OpenVPN

a VPN system that implements techniques to create secure point-to-point or site-to-site connections in routed or bridged configurations and remote access facilities. It implements both client and server applications.

OpenVPN is older, flexible, enterprise-heavy.

Uses:

• TLS
• OpenSSL
• certificates
• TUN/TAP

Can run over:

• UDP
• TCP

Usually UDP is preferred.

OpenVPN mostly runs in userspace. That makes it flexible but slower.

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Why OpenVPN over TCP is Bad

TCP-over-TCP meltdown is a catastrophic performance collapse that occurs when TCP traffic is tunneled through another TCP connection (VPN or SSH tunnel). It causes massive latency, throughput reduction, and connection stalls because both TCP layers try to handle congestion independently, leading to conflicting retransmissions.

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