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ALERT: Prevent your institution from being an unwitting partner in denial of service attacks

This is the technical version of the Alert.
There is also a CIO version.

- - - - - - - - -

May 8, 2013

To: IT Security Staff, and Network and DNS Administrators

REN-ISAC ALERT: Prevent your institution from being an unwitting
partner in denial of service attacks

The REN-ISAC [1] wants to raise awareness and drive change concerning
common network and domain name system (DNS) configurations that fall short
of accepted best practice and which, if left unchecked, open the door for
your institution to be exploited as an unwitting partner to crippling
denial of service attacks against third parties.

Please note important, specific recommended ACTIONS included below.

Although attacks exploiting the network and DNS configuration weaknesses
have been around for a long time, the frequency and impact of attacks have
grown over the past year. These attacks may exploit thousands of
institutional DNS servers to create an avalanche of network traffic aimed
at a third-party victim. The traffic sourced by any single institutional
system may be small enough to go unnoticed at the institution; however, the
aggregate experienced at the target can be crippling. A recent attack [2]
generated over 300 gigabits per second of traffic aimed at the victim
organization. To put that in context, most universities and organizations
connect to the Internet at 1 Gbps or less. In this incident not only was
the intended victim crippled, Internet service providers and security
service providers attempting to mitigate the attack were adversely

Given history and the success of recent attacks, we expect that attacks
will rise in frequency and magnitude in the months ahead.

The network configuration issue concerns the ability for a machine on your
network to send packets marked with a source IP address that doesn't belong
to you ("spoofed") to outside your network. The DNS issue concerns a
configuration that allows outsiders to exploit your DNS servers to send
high volumes of traffic at arbitrary target machines.

=== ACTIONS ===

In a companion note to CIOs [3], the REN-ISAC recommends the following four

1. Distribute a copy of this message to your network administrators,
information security staff, DNS administrators, and other relevant

2. Ensure your institutional network(s) are unable to originate Internet
traffic with spoofed source addresses.

3. Do not permit any DNS server on your networks to answer queries from the
public Internet, with the exception of the institution's authoritative
servers, which should only answer queries about data they are authoritative

4. Investigate rate limiting for your authoritative DNS servers. Rate
limiting becomes even more important for DNSSEC-enabled zones.


===== Overview =====

Open recursive resolvers, authoritative DNS severs (especially when zones
are DNSSEC signed), and networks that do not prevent source address
spoofing create an environment on the Internet where DNS amplification DDoS
attacks [4] of great magnitude can be achieved.

Too many higher education institutions contribute to this known and
avoidable problem.

Unfortunately, this problem goes unsolved because organizations targeted in
the attacks are not the same organizations failing to follow best common
practices and being exploited to conduct the attacks. The exploited
organization often experiences little ill effect; therefore, motivation to
solve the problem depends on good Internet citizenship.

===== Solutions =====

[ DNS ]

  Highly Recommended:

     - Ensure recursive resolvers [5] are accessible only to
     authorized/intended users, such as by limiting access to your
     recursive resolvers to just your enterprise's IP addresses.

     - Manage DNS traffic (port 53 tcp/udp), e.g. by using router ACLs, so
     queries from outside the enterprise can only go to permitted
     authoritative name servers. This mitigates risk from various
     uncontrolled devices, such as Internet appliances that have an
     embedded DNS service.

     - Investigate rate limiting [6][7] for your authoritative name
     servers, and develop a plan for implementation as possible. Rate
     limiting becomes even more important for DNSSEC-enabled zones.

     - Run recursive resolver and authoritative name servers on separate
     machines [8] thereby allowing proper controls for each.


     - Provide a means to monitor DNS traffic, including the ability to
     detect anomalous changes in DNS query patterns.

  Other related good practices:

     - Manage DNS traffic (port 53 tcp/udp), e.g. by using router ACLs, so
     queries from inside the enterprise can only go to intentionally
     permitted enterprise or external (e.g. Google Public DNS or OpenDNS)
     recursive resolvers.

     - Check your DNS configuration for other issues, e.g. using

[ Network ]

  Highly Recommended:

     - Apply BCP38 filtering to prevent spoofed source address traffic from
     leaving your network. [9][10]


     - Collect and store network flow (NetFlow/Sflow/J-Flow) data. Real
     time network flow allows backtracking spoofed network traffic.
     Historical network flow facilitates incident response capabilities.

===== More In-Depth =====

[ Recursive Resolvers ]

  If you allow unrestricted access to your recursive resolvers, those
  resolvers are known as "open recursive resolvers" and are subject to
  abuse by any attacker connected to the Internet.

  To understand how attackers abuse open recursive resolvers, assume
  attacker A wants to flood target T with an overwhelming volume of network
  traffic. Attacker A generates fake DNS queries, pretending (using spoofed
  IP source addresses) to be target T. The attacker sends those queries to
  open recursive resolvers located all over the Internet, potentially
  including yours if it's open for their use. Those open recursive
  resolvers then send answers to the forged DNS queries to target T,
  filling up T's network capacity and potentially knocking T's users off
  the network. The size of the DNS query is much smaller than the size of
  the answer, hence "DNS amplification".

  It is absolutely critical all university recursive resolvers are properly
  configured so they only answer queries for the local users they're meant
  to be serving. You can request a free report of open recursive resolvers
  on your campus from these resources [11][12].

[ Rate Limiting Authoritative DNS Servers ]

  Authoritative DNS servers should be accessible to everyone on the
  Internet; however, authoritative servers can also be exploited for DNS
  amplification attacks, especially with DNSSEC-enabled zones. Rate
  limiting prevents your authoritative server from answering the same
  (spoofed) question tens or hundreds of thousands of times per second. You
  should investigate rate limiting, and implement as possible. See for more information.

[ Network Filtering to Prevent Source-Spoofed Packets ]

  Systems should not be permitted to send spoofed traffic to the Internet,
  pretending to be from some other site's IP addresses. Roughly 80% of all
  networks have already installed filtering rules on their network routers
  to ensure any spoofed network traffic won't hit the Internet, but some
  networks -- including potentially yours -- have not yet done so. We need
  your help. Please ensure your institutional networks prevent traffic with
  spoofed source addresses from leaving your network.

  Blocking spoofed network traffic from leaving your network is an IETF
  Best Common Practice ("BCP"), see: and


The text of this message and the CIO version (along with clobber-free long
URLs) can be found at [3].

We'd appreciate your input on additional means to protect from this threat,
and general feedback concerning the Alert.

If you have any questions, please don't hesitate to e-mail us at

Special thanks go to the members of the REN-ISAC Technical Advisory Group
[13] for their work on this Alert.

On behalf of the REN-ISAC team,

Doug Pearson
Technical Director, REN-ISAC
24x7 Watch Desk +1(317)274-7228


In addition to the references made in the text above, the following may be useful:

DNSSEC and DNS Amplification Attacks

Explaining Distributed Denial of Service Attacks to Campus Leaders

Preventing Use of Recursive Nameservers in Reflector Attacks

US-CERT Alert (TA13-088A) DNS Amplification Attacks


[2] Firm Is Accused of Sending Spam, and Fight Jams Internet
[3] REN-ISAC DNS Amplification Alert [4] What is a DNS Amplification Attack? [5] Section 2 DNS Architectural Components provides definition of recursive resolver [6] Response Rate Limiting in the Domain Name System (DNS RRL) [7] DNS Response Rate Limiting (DNS RRL) [8] Domain Name System (DNS) Security Reference Architecture [9] Network Ingress Filtering [10] Securing the Edge [11] For a list of open resolvers by ASN, e-mail dns-scan /at/ [12] List open resolvers on your network [13] REN-ISAC Technical Advisory Group -o0o-