Definition
OpenVPN is an open-source VPN protocol and software suite first released in 2001 by James Yonan. It uses the OpenSSL library for its cryptography, runs in user space (which makes it portable across operating systems), and can use either UDP (preferred for performance) or TCP (preferred for traversing restrictive networks). It is the most widely deployed open-source VPN protocol in commercial consumer VPNs.
How It Works
An OpenVPN session begins with a TLS handshake — the client and server authenticate with X.509 certificates, negotiate a cipher, and derive session keys. Once established, the data channel encrypts each packet with the negotiated cipher (AES-256-GCM is the modern default) and routes it inside a virtual network interface. UDP mode minimizes latency; TCP mode provides reliability over lossy networks but introduces TCP-over-TCP performance issues.
Strengths and Weaknesses
Strengths: extensively audited, mature, configurable, supported by virtually every commercial VPN provider, can run on any port (including 443 to bypass firewalls). Weaknesses: relatively heavy compared with WireGuard, slower handshake, more CPU-intensive at high throughput, larger codebase that is harder to audit comprehensively. It remains a strong choice and is still the default for many providers, but WireGuard is rapidly displacing it for performance-sensitive use.