Connect to ssh-1 server from recent Ubuntu

[Karl Auer]

On Sun, 2025-06-29 at 22:37 +0100, Liam Proven wrote:
> A chap on a vintage-computing list wants to connect to a SunOS
> machine (pre-Solaris) with SSH from "recent Ubuntu".
> 
> SunOS that old only understands ssh v1. He says that seems not to
> have been available since Ubuntu 10.04.
> 
> Is that right? You can't specify older versions of the protocol?

Not sure if it's the ssh protocol itself, but ssh has withdrawn various
crypto methods over the years as they were revealed to be (or with the
march of time and compute power, became) insecure. I was unable to
connect to some old MikroTik kit because of this. I seem to recall
being able to add the methods back into ssh with KexAlgorithms.

Some likely client configuration item candidates might be:

   HostbasedKeyTypes
   HostKeyAlgorithms
   KexAlgorithms
   PubkeyAcceptedKeyTypes

There are cipher control configuration items in the sshd configuration
too, so if he can inspect what the server has going on, he might be
able to get a clue about what he needs to support client-side. See for
example:

   Ciphers
   HostbasedAcceptedKeyTypes"
   HostKeyAlgorithms
   KexAlgorithms
   PubkeyAcceptedKeyTypes

There is a strong implication in the ssh docs that it still supports
v1, so I'd be optimistic it was a crypto issue.

Getting the output from "ssh -vvvv" would be instructive - it would
certainly reveal whether it's ssh itself or just a crypto method that
is causing the problem. If he's lucky - because it could also be both
:-)

Regards, K.


-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Karl Auer (kauer at biplane.com.au, he/him)
http://www.biplane.com.au/kauer