Script to unlock an Ubuntu or Debian encrypted root filesystem via ssh.

To unlock the target system (after initial setup; see below):

1. Run the following from your workstation:

   unlock-cryptroot target.example.com

2. Enter the password to your ssh key (if there is one).
3. Enter the drive encryption password(s).

For additional options and default file locations, run:

unlock-cryptroot --help

Tested on:

• Ubuntu 18.04 (Bionic)
• Ubuntu 16.04 (Xenial)
• Ubuntu 14.04 (Trusty)

Relevant bug reports:

• Ubuntu bug #595648
• Debian bug #782024

1. Edit /etc/crypttab and add the initramfs option to each device you want to be able to remotely unlock during boot. See man 5 crypttab for details. (The initramfs option is not necessary for the root device or any resume devices, but it doesn't hurt.) Update your initramfs after making any changes (sudo update-initramfs -u).
2. Install the dropbear ssh server into the initramfs:

   sudo apt-get install dropbear-initramfs

3. If you wish to use a non-default IP address or network device, set the ip= kernel boot parameter:
   1. Edit /etc/default/grub
   2. Add your ip= parameter to the GRUB_CMDLINE_LINUX variable
   3. Save your changes
   4. Run sudo update-grub to install the changes
4. Prepare keys for public key authentication:
   1. Generate an ssh key pair for logging in to the initramfs:

      sudo sh -c '(umask 0077 && mkdir -p /etc/initramfs-tools/root/.ssh)'
      sudo ssh-keygen -t rsa -b 4096 -o -a 100 \
          -f /etc/initramfs-tools/root/.ssh/id_rsa

   2. Add the public key to the initramfs's authorized_keys:

      sudo cp /etc/initramfs-tools/root/.ssh/id_rsa.pub \
          /etc/initramfs-tools/root/.ssh/authorized_keys

   3. Update the initramfs:

      sudo update-initramfs -u

5. At boot, Ubuntu's initramfs will create a Netplan config file for your network interface. This config file overrides the default network configuration for that interface. If you do not want this override, install an initramfs script that deletes the Netplan config file after your system is unlocked. See the example etc_* file.

1. Edit /etc/crypttab and add the initramfs option to each device you want to be able to remotely unlock during boot. See man 5 crypttab for details. (The initramfs option is not necessary for the root device or any resume devices, but it doesn't hurt.) Update your initramfs after making any changes (sudo update-initramfs -u).
2. Install the dropbear ssh server into the initramfs:

   sudo apt-get install dropbear-initramfs

3. If you wish to use a non-default IP address or network device, set the ip= kernel boot parameter:
   1. Edit /etc/default/grub
   2. Add your ip= parameter to the GRUB_CMDLINE_LINUX variable
   3. Save your changes
   4. Run sudo update-grub to install the changes
4. Prepare keys for public key authentication:
   1. Generate an ssh key pair for logging in to the initramfs:

      sudo sh -c '(umask 0077 && mkdir -p /etc/initramfs-tools/root/.ssh)'
      sudo ssh-keygen -t rsa -b 4096 -o -a 100 \
          -f /etc/initramfs-tools/root/.ssh/id_rsa

   2. Add the public key to the initramfs's authorized_keys:

      sudo cp /etc/initramfs-tools/root/.ssh/id_rsa.pub \
          /etc/initramfs-tools/root/.ssh/authorized_keys

   3. Update the initramfs:

      sudo update-initramfs -u

1. If you have one or more non-root non-resume partitions that you want to be able to remotely unlock:
   1. Run blkid to get the UUID of each such partition.
   2. Edit /etc/initramfs-tools/conf.d/resume and add a new RESUME=UUID=<uuid> line for each UUID at the top of the file. The last RESUME= line must refer to your resume device. The result should look like this:

      RESUME=UUID=<uuid of non-root non-resume device #1>
      RESUME=UUID=<uuid of non-root non-resume device #2>
      RESUME=UUID=<uuid of resume device>
      

   3. Update the initramfs:

      sudo update-initramfs -u

   By default, initramfs only attempts to unlock the root device and any resume devices. Adding the UUIDs of non-root non-resume devices tricks initramfs into also unlocking those devices. This hack is not needed in Ubuntu 16.04 (Xenial) or later thanks to a new initramfs crypttab option added in Ubuntu 16.04 (Xenial).
2. Install dropbear into the initramfs:

   sudo apt-get install dropbear

3. Set the ip= kernel boot parameter:
   1. Edit /etc/default/grub
   2. Add your ip= parameter to the GRUB_CMDLINE_LINUX variable
   3. Save your changes
   4. Run sudo update-grub to install the changes
4. The kernel ip= parameter conflicts with the system's normal networking configuration, so you must set up a script to deconfigure the interface after the drive is unlocked but before the normal networking configuration is applied. See the example etc_* file.

1. Copy the ssh private key for the target machine's initramfs to the machine that will be doing the remote unlocking:

   T=target.example.com # change as necessary
   scp root@"$T":/etc/initramfs-tools/root/.ssh/id_rsa \
       ~/.ssh/id_rsa.initramfs_"$T"