I encountered this odd issue when I went to SSH from a new CentOS 7 box to an older system, and it just hung with no response. Turning on verbose showed me where it was hanging:
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
I found a number of suggestions on the ‘net as to how to handle this, but the below worked for me. It is a compatibility issue with a newer openSSH connecting to an older one that doesn’t have ciphers as strong.
You need to edit /etc/ssh_config and I added the following to the bottom, so it is under the “host *” items:
SendEnv LANG LC_*