What is SSH?

It is a security shell communication protocol and enables to computers to communicate. SSH is based on a client-server architecture where the user system is the client and the remote system being managed is the server.
Command exemple to connec a server:

Use -v to verbose mode.

SSH server

SSH default port is 22, can be changed on the config file at /etc/sshd_config. It's possible to customize not only the port, but also listen address, max sessions, public keys authentication, password authentication etc.

Authorized keys

Authorized keys are stored at ~/.ssh/autorized_keys. It is possible to add direct at this file a public key or add from client with this command:

Find out more here:
https://manpages.ubuntu.com/manpages/trusty/man1/ssh-copy-id.1.html

SSH client

Guess what? SSH client default port is 22 as well, and can be changed too at /etc/ssh_config. As the server, it's possible to customize a bunch of configuration from port to authentication mode.

It is possible to simplify adding a file named config at ~/.ssh/ for exemple (~/.ssh/config):

Host bastian                //I chose bastian, but you can give the name you want
  Hostname 192.168.0.3      //IP address
  Port 7000                 //Change the port if you did at the server
  User root                 //User name

This way, the command will be:
ssh bastian

SSH keys

I need to be honest, at my fist job as a system administrator I had no idea how ssh works, and droped my jaw seeing a colleague using ssh-key connecting to a server. I needed to find out how it works and than I searched the internet and found a couple of tutorials and one of the best was this one:

Creating keys

How do I know my running key?

To show your identity key:

Error connecting to agent

If the ssh-agent is not running, you need to start it:

What if I have no identity key?

if the agent has no identities, you can set your identity key:

IMPORTANT NOTICE: This image was Image by pch.vector on Freepik