Telnet

Telnet is a application layer protocol where one system can access and use another system over a virtual connection using Telnet Client and Telnet Server by logged into a user's account on port number 23, if they are connected locally.

How telnet works:

Step 1) Install Telnet Client and Telnet server at both client and server side

Step 2) Create a account (Username and password) in client side.

Step 3) Telnet client will send a request to Telnet Server.

Step 4) Telnet server will reply back with asking username and password.

Step 5) If accepted, Telnet Client will established a connection with client.

Step 6) Via virtual terminal, we have access to the client's system.

This all happen when client initiates TCP 3 way-handshake to the server.

Let's see how it does look like, we'll use wireshark for instance.

TCP Handshake before Telent:

Telnet Server IP: 192.168.0.1
Telnet Client IP:  192.168.0.2

 Telnet Server ask for login details to Telnet Client:

Telnet Client response with credentials:

UserID:

Password:

 And Finally Session is closed:

VPN

What is VPN
VPN or Virtual Private Network is a way of connecting a computer to a private network in a public network. It provide us with hard tracking connection by being an anonymous user.

Why We Use VPN
--> Access You Business Network while using public WiFi
--> Access your Home network over VPN while traveling
--> We can Hide our Browsing activity from our ISP
--> We can access Blocked websites
--> We can Download files
--> We can bypass internet over restricted zones...

Setup You Own VPN using OpenVPN: In CentOS

Step 1) Server setup

--> Use this link to select the platform you want to install

https://openvpn.net/index.php/access-server/download-openvpn-as-sw.html 

--> After selecting the OS, choose the OS version.

--> Right click on the OS version and copy link location.

--> Open Terminal and run as root user and type the following command:

--> #wget <Paste the link location>

--> e.g.,
wget "http://swupdate.openvpn.org/as/openvpn-as-2.1.12 CentOS7.x86_64.rpm"

--> After that, enter the following command

--> # rpm -i openvpn-as-2.1.12-CentOS7.x86_64.rpm

--> After some time, you will get server's and client's IP address.

--> For password, type the following command in terminal

--> # passwd openvpn

--> Choose password according to your need.

--> Copy Server's IP address and paste it in the url bar of broswer

--> On the browser you will get login interface, type

--> user : openvpn

--> password: <Your password>

--> Type agree to ULA (User License Agreement)

--> Now you have server up and running.

--> Create a user in the terminal. Use command:

--> # adduser <username>

--> #passwd <password>


Step 2) Client setup

--> In the browser, logout from the admin to come to the connect page and download client openvpn, which will be in the end of interface.

--> Open terminal, become a root user and install client.ovpn file

-->Now type the following command

--> #openvpn client.ovpn

--> # username <type your openvpn username>

--> #passwd <type your openvpn password>

--> Now your openvpn client is up and running

--> Open a new tab in browser and check your IP. It will have new location.


How It Works:

--> As we know, when a computer sends data to another computer through internet, it sends in a series of packets.

--> While in tunneling, the entire packet is send across the internet inside another packer aka encapsulated inside another packet.

--> This encapsulation hides the actual data from external view.

--> VPN creates a secure tunnel across the internet between our system and the our internet destination.

--> This tunnel is first created by authenticating our system with the VPN server (e.g., OpenVPN).

--> Server uses some encryption protocol to secure the data exchange in-between.

Protocols VPN uses:
PPTP: Point To Point Tunneling Protocol
L2TP: Layer 2 Tunneling Protocol
IPSec: Internet Protocol Security
SSL VPN: Secure Socket Layer VPN
SSH: Secure Shell

Other VPN's to use:
1) Openvpn
2) Strong VPN
3) SurfEasy
4) TunnelBear
5) IPVanish VPN
6) HideMyAss
7) Safer VPN

# Although all VPN's are not free, it'll cost you something.