Encrypted root on debian part 2: unattended boot

I want my debian boot to work as follows:

  1. If it’s in my house, it can boot without my being there. To make that happen, I’ll put the root disk key on a USB stick, which I keep in the computer.
  2. If it’s not in my house, it needs a password to boot. This is the normal boot process.

As in part 1, this guide is debian-specific. To learn more about the Linux boot process, see part 1.

First, we need to prepare the USB stick. Use ‘dmesg’ and/or ‘lsblk’ to make a note of the USB stick’s path (/dev/sdae for me). I chose to write to a filesystem rather than a raw block device.

sudo mkfs.ext4 /dev/sdae # Make a filesystem directly on the device. No partition table.
sudo blkid /dev/sdae # Make a note of the filesystem UUID for later

Next, we’ll generate a key.

sudo mount /dev/sdae /mnt
sudo dd if=/dev/urandom of=/mnt/root-disk.key bs=1000 count=8

Add the key to your root so it can actually decrypt things. You’ll be prompted for your password:

sudo cryptsetup luksAddKey ROOT_DISK_DEVICE /mnt/root-disk.key

Make a script at /usr/local/sbin/unlockusbkey.sh

#!/bin/sh
USB_DEVICE=/dev/disk/by-uuid/a4b190b8-39d0-43cd-b3c9-7f13d807da48 # copy from blkid's output UUID=XXXX

if [ -b $USB_DEVICE ]; then
  # if device exists then output the keyfile from the usb key
  mkdir -p /usb
  mount $USB_DEVICE -t ext4 -o ro /usb
  cat /usb/root-disk.key
  umount /usb
  rmdir /usb
  echo "Loaded decryption key from USB key." >&2
else
  echo "FAILED to get USB key file ..." >&2
  /lib/cryptsetup/askpass "Enter passphrase"
fi

Mark the script as executable, and optionally test it.

chmod +x /usr/local/sbin/unlockusbkey.sh
sudo /usr/local/sbin/unlockusbkey.sh | cmp /mnt/root-disk.key

Edit /etc/crypttab to add the script.

root PARTLABEL=root_cipher none luks,keyscript=/usr/local/sbin/unlockusbkey.sh

Finally, re-generate your initramfs. I recommend either having a live USB or keeping a backup initramfs.

sudo update-initramfs -u

[1] This post is loosely based on a chain of tutorials based on each other, including this
[2] However, those collectively looked both out of date and like they were written without true understanding, and I wanted to clean up the mess. More definitive information was sourced from the actual cryptsetup documentation.

Migrating an existing debian installation to encrypted root

In this article, I migrate an existing debian 10 buster release, from an unencrypted root drive, to an encrypted root. I used a second hard drive because it’s safer–this is NOT an in-place migration guide. We will be encrypting / (root) only, not /boot. My computer uses UEFI. This guide is specific to debian–I happen to know these steps would be different on Arch Linux, for example. They probably work great on a different debian version, and might even work on something debian-based like Ubuntu.

In part 2, I add an optional extra where root decrypts using a special USB stick rather than a keyboard passphrase, for unattended boot.

Apologies if I forget any steps–I wrote this after I did the migration, and not during, so it’s not copy-paste.

Q: Why aren’t we encrypting /boot too?

  1. Encrypting /boot doesn’t add much security. Anyone can guess what’s on my /boot–it’s the same as on everyone debian distro. And encrypting /boot doesn’t prevent tampering–someone can easily replace my encrypted partition by an unencrypted one without my noticing. Something like Secure Boot would resist tampering, but still doesn’t require an encrypted /boot.
  2. I pull a special trick in part 2. Grub2’s has new built-in encryption support, which is what would allow encrypting /boot. But grub2 can’t handle keyfiles or keyscripts as of writing, which I use.

How boot works

For anyone that doesn’t know, here’s how a typical boot process works:

  1. Your computer has built-in firmware, which on my computer meets a standard called UEFI. On older computers this is called BIOS. The firmware is built-in, closed-source, and often specific to your computer. You can replace it with something open-source if you wish.
  2. The firmware has some settings for what order to boot hard disks, CD drives, and USB sticks in. The firmware tries each option in turn, failing and using the next if needed.
  3. At the beginning of each hard disk is a partition table, a VERY short info section containing information about what partitions are on the disk, and where they are. There are two partition table types: MBR (older) and GPT (newer). UEFI can only read GPT partition tables. The first thing the firmware does for each boot disk is read the partition table, to figure out which partitions are there.
  4. For UEFI, the firmware looks for an “EFI” partition on the boot disk–a special partition which contains bootloader executables. EFI always has a FAT filesystem on it. The firmware runs an EFI executable from the partition–which one is configured in the UEFI settings. In my setup there’s only one executable–the grub2 bootloader–so it runs that without special configuration.
  5. Grub2 starts. The first thing Grub2 does is… read the partition table(s) again. It finds the /boot partition, which contains grub.cfg, and reads grub.cfg. (There is a file in the efi partition right next to the executable, which tells grub where and how to find /boot/grub.cfg. This second file is confusingly also called grub.cfg, so let’s forget it exists, we don’t care about it).
  6. Grub2 invokes the Linux Kernel specified in grub.cfg, with the options specified in grub.cfg, including the an option to use a particular initramfs. Both the Linux kernel and the initramfs are also in /boot.
  7. Now the kernel starts, using the initramfs. initramfs is a tiny, compressed, read-only filesystem only used in the bootloading process. The initramfs’s only job is to find the real root filesystem and open it. grub2 is pretty smart/big, which means initramfs may not have anything left to do on your system before you added encryption. If you’re doing decryption, it happens here. This is also how Linux handles weird filesystems (ZFS, btrfs, squashfs), some network filesystems, or hardware the bootloader doesn’t know about. At the end of the process, we now have switched over to the REAL root filesystem.
  8. The kernel starts. We are now big boys who can take care of ourselves, and the bootloading process is over. The kernel always runs /bin/init from the filesystem, which on my system is a symlink to systemd. This does all the usual startup stuff (start any SSH server, print a bunch of messages to the console, show a graphical login, etc).

Setting up the encrypted disk

First off, I used TWO hard drives–this is not an in-place migration, and that way nothing is broken if you mess up. One disk was in my root, and stayed there the whole time. The other I connected via USB.

Here’s the output of gdisk -l on my original disk:

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048         1050623   512.0 MiB   EF00  # EFI, mounted at /boot/efi
   2         1050624       354803711   168.7 GiB   8300  # ext4, mounted at /
   3       354803712       488396799   63.7 GiB    8200  # swap

Here will be the final output of gdisk -l on the new disk:

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048          526335   256.0 MiB   EF00  efi # EFI, mounted at /boot/efi
   2         1050624       135268351   64.0 GiB    8200  swap # swap
   3       135268352       937703054   382.6 GiB   8300  root_cipher # ext4-on-LUKS. ext4 mounted at /
   4          526336         1050623   256.0 MiB   8300  boot # ext4, mounted at /boot
  1. Stop anything else running. We’re going to do a “live” copy from the running system, so at least stop doing anything else. Also most of the commands in this guide need root (sudo).
  2. Format the new disk. I used gdisk and you must select a gpt partition table. Basically I just made everything match the original. The one change I need is to add a /boot partition, so grub2 will be able to do the second stage. I also added partition labels with the c gdisk command to all partitions: boot, root_cipher, efi, and swap. I decided I’d like to be able to migrate to a larger disk later without updating a bunch of GUIDs, and filesystem or partition labels are a good method.
  3. Add encryption. I like filesystem-on-LUKS, but most other debian guides use filesystem-in-LVM-on-LUKS. You’ll enter your new disk password twice–once to make an encrypted partition, once to open the partition.
    cryptsetup luksFormat /dev/disk/by-partlabel/root_cipher
    cryptsetup open /dev/disk-by-partlabel/root_cipher root
  4. Make the filesystems. For my setup:
    mkfs.ext4 /dev/disk/by-partlabel/root
    mkfs.ext4 /dev/disk/by-partlabel/boot
    mkfs.vfat /dev/disk/by-partlabel/efi
  5. Mount all the new filesystems at /mnt. Make sure everything (cryptsetup included) uses EXACTLY the same mount paths (ex /dev/disk/by-partlabel/boot instead of /dev/sda1) as your final system will, because debian will examine your mounts to generate boot config files.
    mount /dev/disk/by-partlabel/root /mnt
    mkdir /mnt/boot && mount /dev/disk/by-partlabel/boot /mnt/boot
    mkdir /mnt/boot/efi && mount /dev/disk/by-partlabel/efi /mnt/boot/efi
    mkdir /mnt/dev && mount --bind /dev /mnt/dev # for chroot
    mkdir /mnt/sys && mount --bind /sys /mnt/sys
    mkdir /mnt/proc && mount --bind /dev /mnt/proc
  6. Copy everything over. I used rsync -axAX, but you can also use cp -ax. To learn what all these options are, read the man page. Make sure to keep the trailing slashes in the folder paths for rsync.
    rsync -xavHAX / /mnt/ --no-i-r --info=progress2
    rsync -xavHAX /boot/ /mnt/boot/
    rsync -xavHAX /boot/efi/ /mnt/boot/efi/
  7. Chroot in. You will now be “in” the new system using your existing kernel.
    chroot /mnt
  8. Edit /etc/crypttab. Add:
    root PARTLABEL=root_cipher none luks
  9. Edit /etc/fstab. Mine looks like this:
    /dev/mapper/root / ext4 errors=remount-ro 0 1
    PARTLABEL=boot /boot ext4 defaults,nofail 0 1
    PARTLABEL=efi /boot/efi vfat umask=0077,nofail
    PARTLABEL=swap none swap sw,nofail 0 0
    tmpfs /tmp tmpfs mode=1777,nosuid,nodev 0 0
  10. Edit /etc/default/grub. On debian you don’t need to edit GRUB_CMDLINE_LINUX.
    GRUB_DISABLE_LINUX_UUID=true
    GRUB_ENABLE_LINUX_PARTLABEL=true
  11. Run grub-install. This will install the bootloader to efi. I forget the options to run it with… sorry!
  12. Run update-grub (with no options). This will update /boot/grub.cfg so it knows how to find your new drive. You can verify the file by hand if you know how.
  13. Run update-initramfs (with no options). This will update the initramfs so it can decrypt your root drive.
  14. If there were any warnings or errors printed in the last three steps, something is wrong. Figure out what–it won’t boot otherwise. Especially make sure your /etc/fstab and /etc/crypttab exactly match what you’ve already used to mount filesystems.
  15. Exit the chroot. Make sure any changes are synced to disk (you can unmount everything under /mnt in reverse order to make sure if you want)
  16. Shut down your computer. Remove your root disk and boot from the new one. It should work now, asking for your password during boot.
  17. Once you boot successfully and verify everything mounted, you can remove the nofail from /etc/fstab if you want.
  18. (In my case, I also set up the swap partition after successful boot.) Edit: Oh, also don’t use unencrypted swap with encrypted root. That was dumb.

mon(8)

I had previously hand-rolled a status monitor, status.za3k.com, which I am in the process of replacing (new version). I am replacing it with a linux monitoring daemon, mon, which I recommend. It is targeted at working system administrators. ‘mon’ adds many features over my own system, but still has a very bare-bones feeling.

The old service, ‘simple-status‘ worked as follows:

  • You visited the URL. Then, the status page would (live) kick of about 30 parallel jobs, to check the status of 30 services
  • The list of services is one-per-file in a the services.d directory.
  • For each service, it ran a short script, with no command line arguments.
  • All output is displayed in a simple html table, with the name of the service, the status (with color coding), and a short output line.
  • The script could return with a success (0) or non-success status code. If it returned success, that status line would display in green for success. If it failed, the line would be highlighted red for failure.
  • Scripts can be anything, but I wrote several utility functions to be called from scripts. For example, “ping?” checks whether a host is pingable.
  • Each script was wrapped in timeout. If the script took too long to run, it would be highlighted yellow.
  • The reason all scripts ran every time, is to prevent a failure mode where the information could ever be stale without me noticing.

Mon works as follows

  • The list of 30 services is defined in /etc/mon/con.cf.
  • For each service, it runs a single-line command (monitor) with arguments. The hostname(s) are added to the command line automatically.
  • All output can be displayed in a simple html table, with the name of the service, the status (with color coding), the time of last and next run, and a short output line. Or, I use ‘monshow‘, which is similar but in a text format.
  • Monitors can be anything, but several useful ones are provided in /usr/lib/mon/mon.d (on debian). For example the monitor “ping” checks whether a host is pingable.
  • The script could return with a success (0) or non-success status code. If it returned success, the status line would display in green for success (on the web interface), or red for failure.
  • All scripts run periodically. A script have many states, not just “success” or “failure”. For example “untested” (not yet run) or “dependency failing” (I assume, not yet seen).

As you can see, the two have a very similar approach to the component scripts, which is natural in the Linux world. Here is a comparison.

  • ‘simple-status’ does exactly one thing. ‘mon’ has many features, but does the minimum possible to provide each.
  • ‘simple-status’ is stateless. ‘mon’ has state.
  • ‘simple-status’ runs on demand. ‘mon’ is a daemon which runs monitors periodically.
  • Input is different. ‘simple-status’ is one script which takes a timeout. ‘mon’ listens for trap signals and talks to clients who want to know its state.
  • both can show an HTML status page that looks about the same, with some CGI parameters accepted.
  • ‘mon’ can also show a text status page.
  • both run monitors which return success based on status code, and provide extra information as standard output. ‘mon’ scripts are expected to be able to run on a list of hosts, rather than just one.
  • ‘mon’ has a config file. ‘simple-status’ has no options.
  • ‘simple-status’ is simple (27 lines). ‘mon’ has longer code (4922 lines)
  • ‘simple-status’ is written in bash, and does not expose this. ‘mon’ is written in perl, all the monitors are written in perl, and it allows inline perl in the config file
  • ‘simple-status’ limits the execution time of monitors. ‘mon’ does not.
  • ‘mon’ allows alerting, which call an arbitrary program to deliver the alert (email is common)
  • ‘mon’ supports traps, which are active alerts
  • ‘mon’ supports watchdog/heartbeat style alerts, where if a trap is not regularly received, it marks a service as failed.
  • ‘mon’ supports dependencies
  • ‘mon’ allows defining a service for several hosts at once

Overall I think that ‘mon’ is much more complex, but only to add features, and it doesn’t have a lot of features I wouldn’t use. It still is pretty simple with a simple interface. I recommend it as both good, and overall better than my system.

My only complaint is that it’s basically impossible to Google, which is why I’m writing a recommendation for it here.

Cron email, and sending email to only one address

So you want to know when your monitoring system fails, or your cron jobs don’t run? Add this to your crontab:

MAILTO=me@me.com

Now install a mail-sending agent. I like ‘nullmailer‘, which is much smaller than most mail-sending agents. It can’t receive or forward mail, only send it, which is what I like about it. No chance of a spammer using my server for something nasty.

The way I have it set up, I’ll have a server (avalanche) sending all email from one address (nullmailer@avalanche.za3k.com) to one email (admin@za3k.com), and that’s it. Here’s my setup on debian:

sudo apt-get install nullmailer
echo "admin@za3k.com" | sudo tee /etc/nullmailer/adminaddr # all mail is sent to here, except for certain patterns
echo "nullmailer@`hostname`.za3k.com" | sudo tee /etc/nullmailer/allmailfrom # all mail is sent from here
echo "`hostname`.za3k.com" | sudo tee /etc/nullmailer/defaultdomain # superceded by 'allmailfrom' and not used
echo "`hostname`.za3k.com" | sudo tee /etc/nullmailer/helohost # required to connect to my server. otherwise default to 'me'
echo "smtp.za3k.com smtp --port=587 --starttls" | sudo tee /etc/nullmailer/remotes && sudo chmod 600 /etc/nullmailer/remotes

Now just run echo "Subject: sendmail test" | /usr/lib/sendmail -v admin@za3k.com to test and you’re done!

Mail filtering with Dovecot

This expands on my previous post about how to set up an email server.

We’re going to set up a few spam filters in Dovecot under Debian. We’re going to use Sieve, which lets the user set up whichever filters they want. However, we’re going to run a couple pre-baked spam filters regardless of what the user sets up. Continue reading

Installing email with Postfix and Dovecot (with Postgres)

I’m posting my email setup here. The end result will:

  • Use Postfix for SMTP
  • Use Dovecot for IMAP and authentication
  • Store usernames, email forwards, and passwords in a Postgres SQL database
  • Only be accessible over encrypted channels
  • Pass all common spam checks
  • Support SMTP sending and IMAP email checking. I did not include POP3 because I don’t use it, but it should be easy to add
  • NOT add spam filtering or web mail (this article is long enough as it is, maybe in a follow-up)

Continue reading

XP Boot USB Stick

Most of the following taken from : http://www.msfn.org/board/topic/151992-install-xp-from-usb-without-extra-tools/, just modified to include syslinux support.

Let me know if there are any omissions; it an XP installer bluescreens on boot for me so I can’t actually test.

  1. Obtain an XP iso file
  2. Format drive with one FAT parition, marked bootable.
  3. syslinux -i /dev/sdXX
  4. $ cp /usr/lib/syslinux/bios/mbr.bin >/dev/sdX
  5. $ mount /dev/sdXX /mnt
  6. mkdir /tmp/xp_iso
    mount xp.iso /tmp/xp_iso
    cp -ar /tmp/xp_iso/* /mnt
    umount /tmp/xp_iso
    rmdir xp_iso
  7. cp /usr/lib/syslinux/bios/{chain.c32,libutil.c32,menu.c32,libcom.c32} /mnt
  8. cp /mnt/I386/{NTDETECT.COM,SETUPLDR.BIN,TXTSETUP.SIF} /mnt
  9. Edit /mnt/syslinux.cfg:
    UI menu.c32# Windows XP
    LABEL windows_xp
    MENU LABEL Run Windows ^XP Setup
    COM32 chain.c32
    APPEND fs ntldr=SETUPLDR.BIN
  10. umount /mnt
  11. Boot from the USB stick

Roll-your-own git push-to-deploy, and markdown support

Today I added support for development of za3k.com using git:

# !/bin/sh
# /git/bare-repos/za3k.com/hooks/post-update
cd ~za3k/public_html
env -i git pull
echo "Deployed za3k.com"

and markdown support, via a cgi markdown wrapper someone wrote for apache (yes, I’m still using Apache).

Edit: I ended up wanting support for tables in markdown, so I used Ruby‘s redcarpet markdown gem (the same thing Github uses, supports this style of tables as well as code blocks).

CGI support via http://blog.tonns.org/2013/10/enabling-markdown-on-your-apache.html

Archiving twitter

(Output)

I wanted to archive twitter so that I could

  1. Make sure old content was easily available
  2. Read twitter in a one-per-line format without ever logging into the site

twitter_ebooks is a framework to make twitter bots, but it includes an ‘archive’ component to fetch historical account content which is apparently unique in that it 1) works with current TLS and 2) works the current twitter API. It stores the tweets in a JSON format which presumably matches the API return values. Usage is simple:

while read account
do
    ebooks archive "${account}" "archive/${account}.json"
    jq -r 'reverse | .[] | "\(.created_at|@sh)\t \(.text|@sh)"' "archive/${account}.json" >"archive/${account}.txt"
done <accounts.txt

I ran into a bug with upstream incompatibilities which is easily fixed. Another caveat is that the twitter API only allows access 3200 tweets back in time for an account–all the more reason to set up archiving ASAP. Twitter’s rate-limiting is also extreme (15-180 req/15 min), and I’m worried about a problem where my naive script can’t make it through a list of more than 15 accounts even with no updates.