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[2] However, those collectively looked both out of date and like they were written without true understanding, and I wanted to clean up the mess.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.

Making a hardware random number generator

If you want a really good source of random numbers, you should get a hardware generator. But there’s not a lot of great options out there, and most people looking into this get (understandably) paranoid about backdoors. But, there’s a nice trick: if you combine multiple random sources together with xor, it doesn’t matter if one is backdoored, as long as they aren’t all backdoored. There are some exceptions–if the backdoor is actively looking at the output, it can still break your system. But as long as you’re just generating some random pads, instead of making a kernel entropy pool, you’re fine with this trick.

So! We just need a bunch of sources of randomness. Here’s the options I’ve tried:

  • /dev/urandom (40,000KB/s) – this is nearly a pseudo-random number generator, so it’s not that good. But it’s good to throw in just in case. [Learn about /dev/random vs /dev/urandom if you haven’t. Then unlearn it again.]
  • random-stream (1,000 KB/s), an implementation of the merenne twister pseudo-random-number generator. A worse version of /dev/urandom, use that unless you don’t trust the Linux kernel for some reason.
  • infnoise (20-23 KB/s), a USB hardware random number generator. Optionally whitens using keccak. Mine is unfortunately broken (probably?) and outputs “USB read error” after a while
  • OneRNG (55 KiB/s), a USB hardware random number generator. I use a custom script which outputs raw data instead of the provided scripts (although they look totally innocuous, do recommend
  • /dev/hwrng (123 KB/s), which accesses the hardware random number generator built into the raspberry pi. this device is provided by the raspbian package rng-tools. I learned about this option here
  • rdrand-gen (5,800 KB/s), a command-line tool to output random numbers from the Intel hardware generator instruction, RDRAND.

At the end, you can use my xor program to combine the streams/files. Make sure to use limit the output size if using files–by default it does not stop outputting data until EVERY file ends. The speed of the combined stream is at most going to be the slowest component (plus a little slowdown to xor everything). Here’s my final command line:

#!/bin/bash
# Fill up the folder with 1 GB one-time pads. Requires 'rng-tools' and a raspberry pi. Run as sudo to access /dev/hwrng.
while true; do
  sh onerng.sh | dd bs=1K count=1000000 of=tmp-onerng.pad 2>/dev/null
  infnoise --raw | dd bs=1K count=1000000 of=tmp-infnoise.pad 2>/dev/null
  xor tmp-onerng.pad tmp-infnoise.pad /dev/urandom /dev/hwrng | dd bs=1K count=1000000 of=/home/pi/pads/1GB-`\date +%Y-%m-%d-%H%M%S`.pad 2>/dev/null;
done

Great, now you have a good one-time-pad and can join ok-mixnet ūüôā

P.S. If you really know what you’re doing and like shooting yourself in the foot, you could try combining and whitening entropy sources with a randomness sponge like keccak instead.

Crawling Etiquette

I participate in a mentoring program, and recently one of the people I mentor asked me about whether it was okay to crawl something. I thought I would share my response, which is posted below nearly verbatim.

For this article, I’m skipping the subject of how to scrape websites (as off-topic), or how to avoid bans.

People keep telling me that if I scrape pages like Amazon that I’ll get banned. I definitely don’t want this to happen! So, what is your opinion on this?

Generally bans are temporary (a day to two weeks). I’d advise getting used to it, if you want to do serious scraping! If it would be really inconvenient, either don’t scrape the site or learn to use a secondary IP, so when your scraper gets banned, you can still use the site as a user.

More importantly than getting banned, you should learn about why things like bans are in place, because they’re not easy to set up–someone decided it was a good idea. Try to be a good person. As a programmer, you can cause a computer to blindly access a website millions of times–you get a big multiplier on anything a normal person can do. As such, you can cause the owners and users of a site problems, even by accident. Learn scraping etiquette, and always remember there’s an actual computer sitting somewhere, and actual people running the site.

That said, there’s a big difference between sending a lot of traffic to a site that hosts local chili cookoff results, and amazon.com. You could cause make the chili cookoff site hard to access or run up a small bill for the owners if you screw up enough, while realistically there’s nothing you can do to slow down Amazon.com even if you tried.

Here are a couple reasons people want to ban automated scraping:

  1. It costs them money (bandwidth). Or, it makes the site unusable because too many “people” (all you) are trying to access it at once (congestion). Usually, it costs them money because the scaper is stupid–it’s something like a badly written search engine, which opens up every comment in a blog as a separate page, or opens up an infinite series of pages. For example, I host a bunch of large binaries (linux installers–big!), and I’ve had a search engine try to download every single one, once an hour. As a scraper, you can can avoid causing these problems by
    • rate-limiting your bot (ex. only scraping one page every 5-10 seconds, so you don’t overload their server). This is a good safety net–no matter what you do, you can’t break things too badly. If you’re downloading big files, you can also rate-limit your bandwidth or limit your total bandwidth quota.
    • examining what your scraper is doing as it runs (so you don’t download a bunch of unncessessary garbage, like computer-generated pages or a nearly-identical page for every blog comment)
    • obeying robots.txt, which you can probably get a scraping framework to do for you. you can choose to ignore robots.txt if you think you have a good reason to, but make sure you understand why robots.txt exists before you decide.
    • testing the site while you’re scraping by hand or with a computerized timer. If you see the site do something like load slower (even a little) because of what you’re doing, stop your scraper, and adjust your rate limit to be 10X smaller.
    • make your scraper smart. download only the pages you need. if you frequently stop and restart the scraper, have it remember the pages you downloaded–use some form of local cache to avoid re-downloading things. if you need to re-crawl (for example to maintain a mirror) pass if-modified-since HTTP headers.
    • declare an HTTP user-agent, which explains what you’re doing and how to contact you (email or phone) in case there is a problem. i’ve never had anyone actually contact me but as a site admin I have looked at user agents.
    • probably¬†some¬†more¬†stuff¬†i¬†can’t¬†think¬†of¬†off¬†the¬†top¬†of¬†my¬†head
  2. They want to keep their information secret and proprietary, because having their information publicly available would lose them money. This is the main reason Amazon will ban you–they don’t want their product databases published. My personal ethics says I generally ignore this consideration, but you may decide differently
  3. They have a problem with automated bots posting spam or making accounts. Since you’re not doing either, this doesn’t really apply to you, but your program may be caught by the same filters trying to keep non-humans out.

For now I would advise not yet doing any of the above, because you’re basically not doing serious scraping yet. Grabbing all the pages on xkcd.com is fine, and won’t hurt anyone. If you’re going to download more than (say) 10,000 URLs per run, start looking at the list above. One exception–DO look at what your bot does by hand (the list of URLs, and maybe the HTML results), because it will be educational.

Also, in my web crawler project I eventually want to grab the text on each page crawled and analysis it using the requests library. Is something like this prohibited?

Prohibited by whom? Is it against an agreement you signed without reading with Amazon? Is it against US law? Would Amazon rather you didn’t, while having no actual means to stop you? These are questions you’ll have to figure out for yourself, and how much you care about each answer. You’ll also find the more you look into it that none of the three have very satisfactory answers.

The answer of “what bad thing might happen if I do this” is perhaps less satisfying if you’re trying to uphold what you perceive as your responsibilities, but easier to answer.

These are the things that may happen if you annoy a person or company on the internet by scraping their site. What happens will depend both on what you do, and what entity you are annoying (more on the second). Editor’s note: Some of the below is USA-specific, especially the presence/absence of legal or government action.

  • You¬†may¬†be¬†shown¬†CAPTCHAs¬†to¬†see¬†if¬†you¬†are¬†a¬†human
  • Your¬†scaper’s¬†IP¬†or¬†IP¬†block¬†may¬†be¬†banned
  • You¬†or¬†your¬†scraper¬†may¬†be¬†blocked¬†in¬†some¬†what¬†you¬†don’t¬†understand
  • Your account may be deleted or banned (if your scraper uses an account, and rarely even if not)
  • They may yell at you, send you an angry email, or send you a polite email asking you to stop and/or informing you that you’re banned and who to contact if you’d like to change that
  • You may be sent a letter telling you to stop by a lawyer (a cease-and-desist letter), often with a threat of legal action if you do not
  • You may be sued. This could be either a legitimate attempt to sue you, or a sort of extra-intimidating cease-and-desist letter. The attempt could be successful, unsuccessful but need you to show up in court, or could be something you can ignore althogether.
  • You may be charged with some criminal charge such as computer, wire, or mail fraud. The only case I’m aware of offhand is Aaron Swartz
  • You may be brought up on some charge by the FBI, which will result in your computers being taken away and not returned, and possibly jailtime. This one will only happen if you are crawling a government site (and is not supposed to happen ever, but that’s the world we live in).

For what it’s worth, so far I have gotten up to the “polite email” section in my personal life. I do a reasonable amount of scraping, mostly of smaller sites.

[… section specific to Amazon cut …]

Craigslist, government sites, and traditional publishers (print, audio, and academic databases) are the only companies I know of that aggressively goes after scrapers through legal means, instead of technical means. Craigslist will send you a letter telling you to stop first.

What a company will do once you publicly post all the information on their site is another matter, and I have less advice there. There are several sites that offer information about historical Amazon prices, for what that’s worth.

You may find this article interesting (but unhelpful) if you are concerned about being sued. Jason Scott is one of the main technical people at the Internet Archive, and people sometimes object to things he posts online.

In my personal opinion, suing people or bringing criminal charges does not work in general, because most people scraping do not live in the USA, and may use technical means to disguise who they are. Scrapers may be impossible to sue or charge with anything. In short, a policy of trying to sue people who scape your site, will result in your site still being scraped. Also, most people running a site don’t have the resources to sue anyone in any case. So you shouldn’t expect this to be a common outcome, but basically a small percentage of people (mostly crackpots) and companies (RIAA and publishers) may.