VPN is normally being used to connect two networks, unblock websites and make your connection anonymous. But did you know that a VPN can sometimes give you free internet access. Yes, you read that right a VPN can give you "Free internet access".
VPNs were originally used to join networks together, Large corporations, educational institutions, and government agencies use VPN technology to enable remote users to securely connect to a private network. And then the VPN became a tool to circumvent internet censorship. Tech savvy users use it to protect themselves from network sniffing. Some use it to bypass geo-blocking. And now its being used to get free internet access.
Free Internet Access
A VPN can give you free internet access by using the loop holes present on your network connection or Internet Service Provider(ISP). The VPN simply encapsulates all your network traffic pass it through the loop hole on your network to the VPN server.
Now let's list some loop holes that can normally be exploited so we can get free internet access.
ICMP ExploitThis is the most common loop hole that you can exploit. ICMP protocol is commonly left open because Network administrators use ICMP or what we commonly know as PING to diagnose network problems. If you open your command prompt and ping a website (eg: ping www.google.com) and you receive ping replies without any problem then you can use a VPN that has ICMP protocol to get free internet access.
DNS ExploitThis is another common exploit you can use as most firewalls allow DNS requests to pass freely. You use DNS when you want to resolve a hostname to its corresponding IP address. The DNS protocol uses UDP port 53. Most networks are configured to let any UDP packets that are destined to port 53 to pass freely because they assume that it is a DNS request. If you have a limited internet connection or do not have any data plan but you can still resolve hostnames, chances are you can set your VPN to connect to UDP Port 53 and you will be able to connect your VPN and get free internet access.
Host Header ExploitThis exploit can be used on a TCP vpn connection that allows you to set a custom HTTP header when connecting. This exploit works when your ISP let's you browse certain websites for free. The VPN basically adds an HTTP Host header when connecting to the VPN server, your ISP thinks that you are connecting to the free website but you are actually connecting to your VPN server.
Other open portsISPs have other ports that are still left open. As to why they are left open, I do not know, they are probably being used by their network services for example the famous UDP port 9200 is used for "WAP Connectionless Wireless Session Protocol" (WSP) which is related to WAP services. That is probably the reason why it was always left open by network providers.
There are still other ports that you can probably use. The port number you can use ranges from 1 to 65535. Some network providers also check the source port number or what we sometimes call as Local port number or Bind port number because this is the port that the VPN binds to. Sometimes ISPs only passes UDP/TCP packets that have a specific Source and Destination ports. So if you are looking for an open port number you might want to also mix and match remote and local port numbers. For example you can only connect to UDP port 9200 if you bind to port 53 but not if you let it bind to other port number.