File: //etc/network/if-down.d/resolved
#!/bin/sh
#
# Script fragment to make ifupdown supply DNS infromation to resolved
#
case "$ADDRFAM" in
    inet|inet6) : ;;
    *) exit 0 ;;
esac            
if systemctl is-enabled systemd-resolved > /dev/null 2>&1; then
    interface=$IFACE
    if [ ! "$interface" ]; then
        return
    fi
    ifindex=$(cat "/sys/class/net/$interface/ifindex")
    if [ ! "$ifindex" ]; then
        return
    fi
    mystatedir=/run/network
    statedir=/run/systemd/resolve/netif
    rm -f "$mystatedir/isc-dhcp-v4-$interface" "$mystatedir/isc-dhcp-v6-$interface" "$mystatedir/ifupdown-inet-$interface" "$mystatedir/ifupdown-inet6-$interface" "$statedir/$ifindex"
    if systemctl --quiet is-active systemd-resolved; then
        resolvectl revert "$ifindex" || true
    fi
fi