Category: Domain Name Service (DNS)

Today a new type of ddos has occured, here is the first of it surfacing. I just received this email from the security offices.

Two of our servers are affected. I am working very hard to find a solution for this new kind of attack.
If anybody finds this kind of attack in the bind log files, please let me know about it immediately.

Here a description of how this attack is acting and working:

Chinese Water Torture: A Slow Drip DNS DDoS Attack

A number of our service provider customers around the world are reporting that they see a new type of denial-of-service attack that is using the DNS as the attack vector. The service providers themselves do not appear to be the target of this attack. Instead, the attack tries to overwhelm an outside victim’s authoritative DNS servers. Once the DNS server is taken down, the victim’s domains will appear to be inaccessible.

As a side effect, our service provider customers are seeing a spike in DNS traffic resulting in increased CPU and memory usage. This blogs gives some more details about the attack and suggests what you can do to mitigate the impact of it.

The Attack

It appears that a fairly large botnet is used to send queries for the victim’s domain. Queries are made-up, with random string with up to 16 letters prepended to the victim’s domain, like:

xyuicosic.www.victimdomain.com

A query for this domain is then sent to the service providers DNS server. The DNS server attempts to contact the authoritative nameserver to find the answer. If the authoritative nameserver does not reply (because it is too busy responding to queries from DNS servers all over the world, or perhaps has crashed), the DNS server attempts to contact the next authoritative nameserver and so on. Modern DNS server will make multiple attempts to contact each authoritative nameserver before giving up and responding back to the client with a SERVFAIL response.
The infected client will then repeat the same pattern but this time with another random string prepended, for example:

alkdfasd.www.victimdomain.com

Even though the DNS server was unable to get a response from any of the victimdomain.com authoritative nameservers during the previous query, most DNS servers will still attempt to contact them for this second query.
Now imagine that thousands of bots are sending a relatively small number of queries for such made-up subdomains. This will trigger a large increase in the number of DNS queries sent by the service provider’s DNS servers to the victim’s nameservers.

How to Detect the Attack

While this attack most likely is targeting the authoritative servers for victimdomain.com, it also puts an increased CPU load on the DNS server by forcing it to continually initiate recursive queries and also consumes large amounts of resolver memory resources. More importantly, if the internal resolver resources are fully consumed, the resolver may drop any inbound queries, including queries from legitimate clients.

If the DNS server’s behavior is being monitored, the symptoms of the attack will also show up as:

Increased CPU utilization
Increased number of SERVFAIL responses
Increased number of outbound queries and retransmissions
Increased query latency
Increased number of dropped client queries (if the resolver resources are fully consumed)

One thing all of the victim domains have in common is that they appear to be Chinese sites, perhaps gaming or gambling sites.