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I have a system with an unencrypted / partition on Ubuntu 16.04, but with a LUKS encrypted ZFS zpool on 3 partitions. In order for the system to boot properly I want for the LUKS encrypted volumes to be unlocked before ZFS and other services (like database, web, email etc.) start, and this needs to be able to done remotely, through SSH.

With the 3 partitions added to the /etc/crypttab, the system boots and just after the initramfs stage will wait for unlock (and prompt for passwords). However, the usual way of accomplishing LUKS unlock at boot remotely is done through dropbear in initramfs, however because the 3 partitions are not in fstab the system will 'fall through' initramfs so to speak and continue to systemd. This is undesirable in this case as systemd prioritizes crypttab over the OpenSSH or dropbear meaning remote unlocking is disabled.

A dirty hack that works is simply adding a sleep 300 in initramfs, giving you time to login through dropbear and unlock, however this too is undesirable. I see foresee two options to fix this, but am not sure which would be best and I do not know how to implement either:

  • Changhing systemd boot order, to make sure something like networking and OpenSSH are up before crypttab, enabling remote or local unlocking.

  • Having initramfs wait for non-trivial partitions to be unlocked before proceeding to systemd.

  • Just add/modify unit files so that the decryption happens before the rest. – Ignacio Vazquez-Abrams Apr 2 '18 at 20:38
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I am currently setting up such a system, but in debian stretch. And I am doing my experimentation in a VM before I actually set up the physical computer itself. I have a very similar setup working in the VM.

2 disks in zpool mirror for /, but not /boot. /boot is on md0. zpool is on top of LUKS. Using EFI.

root@zstaging:~# cat /proc/1/comm
systemd

root@zstaging:~# lsblk
NAME            MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT
sda               8:0    0   20G  0 disk
+-sda1            8:1    0  256M  0 part
¦ +-md0           9:0    0  256M  0 raid1 /boot
+-sda2            8:2    0  256M  0 part  /boot/efi
+-sda3            8:3    0 19.5G  0 part
¦ +-disk0_crypt 253:0    0 19.5G  0 crypt
+-sda9            8:9    0    9M  0 part
sdb               8:16   0   20G  0 disk
+-sdb1            8:17   0  256M  0 part
¦ +-md0           9:0    0  256M  0 raid1 /boot
+-sdb2            8:18   0  256M  0 part
+-sdb3            8:19   0 19.5G  0 part
¦ +-disk1_crypt 253:1    0 19.5G  0 crypt
+-sdb9            8:25   0    9M  0 part
sdc               8:32   0   20G  0 disk
sr0              11:0    1  1.8G  0 rom

root@zstaging:~# zpool status
  pool: rpool
 state: ONLINE
  scan: none requested
config:

        NAME             STATE     READ WRITE CKSUM
        rpool            ONLINE       0     0     0
          mirror-0       ONLINE       0     0     0
            disk0_crypt  ONLINE       0     0     0
            disk1_crypt  ONLINE       0     0     0

errors: No known data errors

Relevant files and setup:

root@zstaging:~# cat /etc/fstab
UUID=648bfa4b-1b5f-480a-bb26-b3abffb4a6de /boot auto defaults 0 0
PARTUUID=1673f966-173b-4128-84d5-4e8d5810efef /boot/efi vfat defaults 0 1

root@zstaging:~# cat /etc/crypttab
disk0_crypt UUID=26194846-ba49-4e53-ab0b-857b0dad2021 none luks
disk1_crypt UUID=ef44b66a-8706-4be2-bd12-a30d40de9669 none luks

root@zstaging:~# cat /etc/initramfs-tools/conf.d/cryptroot
target=disk0_crypt,source=UUID=26194846-ba49-4e53-ab0b-857b0dad2021,key=none,rootdev
target=disk1_crypt,source=UUID=ef44b66a-8706-4be2-bd12-a30d40de9669,key=none,rootdev

set CRYPTSETUP=y in /etc/cryptsetup-initramfs/conf-hook

# vi /etc/default/grub
replace GRUB_CMDLINE_LINUX="" with GRUB_CMDLINE_LINUX="boot=zfs"
Remove quiet from: GRUB_CMDLINE_LINUX_DEFAULT
Uncomment: GRUB_TERMINAL=console

# update-initramfs -u -k all
# update-grub
# grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=debian --recheck --no-floppy

Everything works great. Get prompt for unlocking the two disks one at a time, and off we go. It is NOT necessary to list any zfs stuff in /etc/fstab. Yes, initramfs etc. WILL complain about /etc/fstab, but it all works fine, so far

I can even remote unlock it

# apt-get install dropbear-initramfs busybox
# vi /etc/dropbear-initramfs/authorized_keys
Paste my SSH pubkey

# chmod 400 /etc/dropbear-initramfs/authorized_keys
# update-initramfs -u

in another machine that has my SSH keys, add a custom section in ~/.ssh/config:
  Host zstaging_unlock
  HostName <ip_of_zstaging>
  User root
  HostKeyAlias zstaging_unlock

to remote unlock zstaging, from remote machine:
$ ssh zstaging_unlock
and follow prompts

HTH

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