This interview question evaluates requirements, scale assumptions, API/data design, architecture, trade-offs, failure modes, and rollout in a realistic interview setting. A strong answer for Implement CIDR firewall matcher states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.
##### Question
Design and implement a firewall rule matcher that evaluates a single IPv4 address against a list of CIDR rules (each tagged accept/deny). Explain and code the data structures and algorithms you would use. Follow-up: extend the solution to handle matching an incoming CIDR range against the rule set and discuss efficient data structures for range intersection checks.
Quick Answer: This interview question evaluates requirements, scale assumptions, API/data design, architecture, trade-offs, failure modes, and rollout in a realistic interview setting. A strong answer for Implement CIDR firewall matcher states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.
You are to design and implement a firewall rule matcher that decides whether to accept or deny traffic for IPv4 addresses based on a list of CIDR-prefix rules. Each rule is tagged accept or deny. Assume:
IPv4 (32-bit addresses).
Precedence: longest-prefix match wins (typical for CIDR-based routing). For ties (same prefix length), earlier rule definition wins. If nothing matches, default is deny.
Rules can overlap (e.g., 10.0.0.0/8 accept and 10.0.0.0/16 deny).
Tasks
Design and implement a matcher that evaluates a single IPv4 address against a list of CIDR rules (accept/deny). Explain the data structures and algorithms used.
Follow-up: extend the solution to handle evaluating an incoming CIDR range against the rule set. Discuss and propose efficient data structures for range intersection checks.
Inputs/Outputs
Input (single-IP case): list of rules like [("10.0.0.0/8", accept), ("10.0.0.0/16", deny), ...] and a query IP (e.g., "10.1.2.3").
Output: accept or deny according to precedence.
Follow-up input: an incoming CIDR (e.g., "10.1.0.0/17").
Follow-up output: either a single decision if uniform over the range, or a decomposition into subranges with decisions; and a discussion of efficient data structures for range intersection.
Clarifying Questions to Ask Guidance
Clarify users, core use cases, read/write patterns, scale, latency, availability, and data retention.
State explicit assumptions before making sizing or architecture decisions.
Prioritize the functional path first, then address reliability, security, observability, and rollout.
What a Strong Answer Covers Guidance
A scoped requirements summary with concrete non-goals and success metrics.
API, data model, architecture, consistency, capacity, and operations.
Reasoned trade-offs among simple and scalable designs, including bottlenecks and failure modes.
A validation, monitoring, migration, and launch plan appropriate for the risk level.
Follow-up Questions Guidance
What breaks first at 10x traffic or data volume?
How would you degrade gracefully during dependency failures?
What metrics and alerts would prove the design is healthy after launch?