[Cronet startNetLogToFile] accepts the file name that is relative to the app root. An iOS app is not allowed to write to this location; however, it is allowed to write to the Documents directory or the temp directory. So, the current workaround is to prepend "Documents" directory to the path, e.g. [Cronet startNetLogToFile:@"Documents/NetLog.log" logBytes:NO];

Cronet should automatically create the correct absolute path. And pass it to the underlying implementation.

The absolute pass can be obtained by calling
[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory
                                       inDomains:NSUserDomainMask] lastObject];
or NSTemporaryDirectory().