Drop source addresses that cannot legitimately arrive on an ingress interface

Keep this document to one review outcome: Drop source addresses that cannot legitimately arrive on an ingress interface. Only the official source and traced locations below supply facts. Confirm applicability before acting.

Source fact:

The official RFC 2827 — Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing from RFC Editor / Internet Engineering Task Force supports the following bounded statements:

• An aggregation edge restricts downstream transit traffic to source prefixes that the downstream network intentionally advertises, rejecting sources outside those prefixes. The research record locates this support at Sections 1 (Introduction) and 3 (Restricting forged traffic).
• Administrators should log packets rejected by an ingress source-address filter so suspicious activity can be monitored. The research record locates this support at Section 3 (Restricting forged traffic), dropped-packet logging.
• A remote-access server can admit only the address assigned to a single-host customer, while strict return-interface validation is unsuitable where Internet paths are asymmetric. The research record locates this support at Section 4 (Further possible capabilities for networking equipment).

The source support ends with the statements listed above. Use them to examine address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains in the applicable environment, not to imply a wider guarantee.

What the source does not establish

This RFC evidence supports only the named network-protocol decision; it does not select vendor settings, topology, capacity, or an acceptable failure mode. Do not read the source as proof of implementation or permission to change production. Its guidance remains conditional on DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring and the environment’s recorded constraints.

Applicability questions

• For source statement 1 at Sections 1 (Introduction) and 3 (Restricting forged traffic), which observable configuration, record, or test can confirm applicability here?
• For source statement 2 at Section 3 (Restricting forged traffic), dropped-packet logging, which observable configuration, record, or test can confirm applicability here?
• For source statement 3 at Section 4 (Further possible capabilities for networking equipment), which observable configuration, record, or test can confirm applicability here?
• Within address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains, which versions, roles, and configuration states define the review population?
• Could DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring invalidate the test, hide a failure, or change applicability?
• Who owns the decision, and which observation requires stopping, escalation, or rollback?

DSE recommendation:

DSE recommends using the cited source as the evidence anchor for this decision. Make the source, asset scope, owner, and expected outcome explicit in the review record. Record the source location, examined part of address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains, observed and expected states, owner, and reason for deviation.

Translate the conclusion into change control only after documenting dependencies, impact, test method, expected signals, failure signals, and restoration steps. Include DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring, while excluding secrets and sensitive personal or topology data from ordinary tickets.

Verification and evidence

A reviewer should be able to retrace the decision from Sections 1 (Introduction) and 3 (Restricting forged traffic); Section 3 (Restricting forged traffic), dropped-packet logging; Section 4 (Further possible capabilities for networking equipment) through configuration snapshots, route or neighbor state, packet captures, counters, topology records, and controlled failover results. Record what was collected, where, when, by whom, and which system or role it represents.

Keep before-state evidence, approval, test or change result, exceptions, and after-state evidence together. Use an approved lab, window, or nonproduction path for risky tests. Set a recheck trigger for version, architecture, dependency, vendor, incident, or ownership change. A check proves only what was observed.

Official references

• RFC 2827 — Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing — RFC Editor / Internet Engineering Task Force