main.swift

import Foundation
import SystemConfiguration

enum Error: Swift.Error {
    case pppInterfaceNotFound
    case failedToWriteToConfigurationStore
}

let dynamicStore = SCDynamicStoreCreate(nil, "setVPNDNS" as CFString, nil, nil)

func cast<T>(_ input: CFArray?) -> Array<T> {
    guard
        let asAnyObject = input as? [AnyObject],
        let asTypedObject = asAnyObject as? [T]
    else {
        return []
    }

    return asTypedObject
}

func query<T>(_ filter: String) -> [T] {
    return cast((SCDynamicStoreCopyKeyList(dynamicStore, filter as CFString)))
}

func get<T>(_ path: String) -> T {
    return SCDynamicStoreCopyValue(dynamicStore, path as CFString) as! T
}

func set(path: String, value: NSDictionary) -> Bool {
    return SCDynamicStoreSetValue(dynamicStore, path as CFString, value)
}

func getServiceUUID(for desiredInterfaceName: String) throws -> String {

    let serviceKeys: [NSString] = query("State:/Network/Service/(.+)/PPP")

    for serviceKey in serviceKeys {
        let dictionary: [String: AnyObject] = get(serviceKey as String)
        let interfaceName = dictionary[kSCPropInterfaceName as String] as? String

        if (interfaceName == desiredInterfaceName) {
            return serviceKey
                .replacingOccurrences(of: "State:/Network/Service/", with: "")
                .replacingOccurrences(of: "/PPP", with: "") // Quite a naive approach, might explode.
        }
    }

    throw Error.pppInterfaceNotFound
}

func overridePrimary(for serviceUIID: String) {
    let path = "State:/Network/Service/\(serviceUIID)/IPv4"
    var current: [CFString: AnyObject] = get(path)

    current[kSCPropNetOverridePrimary] = 1 as NSNumber
    current[kSCPropInterfaceName] = "en0" as CFString // TODO: Hardcoded for my setup.
    current[kSCPropNetIPv4Router] = "192.168.1.1" as CFString // TODO: Hardcoded for my setup.

    set(path: path, value: current as NSDictionary)
}

func injectDNSServers(for serviceUUID: String, dnsServers: [String]) {
    let path = "State:/Network/Service/\(serviceUUID)/DNS"

    let dictionary = [
        kSCPropNetDNSServerAddresses: dnsServers,
        kSCPropNetDNSSupplementalMatchOrders: [100000 as NSNumber],
        kSCPropNetDNSSupplementalMatchDomains: []
    ] as [CFString : Any]

    set(path: path, value: dictionary as NSDictionary)
}

func makePrimaryService(uuid: String) {
    let globalIPv4Path = "State:/Network/Global/IPv4"
    var current: [CFString: AnyObject] = get(globalIPv4Path)

    current[kSCDynamicStorePropNetPrimaryService] = uuid as CFString

    set(path: globalIPv4Path, value: current as NSDictionary)
}

let interface = "ppp0"
let dnsServers = [
    "10.11.3.241", // VPN DNS
    "10.11.3.242", // VPN DNS
    "83.169.184.33",
    "83.169.184.97"
]

let uuid = try getServiceUUID(for: interface)

injectDNSServers(for: uuid, dnsServers: dnsServers)
overridePrimary(for: uuid)
makePrimaryService(uuid: uuid)