[OpenIndiana-discuss] Newer OpenSSH causes probs for OI ssh

Harry Putnam reader at newsguy.com
Wed Nov 26 23:28:37 UTC 2014

I ran into a failure when attempting to ssh to a debian (jessie) host
on my lan.

(From here on the two hosts are OI-local and DEB-remote)

This is a new problem on machines I have had no problems ssh between
in the past.

The error looks like this:

harry-on-OI-local > ssh DEB-remote

| no common kex alg: client (hp -ed client=OI-local)
|diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1',server (hp -ed server=DEB-remote) 
| curve25519-sha256 at libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1'
| debug1: Calling cleanup 0x807a08c(0x0)

-vv flags give little else that is helpful so not included here.

What it means is that none of the kex algs in use by our version of
ssh are accepted by the DEB-remote host.

OK, it turns out there is a changelog note about the new version of
OpenSSH in use on debian jessie (6.7p1)

| openssh (1:6.7p1-1) unstable; urgency=medium
|   * New upstream release (http://www.openssh.com/txt/release-6.7):
|     - sshd(8): The default set of ciphers and MACs has been altered to
|       remove unsafe algorithms.  In particular, CBC ciphers and arcfour* are
|       disabled by default.  The full set of algorithms remains available if
|       configured explicitly via the Ciphers and MACs sshd_config options.
||  -- Colin Watson <cjwatson at debian.org>  Thu, 09 Oct 2014 14:05:56
|   +0100

So apparently they think all the kex Alg in use on our ssh are
`unsafe' and do not include any in the new defaults.

(The new defaults are the 6 algs mentioned in the error that belong to
the `server' (DEB-remote))

To solve this, I added one of the alg in use on our ssh back into the
offered kex algorithms on DEB-remote (/etc/ssh/sshd_config) like so:

|    KexAlgorithms  curve25519-sha256 at libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1

(followed by a restart of sshd)

I included the existing defaults and added back in, one of alg from our
version of ssh (diffie-hellman-group-exchange-sha1) 

That works.... and I am now able to ssh from OI-local to
DEB-remote. However, now I'm worried that I'm using something that
might create a security hole... my DEB-remote is exposed to ssh from
the internet.. not just the lan.

I did not discover what ssh binary we are using... (is it a Solaris
ssh?) by using pkg tools, other than the pkg name and version:

Can any of you old hands offer some commentary on this?  Is it safe?
Do we need to upgrade the ssh we are using?

More information about the openindiana-discuss mailing list