archlinux-initrd-ssh-cryptsetup/README.md

Personal ArchLinux package combining dropbear and cryptsetup in initrd for unlocking LUKS-encrypted devices either locally (boot console) or remotely over SSH.
The code was reworked from legacy dropbear_initrd_encrypt AUR package.

Installation

After cloning the repo, installation is done as for an AUR package, e.g.:

makepkg -sri

Dropbear

SSH server key need to be generated for dropbear.
Either a new key can be generated with dropbearkey, e.g.:

dropbearkey -t ecdsa -f /etc/dropbear/dropbear_ecdsa_host_key

Or an existing OpenSSH key can be converted with dropbearconvert (useful so that the server fingerprint is the same with both), e.g.:

dropbearconvert openssh dropbear /etc/ssh/ssh_host_ecdsa_key /etc/dropbear/dropbear_ecdsa_host_key

Notes:

• rsa and ed25519 types are also handled
• OpenSSH keys must be in PEM format for dropbearconvert to properly work

If necessary an existing key file can be converted to PEM format using ssh-keygen:

ssh-keygen -A -p -m PEM -f /etc/ssh/ssh_host_ecdsa_key

Configuration

As explained upon installation, the following things need to be done:

• add the authorized SSH public key to /etc/dropbear/initrd.authorized_keys
• add the ip= kernel command parameter to the bootloader configuration (see https://wiki.archlinux.org/index.php/Mkinitcpio#Using_net)
  • e.g. with grub: add ip=:::::eth0:dhcp to GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub, and re-generate the configuration with grub-mkconfig -o /boot/grub/grub.cfg
• in the HOOKS section of /etc/mkinitcpio.conf, add ssh-cryptsetup before filesystems; then rebuild the initramfs: mkinitcpio -p linux
  • when using a non-standard keyboard layout, it is also useful to add the keymap hook before ssh-cryptsetup, and also move keyboard before keymap

The LUKS-encrypted devices to unlock are derived from /etc/crypttab.

Some options can be set in /etc/initcpio/sshcs_env (file is sourced in initrd shell):

• sshcs_opt_debug: whether to be more verbose about ongoing actions
  • default: 0
  • any non-zero value to enable
• sshcs_opt_timeout_ipconfig: time (in seconds) to configure IP
  • default: 10 seconds
• sshcs_opt_listen: SSH listening port
  • default: 22
• sshcs_opt_timeout_poweroff: time (in seconds) to unlock devices before automatic powering off
  • default (and minimum value): 2 minutes
  • negative value to deactivate

For example:

sshcs_opt_timeout_ipconfig=30
sshcs_opt_listen=2222
sshcs_opt_timeout_poweroff=-1

Building notes

1. Modify the sources (features in src, and/or package building files)
2. If src was modified
   • archive the src folder in $pkgname-$pkgver.tar.xz file; e.g.: tar -cJf initrd-ssh-cryptsetup-0.7.tar.xz src
   • upload the archive on the online repository (pointed by PKGBUILD)
3. Update ChangeLog
4. Update PKGBUILD
   • bump pkgver if src was modified, or pkgrel if building files were modified
   • refresh md5sums if necessary (based on md5sum initrd-ssh-cryptsetup-*.tar.xz initrd-ssh-cryptsetup.install output)
5. Delete generated archive file if any