Enable SSH on Ubuntu 18.04 System - How to do it ?

Secure Shell Service known as SSH which is used for secure connection between a client computer and a server. Using SSH you can connect to your system remotely, perform administrative tasks and access files.
Here at LinuxAPT, as part of our Server Management Services, we regularly help our Customers to perform related SSH queries on Ubuntu.
In this context, we shall look into how to enable SSH on Ubuntu to communicate securely with server.

How to Install SSH On Ubuntu ?

Before proceeding on this Installation Procedure, ensure that you are using an account with sudo privileges.
By default in Ubuntu system, SSH server is not installed so you need to install it first. It can be easily install from Ubuntu repository by issuing following commands.
i. First of all you need to update apt package manager by typing:

$ sudo apt update

ii. Now issue the below command to install SSH:

$ sudo apt install openssh-server

It will prompt you to enter password so just enter it and hit Enter key to continue with installation.
SSH service will be started automatically just after finished installation. 
iii. You can verify installation by issuing below command:

$ sudo systemctl status ssh

It should show status as active as given below:

● ssh.service - OpenBSD Secure Shell server
    Loaded: loaded (/lib/systemd/system/ssh.service; enabled; vendor preset: enab
    Active: active (running) since Mon 2019-04-25 05:08:17 UTC; 45s ago
  Main PID: 1001 (sshd)
     Tasks: 1 (limit: 1152)
    CGroup: /system.slice/ssh.service
            └─1001 /usr/sbin/sshd -D

iv. Press q key to go back to terminal.
Finally, you installed the SSH on your Ubuntu system and you can connect it via SSH from any remote machine. 
Linux systems have by default installed SSH client and if you have need to connect it from Windows system you can use PuTTY SSH client.

How to connect to SSH Over LAN ?
i. You can connect your Ubuntu system over LAN using following command:

$ ssh username@ip_address

ii. Change the username with your real Ubuntu user name and ip_address with your Ubuntu system ip address which you want to connect.
iii. If you don't know ip address of your system you can get it by running below command:

$ ifconfig

iv. Now go back to remote machine from where you were trying to connect and run below command:

$ ssh linuxapt@192.168.43.125

When you will connect it for first time, it will prompt you message like below:

The authenticity of host '192.168.43.125 (192.168.43.125)' can't be established.
ECDSA key fingerprint is SHA256:Rcbtm61VXuErBl7nE2+yowF7lgA2bLSiO/33/7qmRJP.
Are you sure you want to continue connecting (yes/no)?
Type yes and hit Enter key to continue. Next you will be prompted to enter password. Once you provide correct password then you will be logged in and show greeting message as below:
Welcome to Ubuntu 18.04.2 LTS (GNU/Linux 4.15.0-1035-aws x86_64)
 Documentation:  https://help.ubuntu.com
 Management:     https://landscape.canonical.com
 Support:        https://ubuntu.com/advantage 
 ...
 ...

How to connect to SSH Over Internet ?
For connect your Ubuntu machine over Internet you must know the public ip address of your machine.
Once you've found the IP address you can login by typing:

$ ssh username@public_ip_address

If you are exposing your machine to the internet you will need to configure the router to accept SSH traffic.

Mostly, to configure your router to accept SSH traffic on a non-standard port and forward it to port 22 on the machine running the SSH service.
You can also secure your connection by setting up a SSH key-based authentication to your Ubuntu machine.

How to Manage SSH Service ?
For any reason you want to stop SSH service then you can do it by execute below command:

$ sudo systemctl stop ssh

To start it you can issue below command and can start it:

$ sudo systemctl start ssh

You can enable SSH service by running following command:

$ sudo systemctl enable ssh

Again, you can disable the SSH service by issuing below command:

$ sudo systemctl disable ssh

[Need urgent assistance in fixing SSH Linux system errors? We can help you. ]

$ sudo apt-get update

$ sudo apt-get install openssh-server

$ sudo service ssh status