static TimedCache<L<S>> detectGateways_cache = new TimedCache(5);

static synchronized L<S> detectGateways() ctex {
  if (detectGateways_cache.has()) ret detectGateways_cache.getNoClean();

  boolean win = isWindows();
  String s = backtick(win ? "ipconfig" : "ip route show");
  new TreeSet<S> ips;
  for (String line : toLines(s))
    if (indexOfIgnoreCase(line, win ? "Gateway" : "via") >= 0)
      ips.addAll(regexpAll("\\d+\\.\\d+\\.\\d+\\.\\d+", line));
  return detectGateways_cache.set(new ArrayList<S>(ips));
}