NSURLErrorDomain Code=-1004 for few seconds after app start up - ios

Question

I'm getting "NSURLErrorDomain Code=-1004" error with Alamofire API calls,
but only for few seconds after app started (or took a rest for few minutes while the app's opened and make a call after that)
If I try to make the same call after few seconds, everything works fine.
I searched all the Stack Overflow questions and checked all the possible causes below:
No problem with Internet connection
"App Transport Security Settings" are correct and server's using https (I also tried "NSAllowsArbitraryLoads=true" but that didn't help)
APIs working fine
My gut feeling is that getting the network settings takes few seconds and when I make an API call before that's done, it just fails immediately. OR.. I'm using a Websocket in background that could be related?
FAILURE: Error Domain=NSURLErrorDomain Code=-1004 "Could not connect to the server." UserInfo={NSUnderlyingError=0x137d39380 {Error Domain=kCFErrorDomainCFNetwork Code=-1004 "(null)" UserInfo={NSErrorPeerAddressKey={length = 16, capacity = 16, bytes = 0x100201bb341a9f540000000000000000}, _kCFStreamErrorCodeKey=-2200, _kCFStreamErrorDomainKey=4}}, NSErrorFailingURLStringKey=[FILTERED], NSErrorFailingURLKey=[FILTERED], _kCFStreamErrorDomainKey=4, _kCFStreamErrorCodeKey=-2200, NSLocalizedDescription=Could not connect to the server.}
Any suggestions?
UPDATED
Found that app makes 4 requests on launch, and 1 or 2 of them randomly fails, and I checked Nginx access and error log and there's no log for the failed calls at all.

Answer 1

We have the same issue here with Nginx 1.10.0 (and 1.9.15), iOS 9.3.1 using HTTP/2 with TLS 1.2.
The issue goes away with HTTP/1.1 and it's also working with HTTP/2 in Nginx version up to 1.9.14.

Answer 2

Nginx 1.11.0 Mainline is now available with the fix included mentioned earlier in this topic;
Change: HTTP/2 clients can now start sending request body
immediately; the "http2_body_preread_size" directive controls size of
the buffer used before nginx will start reading client request body.
I tested it and for me this release now works correctly again.

Answer 3

This seems like a confirmed bug in nginx 1.10. An issue about it can be found on the bug tracker of nginx at https://trac.nginx.org/nginx/ticket/979. The actual issue can be found at https://trac.nginx.org/nginx/ticket/959
You might want to consider switching to the 1.9 branch which has releases which do work. Hopefully nginx will release a 1.10.1 version soon which does not have this bug.
The issue actually only occurs on iOS; Android, Windows and OSX itself seem to have no problems negotiating a valid http2 connection.

Answer 4

I can also confirm that the nginx 1.9.15 not works correctly. Some calls always got "Could not connect to the server", and after revert to nginx 1.9.12 everything works fine.

Answer 5

These are the steps that I would try to follow:
1) test my app in the simulator and device
2) look if https is really needed instead of http
3) configure an alamofire manager and change timeout (for this step i
write some code):
var alamofireManager = Alamofire.Manager.sharedInstance
let configuration = NSURLSessionConfiguration.defaultSessionConfiguration()
configuration.HTTPMaximumConnectionsPerHost = 10
configuration.timeoutIntervalForRequest = 30
configuration.timeoutIntervalForResource = 30
alamofireManager.delegate.taskWillPerformHTTPRedirection = nil
(so whit this last step the next alamofire calls can be for example: alamofireManager.request(etc....)
4) test with hard link like http://www.google.com, if the same thing
dont happened nothing around your swift code are incorrect, try to
set your web server parameters..

Answer 6

Problem solved!!!
versions:
1. Nginx version: 1.10.2
2. IOS version: 9.3.2
When the config like this:
listen 443 ssl;
Have the same problem like you.
But !!!
When the config like this:
listen 443 ssl http2;
Problem solved!!