Reproduction Steps:
1. Open "Bug" project
2. Select "Framework 4.6"
3. Build "test" scene to iOS
4. Deploy to device
5. Connect device to IPv6 network
Note: I was using NAT64 iMac
6. Press UI Button
7. Notice that error is thrown to Xcode's output

Error:
Libinfo call to mDNSResponder on main thread
SocketException: Could not resolve host 'ipv6.google.com'
at System.Net.Dns.Error_11001 (System.String hostName) [0x00000] in <00000000000000000000000000000000>:0
at System.Net.Dns.hostent_to_IPHostEntry (System.String originalHostName, System.String h_name, System.String[] h_aliases, System.String[] h_addrlist) [0x00000] in <00000000000000000000000000000000>:0
at System.Net.Dns.GetHostByName (System.String hostName) [0x00000] in <00000000000000000000000000000000>:0
at System.Net.Dns.GetHostEntry (System.String hostNameOrAddress) [0x00000] in <00000000000000000000000000000000>:0
at System.Net.Dns.GetHostAddresses (System.String hostNameOrAddress) [0x00000] in <00000000000000000000000000000000>:0
at System.Net.Sockets.Socket.BeginConnect (System.String host, System.Int32 port, System.AsyncCallback callback, System.Object state) [0x00000] in <00000000000000000000000000000000>:0
at test.Connect () [0x00000] in <00000000000000000000000000000000>:0
at UnityEngine.Events.InvokableCallList.Invoke (System.Object[] parameters) [0x00000] in <00000000000000000000000000000000>:0
at UnityEngine.EventSystems.ExecuteEvents.Execute[T] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.ExecuteEvents+EventFunction`1[T1] functor) [0x00000] in <00000000000000000000000000000000>:0
at UnityEngine.EventSystems.StandaloneInputModule.ProcessTouchPress (UnityEngine.EventSystems.PointerEventData pointerEvent, System.Boolean pressed, System.Boolean released) [0x00000] in <00000000000000000000000000000000>:0
at UnityEngine.EventSystems.StandaloneInputModule.ProcessTouchEvents () [0x00000] in <00000000000000000000000000000000>:0
at UnityEngine.EventSystems.StandaloneInputModule.Process () [0x00000] in <00000000000000000000000000000000>:0
UnityEngine.Logger:LogException(Exception, Object)
UnityEngine.Debug:LogException(Exception)
UnityEngine.EventSystems.ExecuteEvents:Execute(GameObject, BaseEventData, EventFunction`1)
UnityEngine.EventSystems.StandaloneInputModule:ProcessTouchPress(PointerEventData, Boolean, Boolean)
UnityEngine.EventSystems.StandaloneInputModule:ProcessTouchEvents()
UnityEngine.EventSystems.StandaloneInputModule:Process()

Expected behaviour: No error
Actual result: Error is thrown when connecting to IPv6.

Reproduced on versions: Unity 2017.1.0p2, 2017.1.0p5, 2017.2.0b8, 2017.3.0a5
Reproduced on device: iPhone 5 iOS 10.1.1

Note: Reproduces only with Framework 4.6
Note: Does not reproduce on Android or Editor