I have an old host that works as a NAS basically for backups. It has a linux inside and allows some (by then) fancy stuff out of the box. Recently I was leveraging it as a resource for backups via SSH and I ran into this error. Took me a bit to get the clue and a minimal explanation.

This small post explains it.

The context

So the host I am trying to connect is a WesternDigital MyCloud 3TB that I already talked about. Let's call it backups like how my local DNS knows it (its address is I stopped all services but the mandatory ones to perform the tasks of a NAS to host backups and hold a single share to communicate.

Later I researched a bit more to discover that I can actually log in properly through SSH and the idea was to also make it manage the backups from my fediverse instances with some scripts I'll create.

I am trying to connect from a Raspberry Pi 4 with Bullseye installed and a generated id_ed25519 key (just to be clear, but does not affect the case).

The issue

Trying to acces it from one of my Raspberry Pi machines, it quickly complained:

$ ssh root@backups
Unable to negotiate with port 22: no matching host key type found. Their offer: ssh-rsa,ssh-dss

The fix

A quick internet search throw a post from StackOverflow that suggests the following in a our ~/.ssh/config file:

Host backups
    HostKeyAlgorithms +ssh-rsa

If you don't have this file, simply create it.

Wait, but I need to understand what's actually wrong...

The reason

In the same answer there is a quick explanation:

The traditional ssh-rsa and ssh-dsa algorithms use SHA-1 for signatures, either with RSA or with DSA. SHA-1 is presently considered very weak and practical attacks have been demonstrated on it. In addition, DSA in OpenSSH is limited to 1024-bit keys, which provide an inadequate 80-bit security level (128 bits is the minimum recommended level).

As such, OpenSSH has disabled these algorithms because they are insecure, and if you are using them, the security of your connection cannot be assured. If possible, you should upgrade the firmware on your device to a newer version that will have appropriate patches applied. You should do this anyway, since if the version of the SSH server is old, it will probably have other security problems as well.

If you really cannot upgrade and you must continue to use this, then you can specify the configuration in your ~/.ssh/config.

So I search for the directive and I find its description from the man pages:

  Specifies the protocol version 2 host key algorithms that the client wants to use in order of preference.  The default for this option is:


  If hostkeys are known for the destination host then this default is modified to prefer their algorithms.

... and a command to discover our current accepted host keys:

$ ssh -Q HostKeyAlgorithms

Well, nice, but the error mentions ssh-rsa,ssh-dss that I not only have, also they're part of the default values according to the docs.

Then, what happens?

At this point, trying to get a clue, I read the nice explanation in this post by Ikarus on a similar (and yet more complex) adventure on SSH interconnectivity, but in his case PubKeyAcceptedAlgorithms was the key directive.

Long story short, we can check what HostKeyAlgorithms do we use when connecting to a specific host with:

$ ssh -G root@backups | grep hostkeyalgorithms
hostkeyalgorithms ssh-ed25519-cert-v01@openssh.com,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,sk-ssh-ed25519-cert-v01@openssh.com,sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,sk-ssh-ed25519@openssh.com,sk-ecdsa-sha2-nistp256@openssh.com,rsa-sha2-512,rsa-sha2-256

If you look closely the line, ssh-rsa does not appear, so it is not used. Why? because as explained in the Ikaru's post my OpenSSH has it deprecated and does not actually use it.

When we apply the fix above the answer is:

$ ssh -G root@backups | grep hostkeyalgorithms
hostkeyalgorithms ssh-ed25519-cert-v01@openssh.com,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,sk-ssh-ed25519-cert-v01@openssh.com,sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,sk-ssh-ed25519@openssh.com,sk-ecdsa-sha2-nistp256@openssh.com,rsa-sha2-512,rsa-sha2-256,ssh-rsa

So you can see that ssh-rsa has been added at the end, therefore now usable in the communication.

The observation

I cared to specify a fix that works and does not introduce new issues. And I mention that because some other sites mentioned the following content for the ~/.ssh/config file:

HostKeyAlgorithms ssh-rsa
PubkeyAcceptedKeyTypes ssh-rsa

This fix has 2 interconnected extra issues:

1️⃣ It is replacing the current set up

This means that from now on, all ssh communication will use ssh-rsa discarding the previous set up. At least this should be with a plus + sign so that ssh-rsa is ADDED to the Host Keys and not REPLACING them.

This would make it unable to continue communicating with other systems with Ed25519 keys.

For example, a better approach would have been:

HostKeyAlgorithms +ssh-rsa
PubkeyAcceptedKeyTypes +ssh-rsa

2️⃣ It is applied to ALL ssh communications

Which in my tests it made me loose the connection to GitHub, as I was using Ed25519 to communicate to them.

The proposed fix should have focused to the connection we want to stablish. Taking in account that RSA has been tagged as insecure, we want to use it only where there is no other way, and this means to target the fix only to the intended host and no other one

For example:

Host backups
    HostKeyAlgorithms ssh-rsa
    PubkeyAcceptedKeyTypes ssh-rsa

Also, I found no change/benefit on applying the PubkeyAcceptedKeyTypes, so I ended up discarding the directive when proposing the fix above.

The result

So, finally, when the fix is applied, the communication happens successfully:

ssh root@backups
The authenticity of host 'backups (' can't be established.
RSA key fingerprint is SHA256:1234567890abcdef.
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added 'backups' (RSA) to the list of known hosts.
root@backups's password:
Linux MyCloud-3TB 3.2.26 #1 SMP Thu Jul 9 11:14:15 PDT 2015 wd-2.4-rel armv7l

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.

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