ios - Detecting DNS failure with NSURLSession background config? -

is possible detect dns lookup failures when using nsurlsession background configuration ?

if use default config , completion handler, dns resolution failure reported in error parameter. if use background configuration however, failure never reported , delegate methods never called.

the apple documentation nsurlsessiontaskdelegate protocol says :

server errors not reported through error parameter. errors delegate receives through error parameter client-side errors, such being unable resolve hostname or connect host.

which suggests should seeing dns failure message there. code below illustrates behaviour. have tested on ios 8.4, , 9 on both device , simulator , searched dev forums, , using popular search engines come empty. i'm sure must missing simple.

import uikit  @uiapplicationmain class appdelegate: uiresponder, uiapplicationdelegate, nsurlsessiontaskdelegate {      var window: uiwindow?       func application(application: uiapplication, didfinishlaunchingwithoptions launchoptions: [nsobject: anyobject]?) -> bool {           let bgsesh = nsurlsession(configuration: nsurlsessionconfiguration.backgroundsessionconfigurationwithidentifier("foobarbazqwux"), delegate: self, delegatequeue: nil)         let dfsesh = nsurlsession(configuration: nsurlsessionconfiguration.defaultsessionconfiguration(), delegate: nil, delegatequeue: nil)          let url = nsurl(string: "http://foobarbaz.qwzx")!         let = dfsesh.downloadtaskwithurl(url) { (url:nsurl?, resp:nsurlresponse?, err:nserror?) -> void in             print(err)              /*              optional(error domain=nsurlerrordomain code=-1003             "a server specified hostname not found."              userinfo=0x146e8050 {                 nserrorfailingurlstringkey=http://foobarbaz.qwzx/,                 _kcfstreamerrorcodekey=8,                 nserrorfailingurlkey=http://foobarbaz.qwzx/,                  nslocalizeddescription=a server specified hostname not found.,                  _kcfstreamerrordomainkey=12,                  nsunderlyingerror=0x14693ab0 "the operation couldn’t completed. (kcferrordomaincfnetwork error -1003.)"             })             */         }         a.resume()          let b = bgsesh.downloadtaskwithurl(url)         b.resume()          return true     }     func urlsession(session: nsurlsession, task: nsurlsessiontask, didcompletewitherror error: nserror?) {         // never runs, dns failure not reported background config :-(         print("didcompletewitherror: \(task.taskidentifier) \(error)")     }     // ... other delegate methods ... }