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TA13-088A: DNS Amplification Attacks


Extraordinary Robot
Jun 27, 2006
Original release date: March 29, 2013
[h=3]Systems Affected[/h]
  • Domain Name System (DNS) servers
A Domain Name Server (DNS) Amplification attack is a popular form of Distributed Denial of Service (DDoS) that relies on the use of publically accessible open recursive DNS servers to overwhelm a victim system with DNS response traffic.​
A Domain Name Server (DNS) Amplification attack is a popular form of Distributed Denial of Service (DDoS) that relies on the use of publically accessible open recursive DNS servers to overwhelm a victim system with DNS response traffic. The basic attack technique consists of an attacker sending a DNS name lookup request to an open recursive DNS server with the source address spoofed to be the victim’s address. When the DNS server sends the DNS record response, it is sent instead to the victim. Because the size of the response is typically considerably larger than the request, the attacker is able to amplify the volume of traffic directed at the victim. By leveraging a botnet to perform additional spoofed DNS queries, an attacker can produce an overwhelming amount of traffic with little effort. Additionally, because the responses are legitimate data coming from valid servers, it is especially difficult to block these types of attacks.​
While the attacks are difficult to prevent, network operators can implement several possible mitigation strategies. The primary element in the attack that is the focus of an effective long-term solution is the detection and elimination of open recursive DNS resolvers. These systems are typically legitimate DNS servers that have been improperly configured to respond to recursive queries on behalf of any system, rather than restricting recursive responses only to requests from local or authorized clients. By identifying these systems, an organization or network operator can reduce the number of potential resources that the attacker can employ in an attack.
[h=3]Impact[/h] A misconfigured Domain Name System (DNS) server can be exploited to participate in a Distributed Denial of Service (DDoS) attack.
[h=3]Solution[/h] [h=2]DETECTION[/h]Several organizations offer free, web-based scanning tools that will search a network for vulnerable open DNS resolvers.  These tools will scan entire network ranges and list the address of any identified open resolvers.

Open DNS Resolver Project
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The Open DNS Resolver Project has compiled a list of DNS servers that are known to serve as globally accessible open resolvers.  The query interface allows network administrators to enter IP ranges in CIDR format [1].

The Measurement Factory
Like the Open DNS Resolver Project, the Measurement Factory maintains a list of Internet accessible DNS servers and allows administrators to search for open recursive resolvers [2].  In addition, the Measurement Factory offers a free tool to directly test an individual DNS resolver to determine if it allows open recursion.  This will allow an administrator to determine if configuration changes are necessary and verify that configuration changes have been effective [3].  Finally, the site offers statistics showing the number of open resolvers detected on the various Autonomous System (AS) networks, sorted by the highest number found [4].

Another freely available, web-based tool for testing DNS resolvers is DNSInspect.  This site is similar to The Measurement Factory’s ability to test a specific resolver for vulnerability, but offers the ability to test an entire DNS Zone for several other potential configuration and security issues [5].
[h=4]Indicators[/h]In a typical recursive DNS query, a client sends a query request to a local DNS server requesting the resolution of a name or the reverse resolution of an IP address.  The DNS server performs the necessary queries on behalf of the client and returns a response packet with the requested information or an error [6, page 21].  The specification does not allow for unsolicited responses.  In a DNS amplification attack, the key indicator is a query response without a matching request.  
[h=2]MITIGATION[/h]Unfortunately, due to the overwhelming traffic volume that can be produced by one of these attacks, there is often little that the victim can do to counter a large-scale, DNS amplification-based distributed denial-of-service attack.  While the only effective means of eliminating this type of attack is to eliminate open recursive resolvers, this requires a large-scale effort by numerous parties.  According to the Open DNS Resolver Project, of the 27 million known DNS resolvers on the Internet, approximately “25 million pose a significant threat” of being used in an attack [1].  However, several possible techniques are available to reduce the overall effectiveness of such attacks to the Internet community as a whole.  Where possible, configuration links have been provided to assist administrators with making the recommended changes.  The configuration information has been limited to BIND9 and Microsoft’s DNS Server, which are two widely deployed DNS servers.  If you are running a different DNS server, please see your vendor’s documentation for configuration details.
[h=3]Source IP Verification[/h]Because the DNS queries being sent by the attacker-controlled clients must have a source address spoofed to appear as the victim’s system, the first step to reducing the effectiveness of DNS amplification is for Internet Service Providers to deny any DNS traffic with spoofed addresses.  The Network Working Group of the Internet Engineering Task Force released a Best Current Practice document in May 2000 that describes how an Internet Service Provider can filter network traffic on their network to drop packets with source addresses not reachable via the actual packet’s path [7].  This configuration change would considerably reduce the potential for most current types of DDoS attacks.
[h=3]Disabling Recursion on Authoritative Name Servers[/h]Many of the DNS servers currently deployed on the Internet are exclusively intended to provide name resolution for a single domain.  These systems do not need to support resolution of other domains on behalf of a client, and therefore should be configured with recursion disabled.
[h=4]Bind9[/h]Add the following to the global options [8]:
options {
     allow-query-cache { none; };
     recursion no;
[h=4]Microsoft DNS Server[/h]In the Microsoft DNS console tool [9]:

  1. Right-click the DNS server and click Properties.
  2. Click the Advanced tab.
  3. In Server options, select the “Disable recursion” check box, and then click OK.
[h=3]Limiting Recursion to Authorized Clients[/h]For DNS servers that are deployed within an organization or ISP to support name queries on behalf of a client, the resolver should be configured to only allow queries on behalf of authorized clients.  These requests should typically only come from clients within the organization’s network address range.
[h=4]BIND9[/h]In the global options, add the following [10]:
acl corpnets {;; };
options {
  allow-query { corpnets; };
  allow-recursion { corpnets; };
[h=4]Microsoft DNS Server[/h]It is not currently possible to restrict recursive DNS requests to a specific client address range in Microsoft DNS Server.  The most effective means of approximating this functionality is to configure the internal DNS server to forward queries to an external DNS server and restrict DNS traffic in the firewall to restrict port 53 UDP traffic to the internal server and the external forwarder [11].
[h=3]Rate Limiting Response of Recursive Name Servers[/h]There is currently an experimental feature available as a set of patches for BIND9 that allows an administrator to restrict the number of responses per second being sent from the name server [12].  This is intended to reduce the effectiveness of DNS amplification attacks by reducing the volume of traffic coming from any single resolver.
[h=4]BIND9[/h]On BIND9 implementation running the RRL patches, add the following lines to the options block of the authoritative views [13]:
rate-limit {
    responses-per-second 5;
    window 5;
[h=4]Microsoft DNS Server[/h]This option is currently not available for Microsoft DNS Server.
[h=3]Revision History[/h]
  • March 29, 2013: Initial release

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Syndicated from the United States Security Readiness Team (US-CERT). Link Removed