Recover / reset SSH access without external monitor?

Question

How can I regain SSH access if I:

• Have physical access to the device
• Do not have a way of plugging in an external monitor (or serial console)
• Was previously using SSH

Answer 1

Simple:

1. If you don't already have an SSH private/public keypair, generate one
2. Stick the SD card in the same machine as the SSH keypair
3. Append the public key (.pub extension) to the .ssh/authorize_keys file in home directory of the user you're trying to connect with - see "Appending", "Key location", and "Permissions" below for details.
4. Put SD card back in Pi and start it up
5. Connect using ssh -i (path to private key) (user)@(address)

If you were using SSH previously, you should be presented with a remote terminal. This just worked for me - enjoy.

Appending

In Linux, you can use the following to append one file to another: cat source_file >> detination_file. For me, appending my SSH public key was as simple as: cat ~/.ssh/id_rsa.pub >> /media/rootfs/home/pi/authorized_keys; however, be sure to check the result - sometimes there's a line break missing, and you'll need to insert one manually.

Key location

If you've got the SD card mounted on some other machine, you need to know two things:

1. Where the SD card is mounted
2. For the user you're trying to login with, the location on the SD card of the home directory (often /home/pi)

With these two, the location is: (SD card)/(path to home folder). For me on Pop OS!, this is /media/rootfs/home/pi/authorized_keys

Permissions

• The .ssh directory on the SD card must have be rwx for user ONLY (700)
• The authorized key must be rw for user ONLY

You can run these commands to set permissions correctly:

cd /media/rootfs/home/pi
chmod 700 .ssh
chmod 600 .ssh/authorized_keys