Slow HttpClient operation caused by DNS resolution failure on iOS

[rxwen]

The issue has been discussed in flutter, move it here since it's more relevant.

Our environment is iOS version: 13.3.1. iPhone 6S. (And some customers reported same problems on different devices, iPhone 7, iPhone X, latest iOS system as of now).

As a reference, I created a flutter sample project, with

flutter create --androidx -t app sample

Then, add some code in main.dart to perform a simple http get request, whenever user clicks the + button and display the time spent. (https://gist.github.com/rxwen/009c7ae4328ee799f71013f06e206b13#file-main-dart-L55)
This simplest example, exhibit the same behavior.

Took about 5 second on wifi (On the same device, using safari can open the same page instantaneously under the same wifi connection)
Took about 500 ms on 4G

I run sample on flutter v1.12.13+hotfix.8, v1.9.1 in debug and release mode. All got the similar results.

Further testing showed that it may be related to dns. I tried change the url to its ip address instead of domain name, the speed isn't slow at all.
And if I set the iPhone's dns to manual mode, and remove the ipv6 dns server (which is configured automatically), the problem is gone too.

So, here is the question, how come the ipv6 dns server will affect the flutter app's dns resolution? While other apps are not affected. It's not a feasible solution if we have to ask our app user to configure the dns manually when they encounter this problem. There still should be something different in dart code base that handle the dns in a different way.

[rxwen]

The problem is with dns resolution, I changed the sample to perform dns resolution as:
`
var sw = new Stopwatch()..start();

final ia = await InternetAddress.lookup("baidu.com");

sw.stop();

_counter = sw.elapsed.inMilliseconds;

`
It costs like 5 seconds to run.

The automatic dns configuration is:

If I removed the IPv6 dns server manually, or add a custom dns server (8.8.8.8), the resolution speed is ok.

The dig command test showed the 192.168.1.1 server is ok, and the ipv6 server doesn't.

[rxwen]

The router is a openwrt, it doesn't have upstream ipv6 connection.

Looks like when there are both ipv4 and ipv6 LAN connections on the iOS, the flutter performs dns resolution only with ipv6 (which should fail because there is no ipv6 upstream connection), but ignores the ipv4 dns server. Other applications still respect the ipv4 dns server setting, so they will not fail.

[rxwen]

Capture network traffic on iPhone showed that whenever I asked the app to lookup ip address, it always started a AAAA type lookup and failed. The A type lookup only happened once with a success result, and the result should have been cached.

The InternetAddress.lookup method waited until the type AAAA resolution failed eventually after 5 seconds and then returned. Thought it should be able to return the cached result quicker.

And I'm not sure if the ipv6 ip address is the root cause. As a user of the app reported the slow behavior, while his iPhone didn't obtained a ipv6 address at all.

If https://github.com/dart-lang/sdk/blob/master/runtime/bin/socket_base_linux.cc#L202 is the code for iOS platform, looks like it's implemented on top of getaddrinfo function.

[a-siva]

/cc @zichangg

[zichangg]

From your description, it is likely to be relative to our POSIX socket operations. In dart, we handle IOS and MacOS the same way with read,write and other POSIX sockets methods, which is discouraged by IOS man page.

@rxwen Can you verify that this happens only on IOS but not MacOS?

Some discussion: #41376

[rxwen]

I'm not that familiar with macos app development. So a side question is, do I need to follow this guide (https://flutter.dev/desktop) to try a macos app?

We haven't received any report on android platform with this issue so far.

[zichangg]

You'll need to run the same code on MacOS. Not so familiar with flutter setup either but your link looks good to me.

If you have dart(which should be included in Flutter), you can just run dart path/to/your_program.dart on Mac.

Android's implementation is quite similar but not enough to draw the conclusion. They use different eventhandlers and details are slightly different, which might contain a bug.

[rxwen]

Sure. Will try when I'm in the reproducible environment and let you know.

My initial suspect is could it be possible that it's blocked here till timeout?
If it's caused by the posix socket, does it mean that the flutter code is waiting for io_service's response on a socket without being able to return earlier?

[zichangg]

My initial suspect is could it be possible that it's blocked here till timeout?
If it's caused by the posix socket, does it mean that the flutter code is waiting for io_service's response on a socket without being able to return earlier?

It is possible. For lookup, it is blocking. Connection will be issued after addresses are resolved.
Checkout the implementation here:

sdk/sdk/lib/_internal/vm/bin/socket_patch.dart

Line 456 in 5bf1223

static Future<List<InternetAddress>> lookup(String host,