YouZan Mobile Testing Environment Switching Solution: VpnService + ZanProxy Implementation

This article introduces YouZan's solution for one-click switching of testing environments in mobile app development. The original workflow required complex setup including Charles proxy, Hosts configuration, gateway settings, Wi-Fi proxy configuration, and certificate installation, creating high learning curves for product and operations teams.

The solution combines Android VpnService (or iOS NetworkExtension) with ZanProxy, an open-source Node.js HTTP proxy server. Key technical implementations include:

1. Network Layer Processing: Using badvpn's tun2socks to convert L3 IP packets into SOCKS5 protocol. The IP packet structure contains IP Header and payload (TCP/UDP data), allowing protocol parsing at L3 and L4 layers.

2. SOCKS5 Proxy: Unlike HTTP proxies, SOCKS5 operates at the application layer and can forward any protocol by treating data as raw bytes, supporting both TCP and UDP traffic including custom protocols like IM sockets.

3. Multi-User Management: Using device UUID as username in SOCKS authentication to identify different users and apply different proxy rules.

4. ACL Implementation: Custom DNS server returns fake IPs (198.18.0.0/16) for domains needing proxy, while other traffic connects directly. This reduces server load significantly.

5. DNS Considerations: Both UDP and TCP DNS servers must be implemented since DNS responses over 512 bytes automatically switch to TCP.

The final solution provides a standalone internal app "YouZan Mobile Assistant" with features: one-click environment switching, PC-independent operation, no Wi-Fi proxy configuration needed, single certificate installation, support for packet capture and mocking, no intrusion to business app code, and support for IM socket/WebSocket forwarding.