Add support for internal subnet feature, centralized main panel with multiple edge cluster management, and split DNS for internal vs public websites

[alovvvv]

📌 同步自上游：#849 by @Michael-Tang-2009 (2025-10-04)

Is your feature request related to a problem? Please describe.
Currently, s-ui works well for managing single servers and common use cases. However, in larger deployments where multiple edge servers (public CDN + internal VPN) need to be coordinated, there is no native way to:

• Define and manage internal subnet ranges for VPN users.
• Centrally control multiple servers/clusters from a main management panel.
• Configure DNS resolution rules to split traffic: internal websites → internal servers, public websites → public upstream DNS.

This limits s-ui for enterprise or hybrid deployments where both public-facing services and internal private services need to coexist under the same management umbrella.

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Describe the solution you'd like

1. Internal subnet feature

   • Allow admin to define subnet ranges for VPN users (e.g. 10.10.0.0/24) with granular access rules.
   • Support per-subnet policies for routing, firewall, and monitoring.

2. Main panel with multi-edge management

   • A centralized control plane (main panel) that can connect to and manage multiple edge servers (clusters).
   • Synchronize configurations, users, and statistics across multiple nodes.
   • Edge clusters can serve different roles, e.g. public CDN gateway or internal VPN gateway.

3. Split DNS resolution

   • Ability to configure DNS rules within s-ui panel:

     • Internal domains (e.g. *.corp.internal) → resolve via internal DNS servers.
     • Public domains → resolve via upstream DNS (e.g. DoH/DoT).

   • UI for adding/editing dns.rules and dns.servers.

   • Integration with routing so DNS results are consistently applied.

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Describe alternatives you've considered

• Manually editing sing-box JSON configs or in UI for each server (not scalable, error-prone).
• Using external DNS servers for split resolution (adds complexity, not directly managed in s-ui).
• Third-party orchestration for multi-node sync (overkill for medium deployments).

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Additional context
This feature set would make s-ui suitable for large-scale deployments, bridging both CDN-like public access and private internal VPN networks under unified management.
It would also align with enterprise-grade requirements: centralized control, internal/external traffic separation, and DNS flexibility.

[alovvvv]

💬 原评论 by @Kagashini (2026-01-01)

Approximate implementation for Main panel with multi-edge management: alireza0#954