All network devices transmit and receive information not in a single stream, but in separate network packets. At the same time, each network device is configured for a specific packet size (MTU) that can be transmitted per one session.
The MTU size value can be set independently or is set by the Internet provider. The maximum segment size (MSS) that is transmitted in a single MTU packet is also taken into account.
The most common MTU value is 1500 bytes. However, the operating system can reduce the MTU value if a connection through L2TP, PPTP, or IPsec is used.
Another method is implemented in the OpenVPN protocol, where the MSS value inside the packet changes and the MTU size does not change.
The verification tool analyzes the intercepted MTU and MSS packets and determines if a VPN is used or not. The simplest way to find out whether you are connected to a VPN service is to visit a “What Is My IP” page. If it indicates your actual location, you are not connected to a VPN, if it doesn’t, you are using a VPN.
VPN Fingerprint Verification Accuracy
It is important to consider that the following facts may affect the accuracy of the verification result:
- Different services check their own databases, so the results of determining the VPN fingerprint may vary.
- Internet providers can change the MTU size different from the value of 1500. This is done to coordinate the operation of network equipment from different manufacturers correctly.
- Users can independently adjust the MTU value to troubleshoot file transfer errors, lower ping, etc.
If you use a VPN and are faced with blocking access to the desired site, you could be blocked by VPN fingerprint.