How to use SSH in Linux

Secure Shell, or SSH in short, is a remote connection protocol in Linux as well as other operating systems. It was first introduced as a replacement for telnet, which did not encrypt password information over the remote connection, and hence can be vulnerable to even simplest of attacks. SSH on the other hand uses advanced cryptography techniques to establish connection (Eg. RSA).

Open SSH is a free and open source implementation of SSH protocol in Linux.

Installing `ssh` and `sshd`

On Ubuntu and Debian, the package ssh can be used to install both Open SSH client and Open SSH server.

sudo apt install ssh

On CentOS and Fedora, run:

yum install openssh-server openssh-clients

Start SSH daemon to Allow Remote Connections

sshd is the daemon installed with Open SSH packages. To start the daemon, simply run:

sudo service sshd start

Connect to Remote Computer

To connect to a remote computer using SSH, an SSH daemon must be installed and running on that computer. You need to know the Hostname or IP Address of the computer and the username and its password. Needless to say, the computer should be accessible from your network.

ssh user@hostname

Many a times for automation purposes, there is a need to login to a remote computer without a input prompt for password. To achieve this, we use the RSA authentication method in SSH:

First, generate RSA key for SSH for your user:

ssh-keygen -t rsa

When prompted for a passphrase for this key, you may enter a passphrase as an additional security layer, or leave it empty.

We add this generated key to the authentication agent by running:

ssh-add

Aim is to copy this generated key to the remote computer. Hence, there has to be one login to the target computer/user to copy this generated key to the remote computer’s SSH config. ssh-copy-id copies the current users’ SSH key to a target computer: