Hi everyone. I wanted to share my latest experience with you in case anyone else runs into this.
After applying the 6.0U2 update to a standalone ESXi host I was unable to SSH to the host anymore from PuTTY on my Windows workstation. I know that the 6.0U2 update had some pretty big changes to the SSH daemon so I was pretty sure it was related to that. I searched Google and the VMware knowledge base without much luck. I ran across these articles:
- Connecting to the ESXi host using SSH fails with the error: Network error: Connection Refused (10390...
- Unable to connect to ESXi host using SSH (2041660)
But those articles don't apply to 6.x. 
But they pointed me in a direction to check in the /var/log/auth.log file for any errors. I did indeed find the following errors:
016-06-20T21:52:38Z sshd[40905]: error: Hm, kex protocol error: type 30 seq 1
I did a quick Google search and ran across this article: https://blog.nytsoi.net/2015/07/13/putty-kex-error. The problem (as I understand it) is that the key exchange method "Diffie-Hellman group exchange" is not supported with the version of OpenSSH that is used on the ESXi host. The work-around was fairly easy. The blog article referenced above says:
In PuTTY, go to “Connection → SSH → Kex”. There, select “Diffie-Hellman group exchange” and move it to the bottom of the list, so it is not used.
As soon as I did that, I was able to use PuTTY to connect to the standalone ESXi host.
Hope anyone else finds this information useful. It definitely had me scratching my head for a while.
