Definition
Restricting network traffic originating from a private host or enclave destined towards untrusted networks.
How it works
Outbound traffic, in this context, is network traffic originating from a private host or enclave destined towards untrusted networks. For example:
Filtering is commonly implemented as firewall rulesets to limit outbound traffic permitted to egress a host or network. Firewalls are deployed either directly on hosts through kernel level software implementations or installed in-line directly on network links. There are benefits and disadvantages to each approach.
There are various strategies for developing filtering rulesets:
- An enterprise desktop intranet user connecting to www.example.com
- An internal mail server connecting to an external mail server, mail.example.com
- Block everything by default
- Limit destination hosts
- Limit destination transport or application protocols
- Restrict content outbound (Ex. strings formatted as social security numbers, or proprietary data)
Considerations
- Dynamic IP assignment creates challenges for Outbound Traffic Filtering because users are not necessarily associated with the same IP address. This can be addressed by linking IP address management information with the filtering logic.
- Connections using non-standard transport layer ports may circumvent outbound traffic filtering technology which does not detect application protocol based on traffic content.
- Business requirements typically drive the development of filtering rule sets.
Implementations
- iptables (Linux)
- Windows Firewall
- pf (BSD)
Implementation perspective
Outbound Traffic Filtering 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 Outbound Network Traffic.
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
Show inferred artifact relationship paths (1)
NIST SP 800-53 relationships
The relation label is preserved from the D3FEND ontology. It is not converted into a claim that the control automatically implements or validates this technique.
ATT&CK Enterprise mitigation relationships
These links come from the D3FEND ontology’s ATT&CK mitigation mapping. They help users navigate between the knowledge bases and do not guarantee mitigation effectiveness.
D3FEND note: M1021 scope is broad, touches on an wide variety of techniques in d3fend.
Offensive-technique relationships
These relationships are generated from D3FEND graph paths and are explicitly experimental. They should be treated as hypotheses for defensive analysis—not as proof that the technique prevents, detects, or removes an offensive behavior.
ATT&CK Enterprise (30)
ATT&CK for ICS (2)
Authoritative sources
- Open this technique on the official D3FEND website ↗
- Open the official ontology resources ↗
- Automatically generating rules for connection security ↗Microsoft · Charles D. Bassett; Eran Yariv; Ian M. Carbaugh; Lokesh Srinivas Koppolu; Maksim Noy; Sarah A. Wahlert; Pradeep Bahl · PatentReference
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