flutter - SocketException: Failed host lookup: ‘...com’ (OS Error: nodename nor servname provided, or not known, errno = 8)

Question

We are in a situation where the production app is facing the following socket exception and not able to perform any other network operation after this.?

DioError [DioErrorType.DEFAULT]: SocketException: Failed host lookup: ‘xyz.abc.com’ (OS Error: nodename nor servname provided, or not known, errno = 8)

We are using Dio as a network client, with Retrofit, which internally uses the HttpClient from the dart. With Dio the exception is not reproducible with the simulated environment but using HttpClient directly, the same exception can be reproduced with the following code in iOS simulator.

HttpClient userAgent = new HttpClient();
  bool run = true;
  while (run) {
    try {
      await userAgent.getUrl(Uri.parse('https://www.google.com'));
      print('Number of api executed');
    } catch (e) {
      print(e);
      if (e is SocketException) {
        if ((e as SocketException).osError.errorCode == 8)
          print('***** Exception Caught *****');
      }
    }
  }

Once the exception was thrown, the HttpClient was not able to recover from that stale state and all other API requests were started failing with the same error.

We were able to recover from that stale state by force closing all the previous connections and opening up a new HttpClient.

  HttpClient userAgent = new HttpClient();
  bool run = true;
  while (run) {
    try {
      await userAgent.getUrl(Uri.parse('https://www.google.com'));
      print('Number of api executed');
    } catch (e) {
      print(e);

      if (e is SocketException) {
        if ((e as SocketException).osError.errorCode == 8)
          print('***** Exception Caught *****');
      }
      userAgent.close(force: true);
      print('Force closing previous connections');
      userAgent = HttpClient();
      print('Creating new HttpClient instance');
    }
  }

One interesting fact is after every 236 requests the exception is raising. It could be because of file descriptors over usage but iOS has a limit of 256. ??

With a stable internet connection, this issue reproducible every time in iOS simulator.

Although I am not able to reproduce the issue with Dio client but as in production it is occurring. So I am seeking help to understand the root cause of this issue, also how we can prevent it?

Anyone who has come across this kind of situation and how you have overcome it, please help me.

Thanks in advance.

question from:https://stackoverflow.com/questions/66063913/socketexception-failed-host-lookup-com-os-error-nodename-nor-servname-p

Answer 1

That's a strange error.

This might not answer your question, but may push us towards figuring out what's going on.

The code snippet (copied from question) will open up a new stream with each .getUrl() call and will not close them. (I'm assuming this is intentional to create the socket exception?)

HttpClient userAgent = new HttpClient();
  bool run = true;
  while (run) {
    try {
      await userAgent.getUrl(Uri.parse('https://www.google.com'));
      print('Number of api executed');
    } catch (e) {
      print(e);
      if (e is SocketException) {
        if ((e as SocketException).osError.errorCode == 8)
          print('***** Exception Caught *****');
      }
    }
  }

At some point, a limit (of open streams) is hit. I guess that magic number is 236 in your case.

So at that point, is when you're seeing the nodename or servname provided exception?

(Btw, as an aside, I think that error is coming from the underlying host operating system's DNS service, although I'm not sure if it's due to the request spam, the number of open connections, etc. This may not be relevant info.)

So, if you used the HttpClient in a typical way, making requests & closing those open streams, such as this:

      var request = await userAgent.getUrl(Uri.parse('http://example.com/'));
      var response = await request.close(); // ← close the stream
      var body = await response.transform(utf8.decoder).join();
      // ↑ convert results to text
      // rinse, repeat... 

... Are you still seeing the same nodename or servname provided error pop up?

With this "typical usage" code immediately above, the userAgent can be reused until a userAgent.close() call is made (and the HttpClient is permanently closed. Trying to use it again would throw a Bad State exception).

I'd be interested to hear if the nodename error still occurs with this modified code.

---

Re: the second code snippet from the question.

In the catch block, the HttpClient is closed, then a new HttpClient is created. This effectively closes all the open streams that were opened in the try block (and I assume, resetting the limit of open streams.)

If you adjusted the 2nd code example to use:

      var req = await userAgent.getUrl(Uri.parse('https://www.google.com'));
      userAgent.close(force: true);
      userAgent = HttpClient();
      print('Number of api executed');

Could you run that indefinitely?