Если на ssh подключение получаете Connection closed by, хотя ssh точно работает на удаленной стороне

ssh hostname
Connection closed by hostname

а при -v видно:

debug1: Local version string SSH-2.0-OpenSSH_6.6p1 Ubuntu-2ubuntu1
debug1: Remote protocol version 1.99, remote software version Cisco-1.25
debug1: no match: Cisco-1.25
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-cbc hmac-md5 none
debug1: kex: client->server aes128-cbc hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<3072 div="" sent="">
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP

то необходимо добавить в ~/.ssh/config:

KexAlgorithms=diffie-hellman-group14-sha1,diffie-hellman-group1-sha1