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WinHTTP Web Proxy AutoDiscovery is a feature in the WinHTTP library that allows applications to automatically detect and use web proxy settings without requiring manual configuration. This can be particularly useful for applications that need to work in environments where proxy settings may change frequently, such as corporate networks or public WiFi hotspots.

How It Works

1、Automatic Configuration Script (PAC): The primary method for autodiscovery involves using a PAC file. A PAC file is a JavaScript file that specifies how to determine which proxy server to use based on the URL being accessed.

2、WPAD (Web Proxy AutoDiscovery Protocol): When an application makes an HTTP request, it first checks if there's a PAC file specified in the system's proxy settings. If not, it attempts to retrieve the PAC file from a default location on the network, typicallyhttp://wpad/wpad.dat.

3、Direct Access: If no PAC file is found, the application will fall back to direct access, meaning it will make requests directly to the target server without going through a proxy.

Implementation in Code

Here’s a basic example of how you might set up WinHTTP to use automatic proxy discovery in C++:

#include <windows.h>
#include <winhttp.h>
#include <iostream>
#pragma comment(lib, "winhttp.lib")
int main() {
    HINTERNET hSession = WinHttpOpen(L"A WinHTTP Example Program/1.0",
                                     WINHTTP_ACCESS_TYPE_DEFAULT_PROXY,
                                     WINHTTP_NO_PROXY_NAME,
                                     WINHTTP_NO_PROXY_BYPASS, 0);
    if (!hSession) {
        std::cerr << "Error: Unable to open WinHTTP session." << std::endl;
        return 1;
    }
    // Set up the request
    HINTERNET hConnect = WinHttpConnect(hSession, L"www.example.com",
                                        INTERNET_DEFAULT_HTTP_PORT, 0);
    if (!hConnect) {
        std::cerr << "Error: Unable to connect to server." << std::endl;
        WinHttpCloseHandle(hSession);
        return 1;
    }
    HINTERNET hRequest = WinHttpOpenRequest(hConnect, L"GET", L"/",
                                            NULL, WINHTTP_NO_REFERER,
                                            WINHTTP_DEFAULT_ACCEPT_TYPES,
                                            WINHTTP_FLAG_SECURE); // Use WINHTTP_FLAG_SECURE for HTTPS
    if (!hRequest) {
        std::cerr << "Error: Unable to open request." << std::endl;
        WinHttpCloseHandle(hConnect);
        WinHttpCloseHandle(hSession);
        return 1;
    }
    // Send the request
    BOOL bResults = WinHttpSendRequest(hRequest,
                                       WINHTTP_NO_ADDITIONAL_HEADERS, 0,
                                       WINHTTP_NO_REQUEST_DATA, 0,
                                       0, 0);
    if (!bResults) {
        std::cerr << "Error: Unable to send request." << std::endl;
        WinHttpCloseHandle(hRequest);
        WinHttpCloseHandle(hConnect);
        WinHttpCloseHandle(hSession);
        return 1;
    }
    // End the request
    bResults = WinHttpReceiveResponse(hRequest, NULL);
    if (!bResults) {
        std::cerr << "Error: Unable to receive response." << std::endl;
        WinHttpCloseHandle(hRequest);
        WinHttpCloseHandle(hConnect);
        WinHttpCloseHandle(hSession);
        return 1;
    }
    // Read the data
    DWORD dwSize = 0;
    DWORD dwDownloaded = 0;
    LPSTR pszOutBuffer;
    do {
        // Check for available data.
        dwSize = 0;
        if (!WinHttpQueryDataAvailable(hRequest, &dwSize)) {
            std::cerr << "Error: Unable to query data available." << std::endl;
            break;
        }
        pszOutBuffer = new char[dwSize + 1];
        if (!pszOutBuffer) {
            std::cerr << "Error: Out of memory." << std::endl;
            break;
        }
        ZeroMemory(pszOutBuffer, dwSize + 1);
        if (!WinHttpReadData(hRequest, (LPVOID)pszOutBuffer, dwSize, &dwDownloaded)) {
            std::cerr << "Error: Unable to read data." << std::endl;
            break;
        } else {
            std::cout << pszOutBuffer;
        }
        delete[] pszOutBuffer;
    } while (dwSize > 0);
    // Cleanup
    WinHttpCloseHandle(hRequest);
    WinHttpCloseHandle(hConnect);
    WinHttpCloseHandle(hSession);
    return 0;
}

Key Points

Automatic Proxy Discovery: By settingWINHTTP_ACCESS_TYPE_DEFAULT_PROXY, WinHTTP will attempt to discover and use any configured proxies automatically.

Error Handling: Always check the return values of WinHTTP functions for errors.

Resource Management: Ensure that all handles are properly closed withWinHttpCloseHandle to avoid resource leaks.

This setup should allow your application to adapt to different network environments by leveraging the automatic proxy discovery capabilities provided by WinHTTP.