Windows Network Detector

A lightweight C# .NET 8 console application that detects internet connectivity on Windows machines using a 4-layer fallback strategy — designed to avoid false positives from proxies and false negatives from HTTP-level blocking.

Requirements: Windows 10 or later, .NET 8 runtime.

Problem

Traditional network checks fail in two opposite directions:

• False negatives: HTTP-based checks (e.g. HttpClient.GetAsync("https://www.google.com")) fail even when the network works. HTTP depends on DNS → TCP → TLS → HTTP, and failure at any layer produces a false "no network" result.
• False positives: System proxies (e.g. TUN-based VPN/代理) create virtual network adapters that stay "up" even when the real network is down. TCP connections succeed because they connect to the local proxy process, not the internet.

Solution

Four independent probes cascade sequentially. Each layer validates at a different level, from fastest/lightest to most thorough.

Layer	Method	Protocol	What it verifies	Timeout
1	Windows NCSI	COM API (zero I/O)	OS-level connectivity state	—
2	DNS Query	UDP → public DNS	DNS resolution works	2s/server
3	HTTP GET	HTTP with UseProxy=false	Real internet data (bypasses proxy)	2s/endpoint
4	TCP + HTTP validate	TCP SYN + raw HTTP GET	Connection returns real HTTP response	4s/target

Worst-case total: ~12 seconds (all layers fail).

Usage

Default output

.\NetworkDetector.exe

Output:

Online

Exit code 0 = Online, 1 = Offline.

JSON output

.\NetworkDetector.exe --json

Output:

{"status":"online","layer":1,"latency_ms":0,"detail":null}

Build

cd NetworkDetector
dotnet build
dotnet run

Publish standalone executable

dotnet publish -c Release -r win-x64 --self-contained -p:PublishSingleFile=true -o ../bin

This produces a single NetworkDetector.exe (~50-70 MB) with the .NET runtime bundled — no runtime installation needed on the target machine.

Project Structure

NetworkDetector/
├── Program.cs           # Entry point, arg parsing, layer orchestrator
├── NetworkResult.cs     # Shared result model (enum + record)
├── NcsiProbe.cs         # Layer 1: Windows NCSI COM query
├── DnsProbe.cs          # Layer 2: Manual UDP DNS query (no third-party lib)
├── HttpProbe.cs         # Layer 3: HTTP GET bypassing proxy, verifies response body
└── TcpProbe.cs          # Layer 4: TCP connect + HTTP validate (catches proxy-faked connections)

Design Decisions

• Proxy bypass — Layer 3 sets UseProxy = false and verifies the actual response body (not just status code) to guard against proxies returning fake responses. Layer 4 sends a raw HTTP GET over a raw TCP socket, completely bypassing the .NET HTTP stack and any system proxy configuration.
• China-accessible targets — DNS probes use Alibaba (223.5.5.5), Tencent (119.29.29.29), and 114 (114.114.114.114). HTTP probes use connectivitycheck.gstatic.com and cp.cloudflare.com. TCP targets are Baidu (110.242.68.66) and Alibaba DNS (223.5.5.5).
• No third-party dependencies — pure .NET BCL + P/Invoke for Windows COM.
• No retries within a layer — if a probe fails, move to the next target or layer. This keeps the worst-case latency bounded.

Integration

This is distributed as a standalone exe for testing. To integrate into your WPF/WPF+WebView2 project, reference the NetworkDetector project directly and call:

var result = NcsiProbe.TryDetect()
    ?? DnsProbe.TryDetect()
    ?? HttpProbe.TryDetect()
    ?? TcpProbe.TryDetect()
    ?? new NetworkResult(NetworkStatus.Offline, null, null);

bool isOnline = result.Status == NetworkStatus.Online;