If it is slowly to login ssh server...
the possible reasons for ssh Server
1. add "USEDNS no" in the /etc/ssh/ssh_config
2. remove the IPv6 related settings in the sshd server
the possible reasons for client
1. make sure you have properly configure the dns settings, because after the RSA@ssh negotiations, the ssh client will do some name resolution. If we don't properly configure the dns settings, this will slow down heavily the ssh client login processing (greater than 10 seconds).
If the login to ssh server was failed....
Sometimes while you are connect to the ssh server, you will receive the following information
# ssh 172.17.11.20
ssh_exchange_identification: Connection closed by remote host
This is probably the client IP address was added into the /etc/hosts.deny (such as DenyHosts package)
# DenyHosts: Thu Jun 19 13:26:16 2008 | sshd: 172.17.1.10