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#1 2023-03-04 03:59:33

Registered: 2023-03-04
Posts: 2  

serial console - full disk encryption


I use Devuan for most of my VM work. For servers, I try to keep the install light and minimum. I work mostly in the console, so I usually run my server VMs with serial console output only. More recently I have begun to fully encrypt the whole os, without a separate /boot partition. All went well until I realized that the cryptomount command which is called by Grub, is executed before the serial console initialization and as a result the prompt to enter the passphrase was not being passed to the serial console. I started working to fix the problem only to find out that as far as I could see, there were no simple clear examples of how to solve this specific problem. So here I am trying to detail how I fixed the problem in case this is useful to anyone else.

Cheers, and please feel free to correct/improve my post.


* My setup uses BIOS/MBR, but should also work for UEFI with small tweaks.
* I assume that the reader already knows how to configure grub to display in the serial console.
* Check your grub.cfg carefully for all the modules that need to be preloaded in the core.img ... i.e. luks2 vs luks
* A subsequent grub-install will blow out this configuration and it will no longer work.

I worked out the solution based on information provided by these two links: … ard-layout

----------------- early-grub.cfg ----------------------------------
serial --unit=0 --speed=9600
terminal_output serial
terminal_input serial

cryptomount -u YOUR_UUID

set root=(cryptouuid/YOUR_UUID)
set prefix=/boot/grub
configfile grub.cfg

grub-mkimage -c early-grub.cfg  -o /boot/grub/i386-pc/core.img -O i386-pc biosdisk cryptodisk ext2 gcry_rijndael gcry_sha256 gzio luks part_gpt part_msdos serial terminal configfile keylayouts at_keyboard 
grub-bios-setup -d /boot/grub/i386-pc/ /dev/YOUR_DISK

Last edited by tranquilo (2023-03-05 22:17:20)


#2 2023-03-04 22:34:46

From: Brisbane, Australia
Registered: 2019-07-18
Posts: 596  

Re: serial console - full disk encryption

When I have edits to grub that I want to "stick" from kernel version to new/other kernel version I edit /etc/default/grub.

Each time I do grub-update it uses that template.

I use it for simple stuff, but it may help you as well.

regards, Glenn

pic from 1993, new guitar day.


#3 2023-03-05 21:45:31

Registered: 2023-03-04
Posts: 2  

Re: serial console - full disk encryption

Hi Glenn,

  Unfortunately as far as I know this will not work. The intelligence built into /etc/default/grub does not allow for this "special" scenario. As you might be aware, if you add the GRUB_ENABLE_CRYPTODISK option in that file, grub will do all the magic necessary to be able to "unlock" the disk before reading the grub.cfg. Obviously you can also add GRUB_SERIAL_COMMAND to the same file and you will see the grub boot menu in the serial console, but only AFTER decrypting the disk. That is why I posted this fix, it is a special case (seeing the passphrase prompt in the serial console) for which there are no out of the box solutions. If you have a solution that works without this hack and survives grub-update (which is basically grub-mkconfig -o for grub2) please post it and I will take a look.


P.S. to be clear my hack survives grub-update but NOT grub-install

Last edited by tranquilo (2023-03-05 22:15:14)


#4 2023-03-05 21:52:21

Registered: 2016-11-25
Posts: 3,167  

Re: serial console - full disk encryption

Welcome to Devuan . . . could you please not post line-formatted text to the forum? The forum messages are a fixed width and it causes ugly line breaks. You should be able to go in and edit your post to get rid of them . . . Thanks!


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