OpenSSH client access issues after patching to version 7

After OpenSSH has been patched from vulnerable version 5 to the latest secure version 7.1p, we have encountered some connection issues with some of the clients.

# tail -f /var/log/messages 
fatal: Unable to negotiate with no matching cipher found. 
Their offer: aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc,blowfish-cbc,arcfour [preauth]

Root Cause:
Based on the version 7.1 release note, many ciphers have been disabled due to security issues:

OpenSSH 7.1 release note: 
 * Several ciphers will be disabled by default: blowfish-cbc,
   cast128-cbc, all arcfour variants and the rijndael-cbc aliases
   for AES.

Need to add legacy ciphers to sshd_config in order to support the ssh client:

# vim /etc/ssh/sshd_config
Ciphers aes128-ctr,aes192-ctr,aes256-ctr,,,,blowfish-cbc,aes128-cbc,3des-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc,blowfish-cbc,arcfour

After adding the ciphers and restarting daemon, same client encounter different error:

# tail -f /var/log/messages 
fatal: Unable to negotiate with no matching key exchange method found. Their offer: 
diffie-hellman-group1-sha1,diffie-hellman-group-exchange-sha1 [preauth]

Root Cause:
Based on the version 7.0 release note, some of the key exchange methods have been disabled

OpenSSH 7.0 release note: 
 * Support for the 1024-bit diffie-hellman-group1-sha1 key exchange
   is disabled by default at run-time. It may be re-enabled using
   the instructions at

 * ssh(1), sshd(8): extend Ciphers, MACs, KexAlgorithms,
   HostKeyAlgorithms, PubkeyAcceptedKeyTypes and HostbasedKeyTypes
   options to allow appending to the default set of algorithms
   instead of replacing it. Options may now be prefixed with a '+'
   to append to the default, e.g. "HostKeyAlgorithms=+ssh-dss".

To add the legacy MAC and key exchange algorithms back:

# vim /etc/ssh/sshd_config
MACs hmac-md5,hmac-sha1,,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-sha1-96,hmac-md5-96

KexAlgorithms +diffie-hellman-group1-sha1,diffie-hellman-group-exchange-sha1


  1. I think that we have to be competent and experienced enough in this field in order to freely perform such an action. Anyway, thank you for the description.


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