I just had this happen to me on a XEN host. I had duplicated this host from another, and as is common practice, I deleted the host keys in /etc/ssh after doing this, thinking that new ones would be generated later. But this never happened, and sshd happily started up with no host keys. When attempting to ssh to this host, it would drop out after SSH2_MSG_KEXINIT. All I had to do was create the host keys, which on debian based machine is done this way:
dpkg-reconfigure openssh-server
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
