OpenConnect Protocol Documentation
Welcome to the comprehensive documentation for the OpenConnect Protocol, based on reverse engineering of Cisco Secure Client 5.x+ (formerly AnyConnect).
What is This?
This documentation represents extensive reverse engineering and analysis of the proprietary OpenConnect VPN protocol as implemented by Cisco Secure Client. The goal is to provide:
- Complete protocol understanding for implementing compatible servers
- Security analysis of cryptographic implementations
- Binary analysis techniques for understanding proprietary protocols
- Implementation guidance for WolfSSL-based servers
Key Areas
Protocol Analysis
Deep dive into the protocol internals:
- Cryptographic Analysis - TLS, DTLS, and cipher implementation
- Authentication Methods - OTP, SAML, and multi-factor auth
- Certificate Handling - PKI and certificate validation
- NVM Telemetry - Network Visibility Module analysis
Implementation Guide
Practical guides for building compatible systems:
- WolfSSL Integration - Complete WolfSSL implementation
- Compatibility Guide - Cisco compatibility matrix
- Quick Start - Get up and running quickly
- Deployment Guide - Production deployment
Binary Analysis
Methodology and tools for reverse engineering:
- Decompilation Tools - IDA Pro, Ghidra, Binary Ninja
- Analysis Workflow - Step-by-step RE process
- Advanced Findings - Deep binary analysis results
Features
Protocol features and behaviors:
- DPD and Timers - Dead Peer Detection
- DNS Behavior - DNS handling and split-tunneling
- Optimal Gateway Selection - Gateway selection logic
- Windows Features - Platform-specific features
Target Audience
This documentation is intended for:
- Protocol Implementers - Building OpenConnect-compatible servers
- Security Researchers - Understanding Cisco's VPN security
- Network Engineers - Deploying and troubleshooting VPN infrastructure
- Reverse Engineers - Learning binary analysis techniques
Project Background
This work is part of the wolfguard project, which aims to create a modern, WolfSSL-based OpenConnect VPN server that maintains full compatibility with Cisco Secure Client 5.x+.
Why This Matters
- Open Implementation - Enables open-source VPN infrastructure
- Security Research - Transparent security analysis
- Interoperability - Better client/server compatibility
- Knowledge Preservation - Documents a widely-used but proprietary protocol
Getting Started
If you're new here, start with:
- Overview - High-level protocol overview
- Quick Start - Set up a test environment
- Comprehensive Summary - Executive summary of findings
Contributing
This documentation is a living project. Contributions, corrections, and additional analysis are welcome.
Related Projects:
- wolfguard - Modern OpenConnect server
- OpenConnect - Official OpenConnect client
Legal Notice
This documentation is the result of legitimate reverse engineering for interoperability purposes. All analysis was performed on legally obtained software for the purpose of creating compatible implementations.
Disclaimer: This documentation is provided for educational and interoperability purposes. Use responsibly and in accordance with applicable laws.
Last Updated: October 2025 Protocol Version Coverage: Cisco Secure Client 5.0 - 5.1+ Primary Analysis Platform: Windows, macOS, Linux clients