Problem: You have a computer running Debian 7.0 wheezy with an encrypted root filesystem. At startup you’d like to be able to unlock the
disk over ssh, maybe because it runs headless.
There are some existing solutions to this: early-ssh worked on previous Debian versions. There are other blog posts showing some methods but I found them unwieldy (ie a 35 second delay before you can type your passphrase.)
Here’s how I’m doing it. Unless otherwise mentioned, all of the following commands need to be run as root. The server name in the examples is “debian_headless”
At time of writing I was using versions Debian 7.0 Wheezy with packages cryptsetup 2:1.4.3-4, initramfs-tools 0.109.1 and dropbear 2012.55-1.3.
These steps may work on recent Ubuntu versions as well, please let me know if you try them out.
The very first thing a Debian system runs at startup is a minimal initramfs ramdisk which performs basic initialisation, mounts the main filesystem and starts the rest of the boot process.
This is the stage when an encrypted rootfs gets mounted. We need to customise it so we can ssh in and unlock the rootfs.
apt-get install dropbear initramfs-tools busybox
Dropbear is a slimline SSH server. This package installs into the initramfs and runs during early boot only, allowing you to log in remotely.
Authorise the user(s) you want to be able to login as root and unlock
Hopefully you’re already using SSH Key Authentication, otherwise have a read about it and why it’s a good idea. Set it up for the user(s) on the client systems by having them run
You have to add the public keys for these users to Dropbear’s list of
authorized_keys for login.
For each client user you want to allow to unlock, take their public key file
~/.ssh/id_rsa.pub and append it to
One way of doing it:
scp ~/.ssh/id_rsa.pub myuser@debian_headless:id_rsa.pub ssh myuser@debian_headless sudo sh -c "cat id_rsa.pub >> /etc/initramfs-tools/root/.ssh/authorized_keys" rm id_rsa.pub
Alternative: Have a unique key for root/unlock
If you don’t want to use your usual user-level ssh key and passphrase to provide potential root-level access to your headless machine, generate a separate key (with a different passphrase) and then copy it across to authorized_keys instead:
ssh-keygen -f ~/.ssh/id_rsa_headlessunlock scp ~/.ssh/id_rsa_headlessunlock.pub myuser@debian_headless:id_rsa.pub (etc, as above.)
Add network hardware module
The initramfs doesn’t usually load network hardware modules, so you should add the name of the network hardware module directly to
You can find the module name via sysfs. For example:
$ grep DRIVER /sys/class/net/eth0/device/uevent DRIVER=e1000
So to enable eth0 in this case, you’d add e1000 to
(This may not be necessary on “normal” x86 machines with built-in ethernet, it’s definitely necessary using USB Ethernet on my ARM-based GK802 stick and it’s probably necessary on Raspberry Pi.)
Configure early boot IP Address
Initramfs needs to know the IP address to come up on. You could do this by passing the ‘ip=’ kernel command line parameter, but you can also just set it in an initramfs-tools config file:
/etc/initramfs-tools/conf.d/network_config with contents like this:
<server-ip> can be left blank. Some possible examples:
See the nfsroot kernel docs for full details. Remember if using DHCP that you’ll need a static DHCP entry configured so you can predict what IP your headless machine is going to have on startup.
Annoyance: Interface doesn’t reconfigure
It seems that after the main system boots, the network interface won’t automatically change to whatever is set in
/etc/network/interfaces (because the interface is seen to be already “up” it gets left alone.) To work around this you could put a line ifdown eth0; ifup eth0 in
/etc/rc.local, or just keep the initramfs network settings the same as the ones in
I’ve written a little initramfs-tools hook that generates scripts for easy unlocking of the rootfs.
Download this script file, then read it to make sure you’re not suspicious about what it’s going to do. Then copy it to
/etc/initramfs-tools/hooks/mount_cryptroot and mark it executable:
chmod +x /etc/initramfs-tools/hooks/mount_cryptroot
There’s a section at the top of mount_cryptroot that allows you to set an ALLOW_SHELL variable if you want to be able to drop to a remote root shell for diagnostics. Note that this explicitly allows a remote root shell to anyone with an authorised SSH private key. I can’t guarantee it’s impossible to get to a root shell otherwise, but it’s at least not explicitly welcomed.
Update the initramfs
Update the initramfs so it includes all your changes:
Add client SSH config entry
Because Dropbear has a different SSH host key to the normal system, OpenSSH will (rightly) complain if you try connecting to it on the same IP. The solution is to create a special host definition for the initramfs environment. A custom section in
Host headless_unlock HostName <IP of headless> User root HostKeyAlias headless_unlock
Now you can log in to the unlock stage by simply typing ssh headless_unlock. (Replace headless with whatever name you want to use.)
If you authorized a unique private key to use for unlocking, you can add another line
IdentityFile ~/.ssh/id_rsa_headless or similar.
Putting it all together
Here’s what it looks like to connect to my headless host and unlock the disk:
$ ssh headless_unlock BusyBox v1.20.2 (Debian 1:1.20.0-7) built-in shell (ash) Enter 'help' for a list of built-in commands. Unlocking rootfs... Enter passphrase: Success Connection to 192.168.56.101 closed.
Final word about security
When dealing with remote access and with encryption, it’s always worth thinking about security tradeoffs.
There’s the obvious risk that enabling Dropbear in the initramfs means allowing remote root logins to your system. Even with ALLOW_SHELL turned off there’s no guarantee that someone with access to an authorized SSH key (or Dropbear exploit) can’t log in, pop a root shell somehow, and cause havoc.
In addition, when I was looking for existing techniques to do this I came across a post exclaiming ssh unlocking was a terrible idea, because someone with physical access to your machine can then impersonate you remotely and you’ll never know.
I think this is worth considering, but it’s not as bad as it might seem. All encrypted disk access, with local or remote unlocking, relies on physical security of your device. Yes, if someone gets physical or root access they can steal your Dropbear host key and set up another server that pretends to be you and steals your keys. However, they already have physical or root access. They can just as easily insert malware in the initramfs that quietly captures your passphrase as you type it in in locally.
In either case, you have to be sufficiently sure noone is messing with you. With remote access that’s maybe more likely, but I’m not too worried. I’m using this on a USB stick PC that fits in my pocket, and I don’t want a burglar walking out my door with all my personal data. If some very dedicated person comes after my data, there would be easier ways to get it.
(Also, an obvious disclaimer – all the advice/scripts on this page is worth what you paid for it. I’m not a security professional. Please tell me if it seems broken/unsafe, but don’t blame me if you use it for something important and it isn’t safe enough.)