Definition
Blocking a DNS lookup's answer's IP address value.
How it works
This technique prevents a client from learning IP addresses deemed to be potentially malicious, which would have been delivered via forward resolution responses.
Responses to forward resolution requests (that is, requests where a domain is sent and IP(s) are returned) are collected, and the IP address(es) included as a response are examined. If the IP address(es) are in a range included in the blacklist, then the response is dropped and not forwarded to the client.
The DNS lookup can be blocked by either dropping the network traffic with an inline device, or modifying the value of the response sent by the DNS server. To transparently prevent client applications from hanging on a request, it is common practice to replace malicious values with addresses in the range 127.0.0.0/8 or the address of a honeypot maintained by the network administrators.
Considerations
- This technique does not prevent the client from contacting the blacklisted IP, only from learning about this IP address via a nameserver lookup request.
- DNS Response traffic can be transmitted over many different protocols, which presents a challenge to implementing methods to extract all DNS answer IP address value(s).
- DNS has historically used UDP port 53, with TCP port 53 instead used for responses over 512 bytes or after a lack of response over UDP.
- Usage of new protocols to provide confidentiality for DNS traffic, such as DoH (DNS over HTTPS) and DoT (DNS over TLS), complicates collection of the IP address(es) in DNS responses. These protocols have often been enabled in browser settings transparently after a browser update, with DNS requests proxied over one of these cryptographic protocols through a specified host.
- This technique must be implemented logically between the application that receives the response and the server which sent the response.
- DNS responses sent in an encrypted manner, such as those using DoH or DoT, will require interception of the TLS connections in order to determine the IP address(es) in the response.
- Replacing the response is not effective in the case that the nameserver uses a technique to provide integrity of its responses, such as DNSSEC for DNS responses.
Implementation perspective
Forward Resolution IP Denylisting should be treated as a technical defensive capability rather than a product checkbox. In practice, teams should define the protected scope, the conditions under which the technique acts, and the observable evidence that demonstrates the intended behavior. For this technique, likely engineering context includes the relevant system, activity, and evidence sources.
Use the technique to create a logical or physical boundary that limits access, execution, communication, or movement.
Questions to ask
- What is being isolated, from whom or what, and under which conditions?
- Which flows or operations remain explicitly allowed across the boundary?
- What happens when the isolation mechanism fails or loses policy context?
- How is the boundary tested for alternate paths, bypasses, and unintended dependencies?
Evidence and validation
- Architecture and data-flow diagrams showing the isolation boundary
- Policy, rule, or configuration exports from enforcing components
- Tests of allowed, denied, and failure-state behavior
- Monitoring evidence for boundary violations and policy changes
Common failure patterns
- The intended boundary has undocumented alternate paths or inherited trust relationships.
- Rules accumulate without ownership, review, or removal of obsolete exceptions.
- Isolation is assumed from network location while application, identity, or management paths remain open.
This implementation perspective is original Bare Metal Cyber educational content. It does not replace the official D3FEND definition or establish that a specific product implements the technique.
Technique hierarchy
Top-level family
Parent techniques
Direct child techniques
None listed at this level.
Artifacts and ontology entities
These relationships describe how D3FEND connects a defensive technique to artifacts or other ontology entities. They describe graph semantics, not a product certification.
Explicit technique relationships
Authoritative sources
- Open this technique on the official D3FEND website ↗
- Open the official ontology resources ↗
- Use DNS Policy for Applying Filters on DNS Queries ↗Microsoft · UserManualReference
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