I upgraded macos, xcode and iOS runtime. I updated to Xcode 16, iOS 18 simulator runtime and macOS 15, I made a sample app and I see following error when trying to connect simple http server : Task <8DD928B9-BF28-422A-A96C-E79E8569DB92>.<1> HTTP load failed, 412/0 bytes (error code: -1005 [4:-4]) Task <8DD928B9-BF28-422A-A96C-E79E8569DB92>.<1> finished with error [-1005] Error Domain=NSURLErrorDomain Code=-1005 "The network connection was lost." UserInfo={_kCFStreamErrorCodeKey=-4, NSUnderlyingError=0x600000c88c60 {Error Domain=kCFErrorDomainCFNetwork Code=-1005 "(null)" UserInfo={NSErrorPeerAddressKey=<CFData 0x600002154500 [0x1e007f658]>{length = 16, capacity = 16, bytes = 0x10021f407f0000010000000000000000}, _kCFStreamErrorCodeKey=-4, _kCFStreamErrorDomainKey=4}}, _NSURLErrorFailingURLSessionTaskErrorKey=LocalDataTask <8DD928B9-BF28-422A-A96C-E79E8569DB92>.<1>, _NSURLErrorRelatedURLSessionTaskErrorKey=( "LocalDataTask <8DD928B9-BF28-422A-A96C-E79E8569DB92>.<1>" ), NSLocalizedDescription=The network connection was lost., NSErrorFailingURLStringKey=http://localhost:8000/, NSErrorFailingURLKey=http://localhost:8000/, _kCFStreamErrorDomainKey=4} Also added Sysdiagnose via feedback assistant no. https://feedbackassistant.apple.com/feedback/15379770 Let me know if any additional information is needed.

I have sent a sysdiagnose report via Feedback assistant no. FB15379770

Couple of observations: Nothing shows up in tcpdump when the issue occurs. When there is no issue, I see activity on tcpdump. I have seen this issue in my mobile app that uses URLSession, that is why I also verified in Safari to be sure, before raising this issue. Let me send a send a sysdiagnose report via Feedback Assistant

With additional help, I disabled the network filter. The issue still persists. What other things I could try? Additional logging or tooling I can use to narrow down the places where the resolution is dropped?

I also see following in simulator error logs when issue occurs: error 16:04:42.624021+0530 com.apple.WebKit.Networking WebContent[7071]: dlsym cannot find symbol WKBundleInitialize in CFBundle 0x104888460 </Library/Developer/CoreSimulator/Volumes/iOS_21C62/Library/Developer/CoreSimulator/Profiles/Runtimes/iOS 17.2.simruntime/Contents/Resources/RuntimeRoot/System/Cryptexes/OS/System/Library/PrivateFrameworks/MobileSafari.framework/PlugIns/Safari.wkbundle> (framework, loaded): dlsym(0xf34511, WKBundleInitialize): symbol not found error 16:04:42.624078+0530 com.apple.WebKit.Networking WebContent[7071]: dlsym cannot find symbol WKBundleAdditionalClassesForParameterCoder in CFBundle 0x104888460 </Library/Developer/CoreSimulator/Volumes/iOS_21C62/Library/Developer/CoreSimulator/Profiles/Runtimes/iOS 17.2.simruntime/Contents/Resources/RuntimeRoot/System/Cryptexes/OS/System/Library/PrivateFrameworks/MobileSafari.framework/PlugIns/Safari.wkbundle> (framework, loaded): dlsym(0xf34511, WKBundleAdditionalClassesForParameterCoder): symbol not found error 16:04:42.644778+0530 com.apple.WebKit.Networking Connection 56: received failure notification error 16:04:42.644804+0530 com.apple.WebKit.Networking Connection 56: failed to connect 12:8, reason -1 error 16:04:42.644818+0530 com.apple.WebKit.Networking Connection 56: encountered error(12:8) error 16:04:42.645241+0530 com.apple.WebKit.Networking Task <6243E5E4-5B34-460B-B73A-7E12778ED6D3>.<58> HTTP load failed, 0/0 bytes (error code: -1003 [12:8]) error 16:04:42.645405+0530 com.apple.WebKit.Networking Task <6243E5E4-5B34-460B-B73A-7E12778ED6D3>.<58> finished with error [-1003] <private>