Nasty SSH behaviour on LTS server upgrade

[Nils Kassube]

Nikhil Nair wrote:
> I've just done the latest LTS upgrade from 14.04.5 LTS (I think it
> was) to 16.04.1 LTS, using `sudo do-release-upgrade'.
> 
> I've been SSH'ing in from a Windows machine, using an old version of
> SecureCRT, a commercial SSH client.  I've hda no trouble connecting to
> 14.04.* LTS Ubuntu servers, but as soon as the upgrade to 16.04.1 LTS
> was completed and the machine was rebooted, the SSH client could no
> longer connect.  The message was as follows:
> 
> SecureCRT
> No compatible key exchange method. The server supports these methods:
> curve25519-sha256 at libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecd
> h-sha2-nistp521,diffie-hellman-group-ex
> change-sha256,diffie-hellman-group14-sha1
> No compatible Cipher. The server supports these ciphers:
> chacha20-poly1305 at openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-
> gcm at openssh.com,aes256-gcm at open ssh.com
> 
> I've verified, however, that I'm able to connect to the 16.04.1 LTS
> system *from* another not-yet-upgraded Ubuntu system (i.e. still
> running 14.04.5 LTS).
> 
> While I'm no expert on SSH servers, I'm guessing some compatibility
> options were disabled during the upgrade.  Is that right?  I'd have
> thought a custom sshd.conf (which I definitely had) should have been
> left alone, and that no functionality would be rmeoved from the ssh
> daemon, without a lot of very careful consideration, at least...
> 
> This behaviour wasn't deliberate, was it?

Yes obviously it was intentional. See the xenial release notes [1] - 
several weak chiphers were disabled for the new version. There is also a 
link to upstream instructions to reenable some of them. Of course it 
would be better to upgrade the ssh client to a modern version which can 
use the new chiphers.


Nils

[1] <https://wiki.ubuntu.com/XenialXerus/ReleaseNotes#OpenSSH_7.2p2>