Triggering the Local Network Privacy Alert

Currently there is no way to explicitly trigger the local network privacy alert (r. 69157424). However, you can bring it up implicitly by sending dummy traffic to a local network address. The code below shows one way to do this. It finds all IPv4 and IPv6 addresses associated with broadcast-capable network interfaces and sends a UDP datagram to each one. This should trigger the local network privacy alert, assuming the alert hasn’t already been displayed for your app.

Oh, and if Objective-C is more your style, use this code instead.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@apple.com"

Code Block  
import Foundation
/// Does a best effort attempt to trigger the local network privacy alert.
///
/// It works by sending a UDP datagram to the discard service (port 9) of every
/// IP address associated with a broadcast-capable interface. This should
/// trigger the local network privacy alert, assuming the alert hasn’t already
/// been displayed for this app.
///
/// This code takes a ‘best effort’. It handles errors by ignoring them. As
/// such, there’s guarantee that it’ll actually trigger the alert.
///
/// - note: iOS devices don’t actually run the discard service. I’m using it
/// here because I need a port to send the UDP datagram to and port 9 is
/// always going to be safe (either the discard service is running, in which
/// case it will discard the datagram, or it’s not, in which case the TCP/IP
/// stack will discard it).
///
/// There should be a proper API for this (r. 69157424).
///
/// For more background on this, see [Triggering the Local Network Privacy Alert](https://developer.apple.com/forums/thread/663768).
func triggerLocalNetworkPrivacyAlert() {
    let sock4 = socket(AF_INET, SOCK_DGRAM, 0)
    guard sock4 >= 0 else { return }
    defer { close(sock4) }
    let sock6 = socket(AF_INET6, SOCK_DGRAM, 0)
    guard sock6 >= 0 else { return }
    defer { close(sock6) }
    
    let addresses = addressesOfDiscardServiceOnBroadcastCapableInterfaces()
    var message = [UInt8]("!".utf8)
    for address in addresses {
        address.withUnsafeBytes { buf in
            let sa = buf.baseAddress!.assumingMemoryBound(to: sockaddr.self)
            let saLen = socklen_t(buf.count)
            let sock = sa.pointee.sa_family == AF_INET ? sock4 : sock6
            _ = sendto(sock, &message, message.count, MSG_DONTWAIT, sa, saLen)
        }
    }
}
/// Returns the addresses of the discard service (port 9) on every
/// broadcast-capable interface.
///
/// Each array entry is contains either a `sockaddr_in` or `sockaddr_in6`.
private func addressesOfDiscardServiceOnBroadcastCapableInterfaces() -> [Data] {
    var addrList: UnsafeMutablePointer<ifaddrs>? = nil
    let err = getifaddrs(&addrList)
    guard err == 0, let start = addrList else { return [] }
    defer { freeifaddrs(start) }
    return sequence(first: start, next: { $0.pointee.ifa_next })
        .compactMap { i -> Data? in
            guard
                (i.pointee.ifa_flags & UInt32(bitPattern: IFF_BROADCAST)) != 0,
                let sa = i.pointee.ifa_addr
            else { return nil }
            var result = Data(UnsafeRawBufferPointer(start: sa, count: Int(sa.pointee.sa_len)))
            switch CInt(sa.pointee.sa_family) {
            case AF_INET:
                result.withUnsafeMutableBytes { buf in
                    let sin = buf.baseAddress!.assumingMemoryBound(to: sockaddr_in.self)
                    sin.pointee.sin_port = UInt16(9).bigEndian
                }
            case AF_INET6:
                result.withUnsafeMutableBytes { buf in
                    let sin6 = buf.baseAddress!.assumingMemoryBound(to: sockaddr_in6.self)
                    sin6.pointee.sin6_port = UInt16(9).bigEndian
                }
            default:
                return nil
            }
            return result
        }
}