SSH – How to Fix SSH Dispatch Run Fatal Invalid Format Error

ssh

During the last years I've had various problems with MacOS SSH, but those were quickly resolved searching the net. This is now something different.

This is Mojave -system with lates updates (10.14.4). I do think though that this did begin even before updating with the latest patches.

$ ssh -V
OpenSSH_7.9p1, LibreSSL 2.7.3

The error I get is:

$ ssh <host> -v
---8<----
debug1: no key from blob. pkalg ssh-rsa: invalid format
ssh_dispatch_run_fatal: Connection to <host> port 22: invalid format

I've tried running without defined config-file, running with different options, but with no help. From other systems, such as Arch Linux (OpenSSH_7.9p1) and CentOS7 (OpenSSH_7.4p1) ssh'ing to the same host works well. Especially the same SSH version working on an arch linux tells me that this is something apple-related.

So what is happening now and how to fix this behaviour?

Best Answer

That error sounds like your SSH key may be corrupt. This SF Q&A titled: SSH Suddenly returning Invalid format mentions it as well with that same conclusion.

Your options are to either regenerate a new SSH key or try connecting using just your username + password.

Force SSH password method

To force SSH to use the password method for establishing a connection you can use this alias like so:

$ alias sshno='ssh -o PreferredAuthentications=password -o PubkeyAuthentication=no -o ControlPath=none'

You can then connect to servers with your username + password:

$ sshno <user>@<server>

Problem with SSH key length

In more recent versions of OSX Apple decided to drop support for keys that were < 2048 bits in length. This was apparently done with the migration to Sierra as discussed in this article titled: Upgrading to macOS Sierra will break your SSH keys and lock you out of your own servers.

You can verify your SSH key-pair's length like so:

$ ssh-keygen -lf ~/.ssh/ssh_slm@keynum1_id_rsa.pub
4096 SHA256:mwvSCr2CO5RXjML2EhgERTHgGB23JgRegB23Th34aeT slm@somedom.com (RSA)

This shows that my key's length is 4096 bits. You can generate a SSH key-pair with a specific length by using the -b switch to ssh-keygen.

$ ssh-keygen -b 4096 ....
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