When attempting to ssh from a core with ssh version < 6.7 to a md (or other system) with ssh version >= 6.7, the following error is shown:

no matching cipher found: client aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc server aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com,chacha20-poly1305@openssh.com

reference:

• 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.

fix: at the very list, the Ciphers line in /etc/ssh/ssh_config should be amended to contain "aes128-ctr,aes192-ctr,aes256-ctr"