Is your public DNS server just waiting to participate in a distributed
denial-of-service (DDoS) attack? If it’s using recursion, then the answer
is yes
. DDoS and DNS attacks aren’t new, but they’re
on the rise

Using authoritative
name service
, DNS servers primarily advertise to the world the various records
associated with the domain they serve. Because users prefer common names and
networks prefer numbers, DNS servers handle the translation between what a user
types in a browser—such as—and
the actual IP address the network understands.

The task of answering a query recursively is completely
different. According
to a US-CERT report
, between 75 and 80 percent of all DNS servers can
handle recursive requests.

Recursive DNS provide answers to queries for records by
asking other DNS servers and providing that response to the client that made
the request. Here’s an example:

  1. A user
    enters into
    a Web browser.
  2. The
    computer contacts its local DNS server to determine the IP address of
  3. The DNS
    server looks up
    in its local tables (i.e., its cache) but does not find it listed.
  4. The DNS
    server sends a query to a root server for the IP address of
  5. The
    root server replies with a referral to the top-level domain (TLD) servers
  6. The
    DNS server then contacts the TLD server to determine the IP address of
  7. The
    TLD server replies with a referral to the name server for
  8. The
    DNS server contacts the name server for to determine the IP address.
  9. The
    name server checks a zone file that defines a CNAME record, which shows is an alias of DNS
    returns both the CNAME and the A record for
  10. The
    DNS server sends this response to the original client:
    = (with CNAME record

How can a recursive query become a DDoS attack? For the
attack to work, the attacker needs to be in control of one DNS record.

He or she then populates the TXT field of that record with
information. (The maximum size of the TXT field is approximately 4,200 bytes.) And
then the fun begins. Here’s how:

  1. The attacker
    programs bots to continuously execute requests for this record against
    recursive DNS.
  2. The
    bots spoof the source IP address of these requests, replacing it with the
    DDoS target.
  3. The
    recursive servers take the record from the attacker-controlled zone, and
    send it along to the IP address they think the request came from.

Multiply this by the number of bots participating in the
attack, and you’ve got a DDoS attack. If your DNS server is a target of this
attack, your network will grind to a halt because none of its clients can
resolve an IP address.

What’s the solution? It’s quite simple: Run two different
DNS servers. Let the internal server handle all requests from your network
(even recursive for your clients only).

On the external DNS server, disable recursion. With
recursion disabled, the external DNS server won’t send queries on behalf of
other name servers or clients, which stops attackers from bouncing DoS attacks
off your DNS server by querying for external zones.

Final thoughts

DNS recursion isn’t the problem—it’s a symptom of the problem. IP address
spoofing is the real problem, and this spoofing provides a ready venue for
DDoS, spam, and other headaches.

my opinion, IP address verification is the answer, and the tools already exist
to solve that problem. I know the Internet Engineering Task Force (IETF) is looking
at the issue
, but it needs
to stop investigating and take action.

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Mike Mullins has served as an assistant
network administrator and a network security administrator for the U.S. Secret
Service and the Defense Information Systems Agency. He is currently the
director of operations for the Southern Theater Network Operations and Security