Identifying Fingerprintable Features and Opcode-based Fingerprinting | HackerNoon
Briefly

OpenVPN employs distinct byte patterns and header structures, making it susceptible to fingerprinting. The header's opcode and key ID reveal important message types vital for session handling.
By analyzing packet lengths and server behaviors, researchers identified several fingerprintable features, enabling them to distinguish OpenVPN traffic from other services with high accuracy.
Read at Hackernoon
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