cis-bind9-v301-1-1

DevOps & Security

Use a Split-Horizon Architecture (Manual)

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CIS 1.1 — Use a Split-Horizon Architecture

Profile Applicability

Level 1 - Authoritative Name Server
Level 1 - Caching Only Name Server

Description

Running a Split-Horizon DNS architecture refers to running authoritative DNS servers and services for external DNS queries separate from the internal authoritative DNS servers, which answer all queries originating from within the organization. The external servers are configured to provide only a limited amount of information for the services needed for communication with external clients and services. Typically, the information published in the externally available DNS is the minimal needed for the Internet services such as email, web and gateway systems such as VPNs. The separate internal DNS service typically provides a richer information set typically needed by internal clients.

Rationale

The two goals of Split-Horizon are to:

Minimize the amount and type of externally available information.
Physical and logical separation of external and internal DNS services.

Separating the external and internal DNS servers in this manner adheres to a defense-in-depth approach that limits the potential damage and impact should the external name server be compromised, since it does not service internal clients, nor does it have information on the internal systems and services.

BIND 9 Views can be used to provide different responses based on the source IP address, and have been suggested by some as a means to implement split-horizon without having to separate the internal and external servers. However, the usage of views without separating the servers does not accomplish the second goal. In addition, the usage of views often erroneously assumes that source IP addresses are a reliable security control and cannot be spoofed. Therefore, it is necessary that the internal DNS server be located internally in a way that firewalls and other network controls will ensure external malicious queries will not reach the internal server.

Impact

Not Applicable

Audit Procedure

Review the network and DNS architecture, and identify the external authoritative DNS servers along with the internal authoritative DNS servers to ensure they are separate and serve only external and only internal clients respectively. Review the external DNZ zones to ensure only minimal name information is included in the external zones. Perform a query of an internal only name to the external DNS servers to ensure they do not provide a positive response.

Remediation

Implement Split-Horizon Architecture to separate external and internal DNS services. The external DNS servers should respond only to names of approved external services, such as web, email and VPN services.

Default Value

Not Applicable

References

http://www.deer-run.com/~hal/EUGLUGBINDTalk.pdf

CIS Controls

Controls Version	Control	IG 1	IG 2	IG 3
v6	12 - Boundary Defense	N	Y	Y

MITRE ATT&CK Mappings

Tactic	Technique
Discovery	T1018 - Remote System Discovery
Reconnaissance	T1590 - Gather Victim Network Information

Profile

Level 1 - Authoritative Name Server
Level 1 - Caching Only Name Server