bill and his blog


SSH Oddities

Background

I was trying to debug a service over ssh that offered password-only authentication, but I couldn't seem to get the ssh client to prompt me for the password.

It looked something like this (skip ahead to the conclusion if you're pressed for time):

Troubleshooting

λ ssh root@[redacted]
Unable to negotiate with [redacted] port 22: no matching host key type found. Their offer: ssh-rsa

But the same command was working just fine for my coworker.

I took a closer look with ssh -v root@[redacted], but nothing jumped-out at me. Maybe it's something with my ssh configuration; let's remove that variable:

λ ssh -F /dev/null root@[redacted]
Unable to negotiate with [redacted] port 22: no matching host key type found. Their offer: ssh-rsa

Ah it looks like there's a way to set my preferred authentication method... -- me

λ ssh -F /dev/null -o PreferredAuthentications=password root@[redacted]
Unable to negotiate with [redacted] port 22: no matching host key type found. Their offer: ssh-rsa

Conclusion

Well it turns-out that newer SSH clients disable the ssh-rsa public key signature algorithm because it depends on SHA-1, which is considered insecure.

λ ssh -V
OpenSSH_9.0p1, OpenSSL 1.1.1p  21 Jun 2022

...and according to the ssh -v output, the server is running a pre-COVID(!!!) version of ssh:

debug1: Remote protocol version 2.0, remote software version dropbear_2018.76

So if you don't have time to upgrade the SSH server, and you just want to connect, the following should work because we're opting-into the less secure option:

λ ssh -o HostKeyAlgorithms=+ssh-rsa root@[redacted]