Get MAC address of bluetooth low energy peripheral in iOS

CBPeripheral's identifier property will serve your purpose, available from a still-unconnected device in CBCentralManager's didDiscoverPeripheral delegate method:

- (void)centralManager:(CBCentralManager *)central didDiscoverPeripheral:(CBPeripheral *)peripheral advertisementData:(NSDictionary *)advertisementData RSSI:(NSNumber *)RSSICBPeripheral *peripheral ...NSUUID* serverId = [peripheral identifier];

I have a half dozen LE devices I'm experimenting with, including multiple sets of identical devices. I just confirmed that across two iOS devices, the identifiers for all of these LE widgets were different, but for EACH iOS device, the identifier for each widget was retained across application launches and even across app deletes and reinstalls. I would say this definitively proves that the OS is storing enough info internally that for a given iThing, you'll be able to distinguish between, and re-identify, all the peripherals your app encounters, without actually connecting to them. Also note that the advertisementData, also available before connecting, is full of useful info like CBAdvertisementDataLocalNameKey, CBAdvertisementDataManufacturerDataKey, CBAdvertisementDataServiceUUIDsKey, CBAdvertisementDataSolicitedServiceUUIDsKey, and others, although none as certain to uniquely identify the device as [peripheral identifier] is.

I didn't try doing a device backup and restore to prove the UUIDs were retained, but I'd wager they are, and if they're not, it's something Apple would consider a bug.

Updated Answer :-

After iOS 12 we can get UDID

   print(UIDevice.current.identifierForVendor)   print(UIDevice.current.identifierForVendor?.uuidString)

Before iOS 12**

There is no public API to get this information.

If this is an internal or jailbreak application you can get the value of the kLockdownBluetoothAddressKey key via liblockdown.dylib

Low energy peripherals may use privacy feature which hides the MAC address, so it is not necessarily even possible to get the address before connection or bonding. If you somehow get the MAC address which goes over the air, you need to handle privacy or you have interoperability problems.

Apple uses UUIDs to abstract these privacy features out so users do not need to worry about those.

Correct way to do that like you wrote is to either add some vendor specific data to advertisement packet or use the Device Information service.