How to get unique id in iOS device?

I have used mac address to identify iOS devices in server side. When run my application with iOS 7 unable to retrieve the correct mac address. Alternately i used

NSUUID *oNSUUID = [[UIDevice currentDevice] identifierForVendor];
[strApplicationUUID setString:[oNSUUID UUIDString]];

property. But this value also changed after every new installation of application. Now i want detect particular device in server side? How can i get some unique id per device in iOS?

You can no longer get a unique ID per device. identifierForVendor is the best you're going to get. Apple has systematically disabled identifying a specific device so that users' IDs can't be passed around by marketers.

To get the identifier ID as a string, you can use

let deviceId = UIDevice.current.identifierForVendor?.uuidString

UPDATE

If you want a universal ID then you have the option to use advertisingIdentifier. However, if the user has limited ad tracking in their device settings then this value will simply return all zeroes.

import AdSupport

let identifierManager = ASIdentifierManager.shared()
if identifierManager.isAdvertisingTrackingEnabled {
    let deviceId = identifierManager.advertisingIdentifier.uuidString
}

N.B. Apple recommends that this code only be used by companies building advertisement frameworks, rather than the app developers themselves. If you use this within your code for non-ad-related purposes then prepare to have your app rejected.

The best way to get the device id is identifierForVendor

UIDevice *device = [UIDevice currentDevice];

NSString  *currentDeviceId = [[device identifierForVendor]UUIDString];

UPDATE

For Swift 4.1 use

UIDevice.current.identifierForVendor?.uuidString

Getting device ID in

Swift 3

let device_id = UIDevice.currentDevice().identifierForVendor?.UUIDString

Swift 4:

let device_id = UIDevice.current.identifierForVendor!.uuidString

Getting Device Id in Swift 3.x

let deviceId = UIDevice.current.identifierForVendor?.uuidString

You can use this lib: https://github.com/fabiocaccamo/FCUUID

Please look on the table in the Persistence section. uuidForDevice will cover the most of needed cases and it is the best replacement to udid from old versions of iOS.

Examples in Readme are in Objective-C but it works with Swift too. I have used it since Swift first release until now (Swift 4.1).

For Swift 5

let deviceId = UIDevice.current.identifierForVendor?.uuidString