How to use SSH to connect to a remote server in Linux | ssh Command

Secure Shell, commonly known as SSH, is like a super-secure way to talk to faraway computers, called servers. It's like a secret tunnel on the internet that keeps your conversations safe and private. Imagine you're sending a letter, and instead of sending it openly, you put it in a magic envelope that only you and the person you're sending it to can open. That's what SSH does for your computer talks.

This article is here to help beginners, those who are just starting with this stuff, to understand how to use SSH. We'll show you the steps to use a special command (think of it like a secret handshake) to connect your computer to a faraway server in the world of Linux. By the end of this guide, you'll be more confident in using SSH to make your computer talks safe and secure when dealing with those remote servers.

What is SSH ?

SSH, or Secure Shell, constitutes a cryptographic network protocol designed to enable secure communication between two systems over networks that may not be secure. This protocol is widely employed for remote access to servers and the secure transmission of files between computers. In essence, SSH acts as a secure conduit, establishing a confidential channel for communication in scenarios where the network may pose security risks. This technology is instrumental for professionals seeking a reliable and secure method of managing servers and transferring sensitive data across computers in a controlled and protected manner. ssh runs at TCP/IP port 22. 

Syntax of SSH Command in Linux

The basic syntax for using the SSH command is as follows:

ssh [username]@[hostname or IP address]

Here,

Replace [username] with your remote server username, and [hostname or IP address] with the server's hostname or IP address.

Prerequisites

Before delving into the world of SSH, it's essential to ensure that certain prerequisites are in place. Here's a checklist to guarantee a smooth connection:

1. Remote Computer Status:
   • Make sure the remote computer is turned on and has an active network connection. SSH relies on network connectivity, and the remote server needs to be accessible.
2. Identification Information:
   • Obtain the IP address or the name of the remote machine. This information is crucial for directing your SSH connection to the correct server.
3. Permission to Access:
   • Ensure that you have the necessary permissions to access the remote computer. This typically involves having a valid username and password for the remote server.
4. Firewall Settings:
   • Check the firewall settings on both your local machine and the remote server. SSH connections use a specific port (usually port 22), so it's crucial to ensure that your firewall allows SSH traffic. Adjustments may be required to permit secure communication.

Install SSH Component on Linux

Setting up SSH on Linux may be necessary, as some distributions don't come with it pre-installed. Installing OpenSSH, a widely used SSH implementation, or opting for a graphical user interface (GUI) solution like the PuTTY client for Ubuntu can address this. Here's a step-by-step guide on installing and configuring OpenSSH on both the client and server sides:

Installing it on Both Client and Server

For Debian/Ubuntu-based Systems, open the terminal and run:

sudo apt install openssh-client openssh-server

For Red Hat-based systems like CentOS or Fedora, use either of the following commands:

sudo dnf install openssh-clients openssh-server

or

sudo yum install openssh-clients openssh-server

Note : To check the status of running server after installation we can use this command "systemctl status sshd"

In case the service is not running, run it with the following command:

sudo systemctl start sshd

The command does not print an output.

To have the service start automatically on boot, run:

sudo systemctl enable sshd

Enabling the sshd service starts it during the boot process.

How to Use SSH to Connect to a Remote Server in Linux

In this example we access Ubuntu or Red hat Linux machine via the Windows command prompt using `ssh`

For example: If our IP address is "10.143.90.2" and username is "Jayesh"

Syntax to use ssh to connect to a remote server:

ssh [email protected]

Add your username in place of "Jayesh" and add your IP address in place of "10.143.90.2"

command consists of 3 different parts:

• ssh command instructs the system to establish an encrypted secure connection with the host machine.
• user_name represents the account that is being accessed on the host.
• host refers to the machine which can be a computer or a router that is being accessed. It can be an IP address (e.g., 192.168.1.24) or domain e.g., www.domainname.com).

Note: After logging into the host computer, commands will work as if they were written directly to the host terminal. Using a public-private key pair or SSH key pair to login into the remote host is more secure as compared to using passwords. 

How to create public-private keys?

For generating public-private keys use the command:

ssh-keygen

The private key must remain hidden while the public key must be copied to the remote host. After copying the public key to the remote host, the connection will be established using SSH keys and not the password. 

Options available in ssh

Note: Here instead of user and host add username and IP address you want to connect to. And localhost is IP of our local system.

ssh -1 user@host

ssh -2 user@host

ssh -4 user@host

ssh -6 user@host

ssh -A user@host

ssh -a user@host

ssh -C user@host

ssh -c aes256-cbc user@host

ssh -f user@host command

ssh -g -L 8080:localhost:80 user@host

ssh -n user@host command

ssh -p 2222 user@host

ssh -q user@host

ssh -V

ssh -v user@host

ssh -X user@host

The Three Major Encryption Techniques Used by SSH.

SSH is significantly more secure than the other protocols such as telnet because of the encryption of the data. There are three major encryption techniques used by SSH:

• Symmetrical encryption: This encryption works on the principle of the generation of a single key for encrypting as well as decrypting the data. The secret key generated is distributed among the clients and the hosts for a secure connection. Symmetrical encryption is the most basic encryption and performs best when data is encrypted and decrypted on a single machine.
• Asymmetrical encryption: This encryption is more secure because it generates two different keys: Public and Private key. A public key is distributed to different host machines while the private key is kept securely on the client machine. A secure connection is established using this public-private key pair.
• Hashing: One-way hashing is an authentication technique which ensures that the received data is unaltered and comes from a genuine sender. A hash function is used to generate a hash code from the data. It is impossible to regenerate the data from the hash value. The hash value is calculated at the sender as well as the receiver's end. If the hash values match, the data is authentic.

Conclusion 

In this article we discussed Secure Shell (SSH) which is like a secret, safe tunnel for computers to talk securely over the internet. This guide is for beginners, helping them use SSH to connect their computer to faraway servers in the Linux world. It covers everything from the basic SSH command to prerequisites like checking your internet connection and having the right permissions. You'll learn how to install SSH on Linux, create secure keys, and use them for safer logins. The article also includes frequently asked questions with simple answers, making sure you can confidently and securely manage remote servers using SSH in Linux. It's like giving your computer a secret code to talk safely on the internet!