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19 Apr 2024 06:26

Stupid Linux Tricks: get ssh host keys from new VMs via QR code in the console (also works if host/client is Windows)

One of my standard "tricks" in server admin is to have a brand new VM show its ssh key fingerprint as a QR code in the VM console - then I can just paste it directly into the ssh client prompt, since it's "yes/no/fingerprint" now. That way, I don't have to manually compare a string when I'm connecting to it for the very first time (and have no way to securely connect to it yet).

In the VM, all you need is the 'qrencode' package, which often has no additional dependencies.

Then, you can show small blocks of text as QR codes, drawn via box characters on the text console, like this:

ssh-keygen -lf /etc/ssh/ssh_host_ed25519_key.pub | qrencode -t ansi

To read this on the host system, use any means you like of creating a screen shot, then feed the resulting image into any of several QR code reader utilities. `qtqr` is one such example that can take an image file as input.

I wrote a short Python script to combine all the steps - this takes a screen shot, tries to decode a QR code in it, then spits the resulting text out on stdout (but only if there are no characters in it that might mess up the terminal - note that this will fail if the input text contains a unicode BOM):

#!/usr/bin/env python3

import tempfile
from PIL import ImageGrab
from qrtools import QR
import re
import base64

with tempfile.NamedTemporaryFile(mode="wb",suffix=".png") as tmpimage:

# obtain screen shot of entire screen, and save it to the temp file:
screenshot = ImageGrab.grab()
screenshot.save(fp=tmpimage)

qr = QR()
qr.decode(tmpimage.name)

# only print the string if it does not contain any non-ascii characters:
if not re.search('[^ -~\n\r\t]',qr.data):
print(qr.data)

# remove qrtools's auto-created temporary directory:
qr.destroy()

Usually, installing any QR decoding app will give you the dependencies for this script; if not, search their names in your distro's package manager.

*Bonus: What if the VM is Linux but the client or host system is Windows?*

I recently realised that the stock Windows "Camera" app has a QR code reader in it. You can trick it into introspection by very analogue means: hold a mirror up to your webcam.

...but QR codes can't be read in mirror image (most phones can because they try flipping the image if they can't read it, but the Camera app apparently does not do this).

Note that flipping an image vertically is just as mirrored as flipping it horizontally, and Linux

...so with one very tiny change to my usual command in the VM, I can read the QR code in Windows by holding up a mirror to the camera:

ssh-keygen -lf /etc/ssh/ssh_host_ed25519_key.pub | qrencode -t ansi

becomes

ssh-keygen -lf /etc/ssh/ssh_host_ed25519_key.pub | qrencode -t ansi | tac

('tac' being the command available on every Linux-like system, whose purpose is to read lines and then print them to stdout in reverse order. Its name is 'cat' backwards; 'cat' being the thing that reads and prints whatever is given to it forwards.)

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