使用Dart/Flutter的http.get访问本地IP返回400状态码问题排查

Let’s walk through the most likely fixes for your issue—since your API works with curl and browsers but fails in Dart/Flutter, the problem is almost certainly tied to request formatting or platform-specific network rules.

1. Fix the Request Header (Top Cause for 400 Status) #

Your code uses a header key content instead of the standard Content-Type. Most REST APIs strictly validate header names, and sending an invalid header can trigger a 400 Bad Request.

Even better: GET requests rarely need a Content-Type header (since they don’t include a request body). Try removing it entirely, or use the correct key if you do need it:

// Option 1: No Content-Type header for GET (preferred)
http.Response response1 = await http.get(url);

// Option 2: Correct header key if required by the API
http.Response response1 = await http.get(url, headers: {"Content-Type": "application/json"});

2. Match the Exact URL Path (Trailing Slash Mismatch) #

I noticed your HttpClient call uses a trailing slash (/api/device/state/) while your http.get call doesn’t (/api/device/state). Many web servers treat these as distinct endpoints, which could explain the 400.

Make sure your Dart/Flutter code uses the exact same path as your working curl command. For example, if curl uses http://192.168.0.5/api/device/state (no slash), update all your Dart requests to match that.

3. Fix Flutter’s Platform-Specific Network Restrictions (SocketException) #

The No route to host error is usually a permission or security policy issue on mobile:

Android #

• Add internet permission to android/app/src/main/AndroidManifest.xml:

  <uses-permission android:name="android.permission.INTERNET" />
  

• For Android 9+ (API 28+), cleartext HTTP (non-HTTPS) is blocked by default. Enable it by adding this to the <application> tag:

  android:usesCleartextTraffic="true"
  

iOS #

• Allow HTTP requests to your local device in ios/Runner/Info.plist. Add this section:

  <key>NSAppTransportSecurity</key>
  <dict>
      <key>NSExceptionDomains</key>
      <dict>
          <key>192.168.0.5</key>
          <dict>
              <key>NSIncludesSubdomains</key>
              <true/>
              <key>NSTemporaryExceptionAllowsInsecureHTTPLoads</key>
              <true/>
          </dict>
      </dict>
  </dict>
  

  This is more secure than allowing all arbitrary loads, as it only permits HTTP traffic to your specific local IP.

4. Mirror curl’s Request Headers Exactly #

Use curl -v http://192.168.0.5/api/device/state to see every header curl sends (look for lines starting with >). Then replicate those headers in your Dart code—some APIs reject requests with unusual User-Agent values or missing Accept headers.

Example of matching curl’s headers:

http.Response response = await http.get(
  url,
  headers: {
    "Accept": "*/*",
    "User-Agent": "curl/7.81.0", // Use your curl's User-Agent from the -v output
  },
);

5. Debug HttpClient with Proper Configuration #

Your current HttpClient code doesn’t set any headers, which might be causing issues. Try adding headers and logging more details:

HttpClient client = HttpClient();
client.getUrl(Uri.parse('http://192.168.0.5/api/device/state'))
  .then((request) {
    // Add headers matching curl's output
    request.headers.set('Accept', '*/*');
    return request.close();
  })
  .then((response) {
    print('Status Code: ${response.statusCode}');
    print('Reason Phrase: ${response.reasonPhrase}');
    response.transform(Utf8Decoder()).listen((body) => print('Body: $body'));
  })
  .catchError((error) => print('Error: $error'));

Start with fixing the header and URL path—those are the quickest wins. Then move to platform permissions if you’re still seeing socket errors.

内容的提问来源于stack exchange，提问作者Aleksander Żołądkiewicz