Enabling SSH in Ubuntu

Secure Shell (SSH) is a method primarily on *nix-based operating systems, which enables access to a computer, through a secure, two-way encrypted communication system. It was built as an alternative to telnet and other remote access software, which did not offer a secure means of connection. It is typically used as a way to access a remote system, for development, file sharing, etc. For a network administrator, it is an essential tool and one they should know how to set up, and operate.

SSH in Linux

In Linux, SSH uses a method of encryption called end-to-end encryption, where one user holds a public key, and another a private key. SSH is used via the terminal, hence it has a command-line interface

Installing SSH on Linux

Most Linux distros, and *nix operating systems come pre-installed with an SSH client, typically openSSH. You can check if you have SSH installed by checking the version of SSH on your system, using ssh -V command

1. If you don't have SSH installed on your system, you can install it using your default package manager utility (apt, in our case) of Ubuntu/Debian systems in the following manner

   sudo apt update
   sudo apt upgrade
   sudo apt install openssh-server

2. If you have just installed SSH, to check for a successful install, we can check if the SSH service is up and running in the following manner

   sudo service ssh status

   The output looks like the following

   ? ssh.service - OpenBSD Secure Shell server
       Loaded: loaded (/lib/systemd/system/ssh.service; enabled; vendor preset: enabled)
       Active: active (running) since Mon 2020-06-01 12:34:00 CEST; 9h ago
   ...

   The message you are looking for is Active: active (running).

3. If disabled, then we can start SSH using the following commands

   sudo systemctl enable ssh # Should start SSH when system is turned on
   sudo systemctl start ssh # Turning on SSH

4. If the firewall is enabled, to ensure SSH connections don't fail, we need to allow access through the firewall. The default firewall utility on Ubuntu is ufw. To allow SSH connections, we need to open the port using

   sudo ufw allow ssh

That's it. Now you can successfully connect to your Ubuntu system from any remote machine, using SSH.

Basic SSH Commands

For now, we have seen how to check if SSH is installed, or how to install it. Now we will cover how to enable and run SSH.

To connect to your Ubuntu system over LAN, invoke ssh, with your username and IP address in the following format

ssh username@<IP or Domain>

If you don't know your IP address, you can find it using ip a.

Over here the system IP is 172.31.166.141.

Hence to connect to this system we run

ssh delta@172.31.166.141

For first time connections, you will be greeted with a message in the following format

The authenticity of host '172.31.166.141 (172.31.166.141)' can't be established.
ECDSA key fingerprint is SHA256:Vybt22mVXuNuB5unE++yowF7lgA/9/2bLSiO3qmYWBY.
Are you sure you want to continue connecting (yes/no)?

Type yes, and you will be prompted for a password. Once you enter the password, you will be greeted with the default Ubuntu message

Welcome to Ubuntu 20.04 LTS (GNU/Linux 5.4.0-26-generic x86_64)

 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/advantage
...

SSH Configuarion Options

To configure SSH, all relevant files are stored in /etc/ssh/. Generally, on install, you can change your default port ( precautionary safety measure ), disable root access ( a smart decision ), and/or make other configuration adjustments.

There are generally two files in this directory used for configuration purposes

• ssh_config: used to configure SSH clients. It defines rules used when connecting to a remote host.
• sshd_config: used to configure self's SSH server. Used to define the reachable SSH port, and if root access is allowed and so on.

1. Open your SSH configuration file, using your favourite editor ( command-line or GUI )

   sudo vim /etc/ssh/sshd_config

2. For example, you want to change the default TCP port from 22 to 1337, find the line with Port 22 specified, and edit it to Port 1337.

   
   Also, allow this port using ufw if setup, with the following command

   sudo ufw allow 1337

3. To deny root access, even if the root password is available, find the line with PermitRootLogin, and write a no at the end.
   
4. To ensure the changes take place, restart your SSH server, using the following command

   sudo systemctl restart sshd

Connecting SSH on a specific port

To connect your SSH server using a specific port, you can use the following command

ssh -p <Port Number> username@<IP or Domain>

Exiting or Disabling SSH

To exit your SSH connection, you can just run the command exit, or hit the key combination <Ctrl-D>.

To disable your SSH server, you need to run the following command

sudo systemctl stop ssh # To stop SSH service
sudo systemctl disable ssh # Don't start SSH on system power on

Conclusion

This tutorial covered how to install, and/or enable SSH on your system. After that how to connect, using a port number if needed, and how to edit your configuration files, to ensure better security.