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Alert


CONTROLLING OUTBOUND DNS ACCESS

Last Revised
September 29, 2016
Alert Code
TA15-240A



SYSTEMS AFFECTED

Networked systems


OVERVIEW

US-CERT has observed an increase in Domain Name System (DNS) traffic from client
systems within internal networks to publically hosted DNS servers. Direct client
access to Internet DNS servers, rather than controlled access through enterprise
DNS servers, can expose an organization to unnecessary security risks and system
inefficiencies. This Alert provides recommendations for improving security
related to outbound DNS queries and responses.

Client systems and applications may be configured to send DNS requests to
servers other than authorized enterprise DNS caching name servers (also called
resolving, forwarding or recursive name servers). This type of configuration
poses a security risk and may introduce inefficiencies to an organization.   


IMPACT

Unless managed by perimeter technical solutions, client systems and applications
may connect to systems outside the enterprise’s administrative control for DNS
resolution. Internal enterprise systems should only be permitted to initiate
requests to and receive responses from approved enterprise DNS caching name
servers. Permitting client systems and applications to connect directly to
Internet DNS infrastructure introduces risks and inefficiencies to the
organization, which include:

 * Bypassed enterprise monitoring and logging of DNS traffic; this type of
   monitoring is an important tool for detecting potential malicious network
   activity.
 * Bypassed enterprise DNS security filtering (sinkhole/redirect or
   blackhole/block) capabilities; this may allow clients to access malicious
   domains that would otherwise be blocked.
 * Client interaction with compromised or malicious DNS servers; this may cause
   inaccurate DNS responses for the domain requested (e.g., the client is sent
   to a phishing site or served malicious code).
 * Lost protections against DNS cache poisoning and denial-of-service attacks.
   The mitigating effects of a tiered or hierarchical (e.g., separate internal
   and external DNS servers, split DNS, etc.) DNS architecture used to prevent
   such attacks are lost.  
 * Reduced Internet browsing speed since enterprise DNS caching would not be
   utilized.


SOLUTION

Implement the recommendations below to provide a more secure and efficient DNS
infrastructure. Please note that these recommendations focus on improving the
security of outbound DNS query or responses and do not encompass all DNS
security best practices.  

 * Configure operating systems and applications (including lower-tier DNS
   servers intended to forward queries to controlled enterprise DNS servers) to
   use only authorized DNS servers within the enterprise for outbound DNS
   resolution.
 * Configure enterprise perimeter network devices to block all outbound User
   Datagram Protocol (UDP) and Transmission Control Protocol (TCP) traffic to
   destination port 53, except from specific, authorized DNS servers (including
   both authoritative and caching/forwarding name servers).  
   * Additionally, filtering inbound destination port 53 TCP and UDP traffic to
     only allow connections to authorized DNS servers (including both
     authoritative and caching/forwarding name servers) will provide additional
     protections. 
 * Refer to Section 12 of the NIST Special Publication 800-81-2 for guidance
   when configuring enterprise recursive DNS resolvers. [1]


REFERENCES

[1] Secure Domain Name System (DNS) Deployment Guide


REVISIONS

August 28, 2015: Initial Release

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