How can I replace Raspian SSH host keys if I can't login to pi? ("connection refused")

Question

After configuring a new Raspberry Pi Zero W, I have not been able to ssh into it over WiFi. The IP address is assigned and pingable, but I get 'connection refused' errors.

After some research, I discovered one possible solution is to replace the SSH host keys on the pi.

But it's a Catch-22, because I can't login to the pi to change the keys that I need to change to be able to log into the pi.

What can I do?

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Why do I need to replace host keys?

Short answer: I don't know. I don't even know if that's the only or best solution or just one workaround. Host Keys are configured when ssh is configured. Did that configuration not run, or did it fail for some reason? One thing I did on both Pi's was added a dtoverlay=dwc2 line in config.txt to enable ethernet over USB. Could that have thwarted the boot/initialization sequence? Will run some experiments and post info if I learn anything new.

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Configuration details: The way I configured my Pi's are as follows: Using Etcher.app on macOS, I flashed an SSD card with the full desktop version of Raspian, 2018-11-13-raspbian-stretch-full.img, downloaded from the Raspian site, and installed it on a Pi Zero W and a Pi 3B+, both headless, and used the headless approach to configuring WiFi and ssh (i.e. touched ssh in /boot and created wpa_supplicant.conf in /boot).

I didn't have this problem when I similarly installed Raspian lite (on my Pi 3B, about a month ago, 2018-10-09-raspbian-stretch-lite.img

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Follow-up to @Milliway's comment below: This is the Reddit thread containing post that gave me the idea that regenerating host keys might fix the problem:

https://www.reddit.com/r/raspberry_pi/comments/704v5y/ssh_connection_refused_off_a_headless_setup/

Answer 1

HOW TO REPLACE SSH HOST KEYS ON RASPBIAN W/O BOOTING INTO IT

Note: I had this happen with a recent download and install of Raspian full... it occurred on a headless pi zero W and a headless pi 3B+. This solved it in both cases. Not sure what was wrong with the keys but I suspect something may have been broken in the distribution.

If your ssh "connection refused" error is due to a problem with the Raspbian host's ssh keys (and one way to find out is to update them), you can replace them without booting into Raspbian if you have access to a running Linux host, which I will call a "helper" host. (Note: I went this route because there aren't safe/supported solutions I could find for mounting Linux ext4 partitions in 'writable' mode on macOS).

Your Linux helper host could be Raspbian running on another Pi, Linux running on PC hardware, or if you only have a Windows or Mac host, it could be guest virtual machine running Linux with VirtualBox, etc...

Assuming you have a way to mount the Raspbian media on that host, it's easy to reconfigure the SSH keys by simply overwriting them, because virtually all Linux distributions, (including Raspbian), have openssh installed, which offers the same set of tools.

PROCEDURE

Let's assume you've installed Raspbian on a 16GB micro SD card.

1. Put that micro SD into a USB flash card reader, and insert it into a USB port on your Linux helper host. If you're using a virtual guest you'll have to configure it to intercept the USB device (micro SD card reader), otherwise your host machine may snag that USB host and the VM won't see it.

2. On the Linux helper host, as root, run fdisk -l to find the device associated with your Raspbian media. In particular, you want to find the device associated with Raspbian Linux ext4 partition.

   For example, in the following fdisk output the Linux partition is at /dev/sb2

   $ sudo fdisk -l
   
   Units: sectors of 1 * 512 = 512 bytes
   Sector size (logical/physical): 512 bytes / 512 bytes
   I/O size (minimum/optimal): 512 bytes / 512 bytes
   Disklabel type: dos
   Disk identifier: 0x989b1246
   
   Device     Boot Start      End  Sectors  Size Id Type
   /dev/sdb1        8192    98045    89854 43.9M  c W95 FAT32 (LBA)
   /dev/sdb2       98304 30375935 30277632 14.4G 83 Linux
   

3. Mount the Linux partition, as follows (we'll assume the device is /dev/sb2 and the mount point is /mnt through the rest of the example).

   _{Note: Your actual device and chosen mount point may be different and you'll have to adjust this procedure accordingly.}

   $ sudo mount /dev/sb2 /mnt
   

4. Now you can generate new keys to replace the old keys in your micro SD's (Raspbian OS) ssh directory:

   $ sudo ssh-keygen -f /mnt/etc/ssh/ssh_host_rsa_key -N '' -t rsa 
   $ sudo ssh-keygen -f /mnt/etc/ssh/ssh_host_dsa_key -N '' -t dsa
   $ sudo ssh-keygen -f /mnt/etc/ssh/ssh_host_ecdsa_key -N '' -t ecdsa -b 521
   

5. Now eject the micro SD card, so you can safely physically remove it:

   $ sudo eject /mnt  
   

   _{Note: Eject will fail if any terminal windows are still cd'd into any Raspbian partition directories, so either close any such windows or cd to a different directory.}

6. Re-insert the micro SD into the pi and try booting. Hopefully the ssh connection refused problem will be gone. Of course there are any number of ways ssh might not be working - not installed, out of date, misconfigured, not starting, etc...

Answer 2

This can happen when you have ssh'ed into a pi, then created a new filesystem (FS) (new sd flashed), and then attempt to ssh in again because the host keys are now different than what has been stored into your known_hosts.

Thus, your host that you are attempting to ssh from notes that the host keys of the Pi have changed and correctly sees this as a security issue (man-in-the-middle attack), and will refuse to complete the connection.

So, if you know that the problem is due to a legitimate change in the host key, then what you need to do is update ~/.ssh/known_hosts by removing the old and now incorrect key.

Procedure

While you can manually delete the offend key, it is far easier and safer to use the CLI. The releveant key is identified by the host with which you connected.

Given:

host = raspberrypi.local

Use this command:

$ ssh-keygen -R rasberrypi.local

You may need to delete it by IP.

You can then ssh in but will be warned about the new key.

Watch my YouTube video on what to do to ensure the key's fingerprint is correct.

https://www.youtube.com/watch?v=794sn9nllSM&t=2s