names provided are really wildcards, and by convention/default
all mappers start with luks-, so you can force unlock to try all drives simply
by doing something like 'unlock luks-'.
In all cases, unlock will only consider the process a success only if all
eligible volumes are unlocked successfully. This means:
- All the associated devices must be available at boot / unlock time
- The passphrase must be accepted for all eligible volumes
- cryptsetup luksOpen should not exit for any other reason.
In short, if you have more than one volume in /etc/crypttab, you will need to
be careful about how use this tool.
If the process is successful, unlock will launch the script
/sbin/unlock-reap-success. This will attempt to kill systemd-cryptsetup, and
failing that, attempt to kill cryptroot-ask. On RHEL6 & 7, this aborts the
builtin decrypt password request processes and allows the boot process to
proceed. Note that the plymouth splash screen on RHEL6 (if you happen to be
watching the console...) will still appear to ask for your password, but this
is an artifact. Disable plymouth (rhgb command line) if this annoys you.
You might want to limit access only to the unlock binary, just add command="unlock" to your authorized_keys before the key, e.g.
command="unlock" ssh-rsa .....
The configuration is stored in the crypt-ssh.conf, usually located in /etc/dracut.conf.d/.
The following options are available (see the config file for detailed description):
-
dropbear_port(default:222) - port ssh daemon should listen on -
dropbear_keytypes(default:rsa ecdsa ed25519) - A space-separated list of the SSH key types which will be installed in the initramfs -
dropbear_rsa_key,dropbear_ecdsa_key,dropbear_ed25519_key(default:GENERATE) - Source of the keys, possible options:SYSTEM- copy the private keys from the encrypted system (not recommended)GENERATE- generate a new keys (during the creation of initramfs)- path - key file in OpenSSH format as generared by ssh-keygen (a public file with '.pub' ending must be present too)
If any key type is not included in
dropbear_keytypes, the correspondingdropbear_<type>_keyvariable is ignored -
dropbear_acl(default:/root/.ssh/authorized_keys) - Keys which allowed to login into initramfs
After any configuration change, you have to rebuild the initramfs as the configuration takes effect during the building the initramfs.
By default, dracut-crypt-ssh generates an SSH key whenever the image is built
(GENERATE), which either creates administrative overhead or weakens the
security of the SSH connection as keys will be regenerated transparently during
system updates. It is highly recommended to generate SSH keys specifically
for dracut-crypt-ssh and validate these keys during the initial connection.
The following steps should give you an idea how to set this up. You can change
the directory as you wish. Keep these SSH keys safe, but also keep in mind that
they will be copied to the initramfs on the unencrypted boot partition (where
they may be extracted or changed).
# umask 0077
# mkdir /root/dracut-crypt-ssh-keys
# ssh-keygen -t rsa -m PEM -f /root/dracut-crypt-ssh-keys/ssh_dracut_rsa_key
# ssh-keygen -t ecdsa -m PEM -f /root/dracut-crypt-ssh-keys/ssh_dracut_ecdsa_key
# ssh-keygen -t ed25519 -m PEM -f /root/dracut-crypt-ssh-keys/ssh_dracut_ed25519_key
Point to these keys in the configuration /etc/dracut.conf.d/crypt-ssh.conf:
dropbear_rsa_key="/root/dracut-crypt-ssh-keys/ssh_dracut_rsa_key"
dropbear_ecdsa_key="/root/dracut-crypt-ssh-keys/ssh_dracut_ecdsa_key"
dropbear_ed25519_key="/root/dracut-crypt-ssh-keys/ssh_dracut_ed25519_key"
Remember regenerate the initramfs after this step:
# dracut --force
If things don't work as expected, there are a few ways to find out what is going, get help or report an issue.
With or without crypt-ssh installed, dracut should always prompt for a LUKS password and boot properly when the password is entered. If booting with an interactive password does not work, you need to fix that first: Ensure that the LUKS UUID configured in GRUB and the crypttab in the initramfs are up to date and all modules needed to mount the root filesystem are present in the initramfs.
Refer to the crypto LUKS documentation of dracut
(man dracut.cmdline) and your distribution documentation or help channels.
If you cannot reach the crypt-ssh host via SSH, but you still have interactive console access, you can (re)boot it with a dracut breakpoint.
When GRUB presents the boot options, hit the e key to edit boot options
and add rd.break=pre-mount to the boot options. Remove rhgb and quiet
from the kernel command line, if they are present. Hit Ctrl-x to boot
with the manual configuration and type your LUKS passphrase via the console.
Before mounting the root filesystem, dracut will drop you into a shell.
Is your network adapter present in ip a? If not, does your network adapter
need a module (check lsmod and dmesg | grep net)? Do you have an IP address
configured? Can you use dhclient to acquire an IP address? Is dropbear
running (ps aux)?
If the network adapter (module) is missing, rebuild the initramfs (dracut -f).
If your network configuration is missing, it's probably a configuration
issue. Refer to the Usage section for networking parameters. If the network
comes up, but dracut does not load, that's probably a bug in dracut-crypt-ssh.
Please report it.
If (or rather "when") something goes wrong and you can't access just-booted machine over network and can't get to console (hence sshd in initramfs), don't panic - it's fixable if the machine can be rebooted into some rescue system remotely.
Usually it's some dhcp+tftp netboot thing from a co-located machine (good idea to setup/test in advance) plus whoever is there occasionally pushing the power button, or maybe some fancy hw/interface for that (e.g. hetzner "rescue" interface).
To see what was going on during initramfs, open "modules.d/99base/rdsosreport.sh" in dracut, append this (to the end):
set -x
netstat -lnp
netstat -np
netstat -s
netstat -i
ip addr
ip ro
set +x
exec >/dev/null 2>&1
mkdir /tmp/myboot
mount /dev/sda2 /tmp/myboot
cp /run/initramfs/rdsosreport.txt /tmp/myboot/
umount /tmp/myboot
rmdir /tmp/myboot
Be sure to replace /dev/sda2 with whatever device is used for /boot, rebuild
dracut and add rd.debug to cmdline (e.g. in grub.cfg's "linux" line).
Upon next reboot, wait for at least a minute, since dracut should give up on trying to boot the system first, then it will store full log of all the stuff modules run ("set -x") and their output in "/boot/rdsosreport.txt".
Naturally, to access that, +1 reboot into some "rescue" system might be needed.
In case of network-related issues - e.g. if "rdsosreport.txt" file gets created
with "rd.debug", but host can't be pinged/connected-to for whatever reason -
either enable "debug" dracut module or add dracut_install netstat ip line to
install() section of "modules.d/60dropbear-sshd/module-setup.sh" and check
"rdsosreport.txt" or console output for whatever netstat + ip commands above
(for "rdsosreport.sh") show - there can be no default route, whatever interface
naming mixup, no traffic (e.g. unrelated connection issue), etc.
If you suspect a bug in the software, please report it via our issue tracker.
The integrity, confidentiality and authenticity of your encrypted data relies on the physical integrity of your device. If someone else has access to the device that you are unlocking, it is entirely possible to replace the executable files handling your key material, or steal your initramfs's SSH private keys. Arguably, this kind of attack is possible without the "crypt-ssh" module, but using automated or remote access could make such an attack easier to conceal. If this is a concern for you, consider keeping your devices offline and on your person. If this is not a concern for you, i.e., you place a certain amount of trust in your hosting provider or physical integrity, this tool might still protect against accidental data leaks (i.e. VM deprovisioning, replaced hard disks).