unlock a deny host

1.
$ service denyhosts stop

2.
$ vim /etc/hosts.deny
delete the IP from the file

3.
$ cd /va/lib/denyhosts
$ grep -iRH IP ./*

[root@server denyhosts]# grep -iRH IP ./*

./hosts:IP:0:Sun Feb  9 11:27:57 2014

./hosts-restricted:IP:0:Sun Feb  9 11:27:57 2014

./hosts-root:IP:0:Sun Feb  9 11:27:57 2014

./hosts-valid:IP:11:Sun Feb  9 11:31:57 2014

./users-hosts:username - IP:11:Sun Feb  9 11:31:57 2014

delete the IPs from all of the files. 

4. 

$ service denyhosts start

Generate SSH public key authentication

$ ssh-keygen
You can just press enter to create a rsa key pair;
.ssh/id_rsa is your priviate key stays in this machine
.ssh/id_rsa.pub is the public key you need to copy to the remote machine
I would suggest to use a passphrase, which only need to enter once for each OS boot

$ ssh-copy-id username@remote.machine.com
It copies the public key from local machine to a remote machine, and stores it at .ssh/authorized_key file

SSH into a computer behind NAT

Go to your router settings page, e.g 192.168.1.1
Go to the port forwarding setting page, configure following settings

External Port 2225	Internal Port 22	Protocol tcp/udp	To IP Address 192.168.1.102

You may want to set a normal port, because attacked cannot guess that. If you set 22 to 22, you may have may brute force attacks from outside. Set external port as normal port is more secure. 

Then: 

ssh username(AT)corsairxxv(DOT)gotdns(DOT)com -p 2225

or
scp -r -P 2225 username(AT)corsairxxv(DOT)gotdns(DOT)com:~/work/workspace/ChestTest ~/
scp -P 2225 file.zip corsairxxv.gotdns.com:~/