Installation

We're going to use Arch Linux ARM not because we like to move fast and break things, but because it is a minimal base that will let us make the most of the diminuitive hardware and learn more about Linux, the operating system, and commonly used programs.

Initial Install

Raspberry Pi versions 1 - 3B+ (all of them, circa 2018) does not support hardware encryption, meaning full disk encryption will use significant CPU % and decrease disk performance.

  • [ ] Test.

The initial installation is carried out on our computer following the correct variant of instructions:

See File System Variations. It's easier to vary this now rather than later.

bla bla bla.

This is your opportunity to encrypt the root partition (excluding the boot drive) or use f2fs with the root partition. I did neither at this point.

Encryption is tricky because you'll need to physically enter the password if you reboot.

Encryption after SSH? Worth looking into:

Create an image for easy future use... (after initial setup... hmm)

Note: badblocks -wsv /dev/... can be used to check each storage device you are using is okay.

WARNING -w = destructive read-write test.

List bad blocks: dumpe2fs -b /dev/sdX1

Find new bad blocks: fsck -vcck /dev/sdX1

Steps

  1. Follow the instructions on the wiki page, using 250 MB for boot, 16 GB for root, and keeping the rest as spare.

Hardware Optimisation and Issues

Raspberry Pi 3

Edit /boot/config.txt with some or all of these options.

# Reduce memory allocation to unused GPU, increasing RAM available to OS
gpu_mem=16

# Disable unused WiFi and Bluetooth hardware to save power
dtoverlay=pi3-disable-wifi
dtoverlay=pi3-disable-bt

# Disable unused HDMI port to save power (undocumented - need source link)
hdmi_blanking=2
# Force normal boots without HDMI cable connected
hdmi_force_hotplug=1
# Disable DVI mode over HDMI
hdmi_drive=2
# Disable overscan for TVs
disable_overscan=1

# Improve the boot time
disable_splash=1
boot_delay=0

# Reduce minimum frequency of processor (to save power?)
arm_freq_min=300

Note that the status of the HDMI port can be checked at /opt/vc/bin/tvservice.

Booting up with HDMI plugged in

This likely affects ARMv6 devices including the Raspberry Pi, Raspberry Pi Zero, and Raspberry Pi Zero W.

Some Raspberry Pi boards including the Raspberry Pi Zero W will not boot up properly without a HDMI display plugged in. To fix this, add this line to /boot/config.txt

hdmi_force_hotplug=1

Power

Watch out for Under-voltage detected! system logs.

File System Variations

There is a lot of flexibility in how we configure the partitions.

The /boot partition is fairly inflexible (confirm encryption?), it must be FAT32. It is possible to boot over the network, or from a USB flash drive.

The root / partition is more flexible, and you can install with other file systems from the get-go, e.g. f2fs or btrfs. Edit cmdline.txt.

It is easy to use RAID, LUKS, LVM and whatever file system you want on pure data devices.

It is much more tricky to install if you want to do those things on your main root / partition.

We will probably use 3-4 GB on / for programs and config files, and all data will be on a separate partition. I would recommend 4x this for / , i.e. 16GB, to leave room for future growth, and significant spare space to extend the longevity of this part of the SD card.

RAID + LUKS + LVM

Could be useful if you have externally-powered USB storage devices.

Note LVM can actually raid, but the tooling is more opaque and there is less community support - lvmraid.

Btrfs

Well we're using Arch, so why not a slightly dodgy filesystem? It has cool features like copy-on-write, snapshots, fast backups, and... RAID5 corrupting your data.

https://github.com/NicoHood/NicoHood.github.io/wiki/Raspberry-Pi-Encrypted-Btrfs-Root

f2fs

f2fs encryption info: https://www.kernel.org/doc/html/v4.15/filesystems/fscrypt.html

Per-Directory. Encrypt file + filename. Not file size, timestamps, permissions, extended attributes. Uses fscrypt, kernel-tool.

Example Installation: Headless ARMv7 Encrypted Root on RPi 3B+

In this brief example of an installation, we will install Arch Linux ARMv7 on a Raspberry Pi 3B+ using an ext4 filesystem installed headlessly via ethernet. We start up by following the standard instructions.

  1. Verify the SD Card: badblocks -wsv /dev/sdX.

  2. Partition the SD Card: fdisk /dev/sdX

    1. 250 MB W95 FAT32 (LBA) primary partition.

    2. 3750 MB primary partition.

    3. The remainder should be another primary partition; this will be the encrypted root.

  3. Create and mount the filesystems:

    mkfs.vfat -n BOOT /dev/sdX1
    mkdir boot
    mount /dev/sdX1 boot
    
    mkfs.ext4 -L clearroot /dev/sdX2
    mkdir root
    mount /dev/sdX2 root
    
  4. Download and install Arch Linux ARM:

    wget http://os.archlinuxarm.org/os/ArchLinuxARM-rpi-2-latest.tar.gz.sig
    wget http://os.archlinuxarm.org/os/ArchLinuxARM-rpi-2-latest.tar.gz
    gpg --verify ArchLinuxARM-rpi-2-latest.tar.gz.sig
    
    bsdtar -xpf ArchLinuxARM-rpi-2-latest.tar.gz -C root
    sync
    mv root/boot/* boot
    
  5. Edit boot/config.txt with the additional lines:

    # Ensure we can boot without a HDMI cable connected
    hdmi_force_hotplug=1
    
  6. As your own user, generate and copy over the SSH key we will use to login to the PiServer in order to decrypt the root partition:

    ssh-keygen -t rsa -b 4096 -f ~/.ssh/piserver_decryptor_key
    
  7. Umount, disconnect, and plug everything into the RPi:

    umount boot root
    
  8. Determine the IP address: sudo nmap -sT --open -p 22 192.168.1.0/24.

  9. Copy over the decryptor SSH key (alarm user password is alarm):

    ssh-copy-id -i ~/.ssh/piserver_decryptor_key.pub [email protected]<ip-address>
    
  10. Connect and login to the RPi: ssh [email protected]<ip-address>.

  11. Do the minimal steps to be able to configure the encrypted partition:

    # The root password is root
    su - root
    
    pacman-key --init
    pacman-key --populate archlinuxarm
    pacman -Syu
    pacman -S --needed sudo git rsync base-devel dropbear
    
  12. Add the line alarm ALL=(ALL) ALL using to sudoers:

    visudo
    
  13. Backup some files... Just in case:

    cp /etc/mkinitcpio.conf /etc/mkinitcpio.conf.clearroot
    cp /boot/cmdline.txt /boot/cmdline.txt.clearroot
    cp -r /boot /root/boot_clearroot
    
  14. Edit /etc/mkinitcpio.conf, ensuring the following configuration is included:

    MODULES=(g_cdc usb_f_acm usb_f_ecm smsc95xx g_ether)
    FILES=(/usr/lib/libgcc_s.so.1)
    HOOKS=(base udev autodetect modconf block sleep netconf dropbear encryptssh filesystems keyboard fsck)
    

    (https://github.com/grazzolini/mkinitcpio-utils/issues/13)

  15. Copy over the SSH key for the initial decryption:

    mkdir /etc/dropbear
    cp /home/alarm/.ssh/authorized_keys /etc/dropbear/root_key
    
  16. Configure at least one SSH key for the dropbear SSH server, install dropbear, and regenerate the boot image:

    cd /etc/ssh
    rm ssh_host_rsa_key
    ssh-keygen -t rsa -b 4096 -f ssh_host_rsa_key -N "" -m PEM < /dev/null
    
  17. Now return to alarm:

    git clone https://aur.archlinux.org/yay.git
    cd yay
    makepkg -sirc
    yay -S mkinitcpio-utils mkinitcpio-netconf mkinitcpio-dropbear
    
  18. Now return to root, and setup the encrypted partition:

    cryptsetup luksFormat --cipher aes-xts-plain64 --key-size 512 --hash sha512 --iter-time 1000 --use-random /dev/mmcblk0p3
    cryptsetup luksOpen /dev/mmcblk0p3 cryptroot
    mkfs.ext4 -L cryptroot /dev/mapper/cryptroot
    mount /dev/mapper/cryptroot /mnt
    
    rsync --info=progress2 -axHAX / /mnt/
    
  19. Adjust the partition in cryptroot:

    # /mnt/etc/fstab
    /dev/mapper/cryptroot / ext4 defaults,noatime,lazytime 0 1
    # /mnt/etc/crypttab
    cryptroot /dev/mmcblk0p3 none luks
    
  20. Configure the bootloader /boot/cmdline.txt:

    cryptdevice=/dev/mmcblk0p3:cryptroot root=/dev/mapper/cryptroot ip=::::piserver_decryptor:eth0:dhcp rw rootwait console=ttyAMA0,115200 console=tty1 selinux=0 plymouth.enable=0 smsc95xx.turbo_mode=N dwc_otg.lpm_enable=0 kgdboc=ttyAMA0,115200 elevator=noop
    

That's it. Reboot; Find piserver_decryptor; Login to [email protected] using the piserver_decryptor SSH key; Decrypt; You will be disconnected as it reboots - now, login again into the encrypted partition.

Move on to the Initial Setup!

Note: No additional partition, using QEMU - https://wiki.polaire.nl/doku.php?id=archlinux-raspberry-encrypted.

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