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Installing a cloud server with full disk encryption

Making spying a bit harder for your cloud provider

Installing a cloud server with full disk encryption

I am installing a new server on Kimsufi. I want full-disk & swap encryption. Let’s do it.

Guides:

Overview of steps

  • Boot the server in rescue mode.

  • Wipe filesystem, format partitions:

  • Install and configure Debian (including disk cryptography and decryption through SSH).

  • Boot the server & setup automated decryption from a remote (in case of reboot).

Detailed process

Boot the server in rescue mode

  • Go to the Kimsufi admin panel, click “NetBoot”, select “Rescue”, pick “rescue64-pro”, clock “Next”, “Confirm”, and then click the “Reboot” button on the admin panel.

  • Kimsufi will mail you the root password for you to log into the server in rescue mode.

  • Will likely cause an error from your SSH client, since the fingerprint of the server differs from the usual one.

Wipe filesystem & format partitions

Target partition table:

        +-----------+--------------------+-----------+--------------------+
Name:   | /dev/sda1 | /dev/sda2          | /dev/sda3 | /dev/sda4          |
        |           |                    |           |                    |
        |           |                    |           |                    |
        |           | +------------------+           | +------------------+
        |           | | /dev/mapper/swap |           | | /dev/mapper/main |
Format: | none      | | swap             | ext4      | | ext4             |
Size:   | 1MiB      | | 8GiB             | 512MiB    | | remaining space  |
Flags:  | bios_grub | |                  |           | |                  |
Mount:  |           | | swap             | /boot     | | /                |
        +-----------+-+------------------+-----------+-+------------------+

We use GPT partition table layout (without UEFI), which demands a small bios_grub partition at the beginning of the drive (stores GRUB’s core.img). Hence /dev/sda1. See:

  • https://en.wikipedia.org/wiki/BIOS_boot_partition
  • https://www.gnu.org/software/grub/manual/grub/html_node/BIOS-installation.html#BIOS-installation
  • https://wiki.archlinux.org/index.php/partitioning#Partition_table
  • https://wiki.gentoo.org/wiki/Handbook:AMD64/Installation/Disks#Partition_tables
  1. Wipe filesystem, format partitions

    • Wipe all information about previous filesystem:

      DANGER!!! DANGER!!! WIPES EVERYTHING FROM YOUR DISK!!!

      wipefs -a /dev/sda
      
    • Create GPT disk layout

      # Create GPT layout
      parted -a optimal /dev/sda mklabel gpt
      # Create first 1MiB bios_grub partition 
      parted /dev/sda -a optimal mkpart bios_grub 0% 1MiB
      parted /dev/sda set 1 bios_grub on
      # Create third 8GiB swap partition
      parted /dev/sda -a optimal mkpart swap 1MiB 8001MiB
      # Create second 512MiB /boot partition
      parted /dev/sda -a optimal mkpart boot 8001MiB 8513MiB
      # Create last / partition using all remaining space 
      parted /dev/sda -a optimal mkpart main 8513MiB 100%
      # Set first partition as bootable
      
    • Format partitions

      • The ext4 boot partition

        Format it like so:

        mkfs.ext4 /dev/sda3
        
      • The encrypted swap partition

        All configuration of the encrypted swap partition comes after the OS installation.

      • The encrypted main partition

        # Set it up using cryptsetup (I searched for good parameters):
        cryptsetup --type luks2 --cipher aes-xts-plain64 --hash sha256 --iter-time 2000 --key-size 512 luksFormat /dev/sda4
        
        # Get the UUID, save it for later:
        cryptsetup luksDump /dev/sda4 | grep UUID | awk '{print $2}'
        # Example output: 09762476-ba7c-4732-8856-44a716c23339
        
        # Decrypt the partition:
        cryptsetup open /dev/sda4 main
        
        # Format it to ext4:
        mkfs.ext4 /dev/mapper/main
        
  2. Install and configure Debian

    • Mount the partitions

      # Decrypt the main partition (if not already done):
      cryptsetup open /dev/sda4 main
      # mount it:
      mount /dev/mapper/main /mnt 
      # Mount the boot partition:
      mkdir /mnt/boot
      mount /dev/sda3 /mnt/boot
      
    • Bootstrap latest stable Debian into mounted partitions

      debootstrap --arch amd64 stable /mnt http://ftp.fr.debian.org/debian/
      
    • Mount system partitions

      mount -o bind /dev /mnt/dev
      mount -t proc proc /mnt/proc
      mount -t sysfs sys /mnt/sys
      mount -t devpts devpts /mnt/dev/pts
      
    • Chroot

        chroot /mnt /bin/bash
      
    • Fill out crypttab and fstab (UUID is the output of above cryptsetup luksDump)

      /etc/crypttab

      cat << EOF > /etc/crypttab
      # <name> <device>                                  <password>   <options>
      main     UUID=09762476-ba7c-4732-8856-44a716c23339 none         luks
      swap     /dev/sda2                                 /dev/urandom swap,noearly,cipher=aes-xts-plain64,size=512
      EOF
      

      /etc/fstab

      cat << EOF > /etc/fstab
      # <filesystem>   <dir> <type> <options>          <dump> <pass>
      /dev/mapper/main /     ext4   errors=remount-ro  0      1
      /dev/sda3        /boot ext4   defaults           0      2
      /dev/mapper/swap none  swap   sw                 0      0
      EOF
      
    • Do the /proc/mounts -> /etc/mtab symlink

      ln -s /proc/mounts /etc/mtab
      

      Why? Linux From Stratch and our guide on OpsBlog both say it’s needed.

    • Choose a hostname & DNS domain

      hammerhead & hammerhead.luxeylab.net

    • Fill in network-related files

      Beware of your ethernet interface name! It’s only after failing with eth0 that the boot logs informed me that the iface name was eno1.

      /etc/network/interfaces

      cat << EOF > /etc/network/interfaces
      ######################################################################
      # /etc/network/interfaces -- configuration file for ifup(8), ifdown(8)
      # See the interfaces(5) manpage for information on what options are
      # available.
      ######################################################################
      
      # loopback interface
      auto lo
      iface lo inet loopback
      
      # eno1 through DHCP
      auto eno1
      iface eno1 inet dhcp
      EOF
      

      /etc/resolv.conf

      cat << EOF > /etc/resolv.conf 
      # OVH public DNS
      nameserver 213.186.33.99
      # https://servers.opennicproject.org/edit.php?srv=ns2.he.de.dns.opennic.glue 
      nameserver 172.104.136.243
      EOF
      

      /etc/hosts

      cat << EOF > /etc/hosts
      127.0.0.1 localhost
      127.0.1.1 hammerhead hammerhead.luxeylab.net 
      
      # The following lines are desirable for IPv6 capable hosts
      ::1     localhost ip6-localhost ip6-loopback
      fe00::0 ip6-localnet
      ff00::0 ip6-mcastprefix
      ff02::1 ip6-allnodes
      ff02::2 ip6-allrouters
      ff02::3 ip6-allhosts
      
      213.186.33.116 ws.ovh.com
      198.245.48.4 ws.ovh.ca
      5.135.179.11 hammerhead hammerhead.luxeylab.net 
      EOF
      

      /etc/hostname

      echo "hammerhead" > /etc/hostname
      

      /etc/timezone

      dpkg-reconfigure tzdata
      
    • Update APT repositories in /etc/apt/sources.list

      cat << EOF > /etc/apt/sources.list
      deb http://ftp.fr.debian.org/debian buster main
      deb-src http://ftp.fr.debian.org/debian buster main
      
      deb http://ftp.fr.debian.org/debian-security/ buster/updates main
      deb-src http://ftp.fr.debian.org/debian-security/ buster/updates main
      
      deb http://ftp.fr.debian.org/debian buster-updates main
      deb-src http://ftp.fr.debian.org/debian buster-updates main
      EOF
      
    • Configure locales & keyboard

      # Refresh packages list
      apt update
      # Install & configure locales 
      apt install locales && dpkg-reconfigure locales
      # Install & configure keyboard 
      apt install console-setup console-data && dpkg-reconfigure keyboard-configuration
      
    • Pick up a kernel & install it

      apt-cache search linux-image
      # I selected the meta-package `linux-image-amd64` which is the latest 
      apt install linux-image-amd64 linux-headers-amd64
      
    • Install additional necessary software

      apt install cryptsetup dropbear grub-pc man-db ssh 
      

      Because we use a GPT table layout but no UEFI, we install grub-pc and not grub-efi-amd64.

      Do think about installing bash-completion and ufw someday.

    • Set up SSH keys and the rest (where <PUBLIC_SSH_KEY> is your client SSH public key)

      mkdir /root/.ssh && chmod 600 /root/.ssh
      echo "<PUBLIC_SSH_KEY>" > /root/.ssh/authorized_keys
      echo "<PUBLIC_SSH_KEY>" > /etc/dropbear-initramfs/authorized_keys
      
    • Configure dropbear-initramfs (SSH on boot)

      sed -i 's/#DROPBEAR_OPTIONS=/DROPBEAR_OPTIONS=\"-p 2222 -c \/bin\/cryptroot-unlock\"/' /etc/dropbear-initramfs/config
      

      This changes the SSH listen post to 2222, and enforces that the only command run from dropbear-initramfs is cryptroot-unlock.

    • Configure sshd custom listen port (SSH after boot)

      sed -i 's/#Port 22/Port 2223/' /etc/ssh/sshd_config
      
    • [Optional] Authorize root password login

      # Remove -s (disable password login) from dropbear-initramfs
      sed -i 's/local flags=\"Fs\"/local flags=\"F\"/' /usr/share/initramfs-tools/scripts/init-premount/dropbear
      
    • Update GRUB and initramfs

      update-grub && update-initramfs -u
      
    • Clean up before leaving

      exit
      umount /mnt/{boot,dev/pts,dev,proc,sys}
      umount /mnt
      cryptsetup luksClose main
      
  3. Boot the server & setup automated decryption from a remote

    • I configured my laptop’s ~/.ssh/config like so:

      Host hammerhead-decrypt
          User root
          Hostname <SERVER_URL_OR_IP>
          Port 2222
          IdentityFile <PUBLIC_SSH_KEY_PATH>
          # Prevents signatures mismatch
          UserKnownHostsFile ~/.ssh/known_hosts_hammerhead-decrypt
      Host hammerhead
          User root
          Hostname <SERVER_URL_OR_IP>
          Port 2223
          IdentityFile <PUBLIC_SSH_KEY_PATH>
      
    • And here is how I connect to the server:

      laptop$ ssh hammerhead-decrypt 
      Please unlock disk main: <PASSWORD>
      # [...]
      Connection to <SERVER_URL_OR_IP> closed.
      laptop$ ssh hammerhead 
      # [...]
      root@hamerhead:~# echo A winner is you!
      A winner is you!
      

Automatically decrypt the drive from a remote server

It is desirable to have a daemon running on a remote server, to automatically decrypt the drive when the encrypted server reboots without warning.

The remote server is called a key escrow. One must be particularly careful about the escrow’s security, since it holds the decryption keys for our server.

Trinity recommends Tang and Clevis.