:local curDNS [/ip dns get server]
:local vpnDNS "1.1.1.1"
:local oriDNS "203.142.82.222,203.142.84.222"
:local testDomain "www.google.com"

:local isDown "no"

:if ($curDNS = $vpnDNS) do={
    :do {
        :resolve $testDomain server=$vpnDNS

    } on-error={
        :set isDown "yes"
        :log warning "VPN is seems down, checking number 1"
        :delay 1
    }

    :for i from=2 to=5 do={
        :if ($isDown = "yes") do={
            :do {
                :resolve $testDomain server=$vpnDNS
                :set isDown "no"
            } on-error={
                :log warning "VPN is seems down, checking again number $i"
                :set isDown "yes"
                :delay 1
            }
        }
    }

    :if ($isDown = "yes") do={
        :log warning "VPN DNS Offline"

        /ip dns set servers=$oriDNS
    }

} else={
    :do {
        :resolve $testDomain server=$vpnDNS
        :log warning "VPN DNS Online"

        /ip dns set servers=$vpnDNS

    } on-error={}
}