The spate of huge DNS DDoS amplification attacks of the last year probably had a previously undetected helping hand from millions of poorly-configured home routers, ISP security outfit Nominum has discovered.

Using figures from its own network of Vantio ThreatAvert boxes inside ISPs, the firm estimates that as many as 24 million home routers across Asia, parts of Eastern Europe and Central America could be operating as open DNS proxies, with 5.3 million detected as having been used in DDoS attacks in February alone.

Exactly how much traffic the company is reluctant to estimate but it could be anything from tens of Gigabits to possibly hundreds of Gigabits if corralled at one time.  It was enough for DNS amplification to account for 70 percent of all traffic on one provider’s network during January.

The company doesn’t spell it out but this is presents a tricky issue; unlike managed infrastructure-level routers, home routers sit on people’s desks and are left to rot into obsolescence. The chances of any of these devices being patched or fixed are vanishingly low, giving DDoS criminals a large population of devices to exploit in attacks more or less indefinitely.

“We have no way of knowing what kind of home routers these are,” said Nominum’s director of product marketing, Bruce van Nice. Nominum could detect the traffic and had worked out a way to see the IP addresses it had come from but not which target it was being aimed at, he said.

“The consumer has no visibility on this,” agreed van Nice.

As to why routers were operating as DNS proxies, he suspected that the exploited devices had probably not been hit by any specific vulnerability. More likely, they had come out of the box with a configuration issue from day one.

It’s still pretty mysterious why a home router would behave in this way, basically forwarding  small DNS queries to multiple ISPs resolution servers, which in turn forward them to the intended target.  Home routers are small fry in Internet terms but they are still routers. A router behaving as an open proxy is like a post office that just keeps sending vast number of chain letters to other post offices in the expectation each will deliver its load to one address.

This is a good analogy for what Spamhaus went through last March during a DNS amplification attack that made headlines thanks to its record-breaking size. Unfortunately, Spamhaus didn’t have a big enough mail box to receive billions of letters and so passed them on to a firm called CloudFlare that did.

In theory, the CloudFlares of the world are able to throw all this unwanted garbage into a giant virtual hole and forget about it.

Given the difficulty of remediating home routers whose owners can’t easily be identified, Nominum’s pragmatic alternative is to configure ISP-level DNS servers to drop this kind of traffic before it even gets to that level. Without this approach ISPs as well as targets would suffer the real consequences, he said.

The possibility that home routers could have been a hidden part of the DNS amplification DDoS phenomenon remains an intriguing story. If correct, the issue could be larger and harder to fix than experts have realised.

Amplification remains the new black for DDoS attackers, covering not only DNS but, more recently, a monster NTP attack on a customer of CloudFlare.  For some reason, the attackers have it in for this relatively small but now quite famous firm or perhaps for its growing band of anxious customers.

Most home users will not find their routers behaving as DNS proxies but anyone that wants to check can test their device.