ssh-keyscan broke when we upgraded OpenSSH

[Damien Miller]

On Fri, 1 Apr 2022, Pete Siemsen wrote:

> About 2 weeks ago, we upgraded a Debian Linux system to "OpenSSH_8.4p1
> Debian-5, OpenSSL 1.1.1n  15 Mar 2022". On that system, ssh-keyscan changed
> for the worse. It stopped working for about 30 of our network devices. When
> it fails, the "-v" command-line option shows that the reason is
> 
> debug1: kex: algorithm: (no match)
> 
> It seems that the upgraded version of ssh-keyscan doesn't like devices that
> use diffie-hellman-group-exchange-sha1. The ssh command continues to work
> with these devices, but the ssh-keyscan does not.
> 
> We are familiar with addressing this problem for the ssh command by setting
> KexAlgorithms lines in our ~/.ssh/config files. It seems that ssh-keyscan
> doesn't even look in the ~/.ssh/config file at all, so we can't do that.
> 
> We have a Python program that runs ssh-keyscan on 192 devices. Our
> work-around is to manually maintain a list of the devices that now don't
> work with ssh-keyscan. We use ssh to get the keys for those devices into a
> temporary ssh_known_hosts file, and merge that file with the dynamic one.

Yes, ssh-keyscan currently uses the ssh default key exchange algorithm
list and has no way of overriding it.

I guess we either need to include all KEX algorithms in ssh-keyscan's
list or to provide an option to do this.