Security Alert!

April 30, 2016 in Security Alert! by R Bennett  |  No Comments

TA14-017A:UDP-Based Amplification Attacks

Original release date: January 17, 2014 | Last revised: April 13, 2016

Systems Affected

Certain application-layer protocols that rely on User Datagram Protocol (UDP) have been identified as potential attack vectors:

  • DNS
  • NTP
  • SNMPv2
  • NetBIOS
  • SSDP
  • CharGEN
  • QOTD
  • BitTorrent
  • Kad
  • Quake Network Protocol
  • Steam Protocol
  • RIPv1
  • Multicast DNS (mDNS)
  • Portmap/RPC


A Distributed Reflective Denial of Service (DRDoS) attack is a form of Distributed Denial of Service (DDoS) that relies on the use of publicly accessible UDP servers, as well as bandwidth amplification factors, to overwhelm a victim system with UDP traffic.


UDP, by design, is a connection-less protocol that does not validate source IP addresses. Unless the application-layer protocol uses countermeasures such as session initiation in VOIP (voice over IP), it is very easy to forge the IP packet datagram to include an arbitrary source IP address [1]. When many UDP packets have their source IP address forged to the victim IP address, the destination server (or amplifier) responds to the victim (instead of the attacker), creating a reflected Denial of Service (DoS) Attack.

Recently, certain UDP protocols have been found to have particular responses to certain commands that are much larger than the initial request. Previously, attackers were limited linearly by the number of packets directly sent to the target to conduct a DoS attack; now a single packet can generate tens or hundreds of times the bandwidth in its response. This is called an amplification attack, and when combined with a reflective DoS attack on a large scale using multiple amplifiers and targeting a single victim, DDoS attacks can be conducted with relative ease.

To measure the potential effect of an amplification attack, a metric called the bandwidth amplification factor (BAF) is used. BAF can be calculated as the number of UDP payload bytes that an amplifier sends to answer a request, compared to the number of UDP payload bytes of the request [2] [3].

The list of known protocols—and their associated bandwidth amplification factors—are listed below. US-CERT offers thanks to Christian Rossow for providing this information. For more information on bandwidth amplification factors, please see Christian’s blog and associated research paper.

Protocol Bandwidth Amplification Factor Vulnerable Command
DNS 28 to 54 see: TA13-088A [4]
NTP 556.9 see: TA14-013A [5]
SNMPv2 6.3 GetBulk request
NetBIOS 3.8 Name resolution
SSDP 30.8 SEARCH request
CharGEN 358.8 Character generation request
QOTD 140.3 Quote request
BitTorrent 3.8 File search
Kad 16.3 Peer list exchange
Quake Network Protocol 63.9 Server info exchange
Steam Protocol 5.5 Server info exchange
Multicast DNS (mDNS) 2 to 10 Unicast query
RIPv1 131.24 Malformed request
Portmap (RPCbind) 7 to 28 Malformed request

In March 2015, Software Engineering Institute CERT issued Vulnerability Note (VU#550620) describing the use of mDNS in DRDoS attacks. Attackers can leverage mDNS by sending more information than can be handled by the device, thereby causing a DoS. [6]

In July 2015, Akamai Technologies’ Prolexic Security Engineering and Research Team (PLXsert) issued a threat advisory describing a surge in DRDoS attacks using the Routing Information Protocol version one (RIPv1). Malicious actors are leveraging the behavior of RIPv1 for DDoS reflection through specially crafted request queries [7].

In August 2015, Level 3 Threat Research Labs reported a new form of DRDoS attack that uses portmap.  Attackers leverage the behavior of the portmap service through spoofed requests and flood a victim’s network with UDP traffic. [8]


Attackers can utilize the bandwidth and relative trust of large servers that provide the above UDP protocols to flood victims with unwanted traffic, a DDoS attack.



Detection of DRDoS attacks is not easy because of their use of large, trusted servers that provide UDP services. Network operators of these exploitable services may apply traditional DoS mitigation techniques. To detect a DRDOS attack, watch out for abnormally large responses to a particular IP address, which may indicate that an attacker is using the service.

If you are a victim of DRDoS attack, there are a few things you can do to detect such activity and respond:

  1. Detect and alert large UDP packets to higher order ports.
  2. Detect and alert on any non-stateful UDP packets. (A simple snort example is below. You will need to customize this approach to your environment with whitelist and known services.)
Simple Snort rule example for stateless UDP check
var HOME_NET []
preprocessor stream5_global: track_ip yes, track_tcp yes,track_udp yes,track_icmp no,max_tcp 262144, max_udp 131072
preprocessor stream5_ip: timeout 180
preprocessor stream5_tcp: policy first, use_static_footprint_sizes
preprocessor stream5_udp: timeout 180, ignore_any_rules
alert udp HOME_NET 1024: -> any any (msg:”UDP Session start”; flowbits:set,logged_in; flowbits:noalert; sid: 1001;)
alert udp any any -> HOME_NET 1024: (msg:”UDP Stateless”; flowbits:isnotset,logged_in; sid: 1002)
  1. If you are an upstream provider maintain updated contacts and methods with downstream customers to send alerts by network.

In general, network and server administrators for Internet service providers (ISPs) should use the following best practices to avoid becoming amplifier nodes:

  1. Detect spoofed packets using network flow. (In order to validate before blocking it, read more in the Mitigation section on blocking spoofed traffic.)
  2. Monitor for an unusual number of requests to UDP services at risk using network flow or other summarized network data.
  3. Use network flow to detect service anomalies (bytes-per-packet, packets-per-second anomalies).


If you are a victim of DRDoS attack there are a few things you can do to mitigate this attack:

  1. Use stateful UDP inspection to reduce impact on critical services on your border firewall or border router (like reflexive ACL [9])
  2. Using Border Gateway Protocol (BGP), create a Remotely Triggered Blackhole, preferably in coordination with your upstream provider or ISP. [10]
  3. Maintain a list of primary upstream provider emergency contacts to coordinate response to the attack. If you are an upstream provider, conduct mitigation in coordination with your downstream customers.

In general, network and server administrators for Internet service providers (ISPs) should use the following as best practices to avoid becoming amplifier nodes:

  1. Keep your software and configuration up to date to deny or limit abuse (e.g., DNS response rate limit [11] [12] [13])
  2. Disable and remove unwanted services or deny access to local services over the Internet.
  3. Enable network-based rate-limiting to legitimate services you provide over the Internet using UDP-based protocols (e.g., quality of service (QoS) on switching and routing devices)
  4. Work with Customer Provider Edge manufactures for secure configuration and software [14]

As a service provider, to avoid any misuse of Internet resources

  1. Block spoofed packets by using ingress filtering (the Spoofer Project [15] and IETF BCP 28 and BCP 39 guidelines [16])
  2. Use traffic shaping on UDP service requests to ensure repeated access to over-the-internet resources is not abusive.[17] [18]



  • February 9, 2014 – Initial Release
  • March 7, 2014 – Updated page to include research links
  • July 13, 2015 – Added RIPv1 as an attack vector
  • August 19, 2015 – Added Multicast DNS (mDNS) and Portmap (RPCbind) as attack vectors
  • April 13, 2016 – Updated detection and mitigation information
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