Dns

DNS

DNS is the global front door: it turns a name (api.example.com) into an address before any request reaches your servers. It is also a routing layer — the same lookup can hand different clients different answers (by region, latency, health, or weight). Get it wrong and clients reach a dead region, fail over in minutes instead of seconds, or cache a bad answer for hours. It is not a load balancer and not a CDN; it decides which endpoint a client is told to use, not how bytes are balanced inside that endpoint.

When to reach for this

A service is reachable by a stable hostname; traffic must be steered to the closest or healthiest region/data center; or an endpoint's IP can change (a new load balancer, a failover site) and clients must follow without code changes. Multi-region or multi-data-center designs need DNS to pick the entry point; single-region designs still need it for a stable, movable name.

When NOT to

Routing inside one region — that is a load balancer's job (→ load-balancing), which reacts in milliseconds, not TTL-bound minutes. Fine-grained per-request or session-aware steering — DNS answers per lookup, then the client caches it. Fast failover with sub-second recovery — DNS failover is gated by TTL and resolver caching; do not promise instant cutover from the name layer. And do not reach for exotic routing policies before a number shows users are spread across regions (YAGNI) — a single A/ALIAS record is the right default for one region.

Clarify first

• Geographic spread — one region or many? Where are the users? (→ requirements-scoping)
• Failover target — recovery time objective for losing a region: seconds, or "a few minutes is fine"?
• How dynamic is the endpoint — fixed IPs, or an LB hostname that changes? (drives A vs CNAME/ALIAS)
• Steering goal — closest by latency, by user geography, by weight (canary/A-B), or just round-robin?
• Staleness tolerance for the mapping — how long can a client keep a stale answer? (sets the TTL → back-of-the-envelope)

The options

Record type (what the name returns)

• A / AAAA — name → IPv4 / IPv6. Use when the endpoint has a stable IP.
• CNAME — name → another name. Use to alias onto a provider hostname; never at the zone apex (example.com).
• ALIAS / ANAME (provider-specific) — apex alias to a hostname. Use to point example.com at an LB/CDN hostname.
• NS / MX / TXT — delegation, mail, verification. Use as the domain requires; not traffic steering.

Routing policy (which answer a client gets)

• Simple — one answer for everyone. Use for a single endpoint.
• Weighted — split by percentage. Use for canary, A/B, or shifting between clusters.
• Latency-based — answer the region with lowest measured latency to the resolver. Use to minimize round-trip.
• Geolocation / GeoDNS — answer by the client's location (data residency, localized content). Use when geography, not latency, must decide.
• Failover — primary while healthy, else secondary. Use for active-passive DR.
• Multivalue — return several healthy IPs; client picks. Use for cheap client-side spread with health pruning.

Reachability

• Anycast — one IP advertised from many sites; the network routes to the nearest. Use for resolver/CDN front ends and DDoS resilience (concept lives in content-delivery).

Trade-offs

| Option | What it solves | What it worsens | Change it when | |---|---|---|---| | Simple A/ALIAS | Trivial; one stable name | No steering, no failover; SPOF if the IP dies | Traffic spans regions or needs DR → weighted/latency/failover | | Weighted | Gradual shifts, canary, cluster balancing | Manual/slow; not health-aware unless paired with checks | You need automatic closeness → latency; or automatic cutover → failover | | Latency-based | Lowest RTT per user automatically | Routes to resolver location, not user; needs per-region endpoints | Data residency matters more than speed → geolocation | | Geolocation | Compliance, localized answers | Misroutes via VPN/forwarding resolvers; coarse map | Speed matters more than geography → latency-based | | Failover | Automatic active-passive cutover | Recovery bounded by TTL + caching; cold standby risk | RTO must be sub-second → in-region LB, not DNS | | Anycast | Nearest entry + absorbs DDoS | Operationally heavy (BGP); flap on route changes | You only have one site → not worth it |

Behavior under stress

DNS fails in slow, wide ways — a bad answer or an authoritative outage affects everyone who looks up next, and lingers as long as TTLs allow.

• TTL caching delays recovery and propagation. Resolvers and OS/browser

caches hold answers for the TTL; some ISPs over-cache. A failover or IP change only takes effect as old entries expire — the effective recovery time is TTL plus stragglers, not zero.

• Authoritative DNS as a SPOF. If your authoritative servers (or a single

managed zone) go down, nothing resolves, even though your servers are healthy. Use a provider on anycast across many sites; consider a secondary DNS provider.

• Health-check stampede / flapping. Aggressive health checks across many

endpoints can hammer targets, and a flapping check can oscillate answers, scattering clients. Tune interval, failure threshold, and require N consecutive failures (→ resilience-failure owns health-check tuning).

• Low-TTL load. Cutting TTL to seconds for fast failover multiplies lookup QPS

against authoritative servers and removes the cache cushion during an attack.

• DDoS on the resolver tier. DNS is a classic amplification/DDoS target; an

overwhelmed resolver makes your whole property unreachable.

Monitor: resolution latency and error/SERVFAIL rate, query QPS per record, health-check status per endpoint, time-to-propagate after a change, and the share of traffic each region/answer actually receives.

How to apply

1. Clarify the inputs — geographic spread, RTO for a region loss, endpoint

stability, steering goal, and mapping staleness (see Clarify first). One region with a fixed entry point needs only a simple record — stop there.

1. Pick the policy from the trade-off table — choose a record type (ALIAS

at the apex onto an LB/CDN hostname is the common default) and a routing policy keyed to the steering goal: weighted for canary, latency for speed, geo for residency, failover for active-passive DR.

1. Set the key knobs — TTL (balance fast failover against lookup load and

propagation), health-check interval and failure threshold, and the failover target. Decide secondary-DNS / anycast posture up front.

1. Stress-test the choice — walk Behavior under stress: confirm the TTL

gives an acceptable effective recovery time, the authoritative tier is not a SPOF, and health checks won't flap or stampede.

1. Size it with numbers — estimate lookup QPS at the chosen TTL and the

propagation window a change implies (→ Numbers that matter).

1. Pick a provider — default to the generic recipe; open a provider file only

if the user named a cloud (see Choosing a provider).

Dos and don'ts

Do

• Put steering at DNS for cross-region choice and let an LB balance within a region.
• Use ALIAS/ANAME at the apex onto an LB/CDN hostname so IP changes need no edits.
• Pair routing policies with health checks so dead regions stop being handed out.
• Run authoritative DNS on anycast across many sites; consider a second provider.
• Set TTL deliberately — short enough for your RTO, long enough to survive load.

Don't

• Don't expect DNS failover to be instant; it is bounded by TTL plus resolver caching.
• Don't put a CNAME at the zone apex — it is invalid; use ALIAS/ANAME.
• Don't trust geolocation for security or precise placement (VPNs and forwarders lie).
• Don't run a single authoritative zone as a SPOF for an otherwise multi-region design.
• Don't drop TTL to seconds everywhere "just in case" — it multiplies query load.

Numbers that matter

The quantities to estimate: the TTL, which sets both the propagation window and the effective failover time (a 60 s TTL means up to ~60 s-plus of staleness, not instant); the lookup QPS the TTL implies against authoritative servers (shorter TTL → more queries); and a resolution latency budget (one extra round trip, largely hidden by caching). Anchor these with rules of thumb and peak-multiplier math in back-of-the-envelope — don't restate its tables here.

Interface sketch

A DNS record is a contract: name (api.example.com), type (A/AAAA/ CNAME/ALIAS), value (IP or target hostname), TTL (seconds), and an optional routing policy + health check. Example as a zone line:

api.example.com.   60   IN   A   203.0.113.10     ; latency-policy, region us-east, health-check id hc-1

Decide per record: which policy selects it, what health check gates it, and the TTL that bounds its staleness.

Choosing a provider

Default to the generic recipe above. If the user names a cloud, read references/providers/.md for the managed-service mapping, quotas/limits, and provider-specific trade-offs. If no file exists for that provider, the generic recipe is the answer.

Diagram

To visualize the resolution path (client → recursive resolver → root/TLD/ authoritative → answer) or a geo/latency-steered multi-region front door, use the in-plugin architecture-diagram skill. A quick inline sketch: client → resolver → authoritative (policy: latency) → region A | region B.

Related building blocks

• load-balancing — complements this: DNS spreads traffic across regions/endpoints, while an LB spreads requests within a region and reacts in milliseconds.
• content-delivery — pairs with this: CDNs route users to edge POPs via anycast and DNS, and the anycast/edge-caching concept is owned there.
• resilience-failure — feeds into this: health-check tuning, failover, and degradation patterns (and rate limiting) are owned there and gate DNS failover.
• back-of-the-envelope — feeds into this: it supplies the TTL/QPS/latency math that sizes the record set and propagation window.
• system-design — owned-concept lives in the orchestrator: the reasoning loop, the trade-off method, and the ten failure modes.

References

• references/deep-dive.md — resolution hierarchy (recursive vs authoritative, root/TLD), the full record-type set, how TTL/propagation really behaves, anycast mechanics, and health-checked failover internals. Read when designing the DNS layer in detail.
• references/providers/{generic,aws,azure,gcp}.md — service mappings, limits, and pitfalls per environment.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: proyecto26
• Source: proyecto26/system-design-skills
• License: MIT
• Homepage: https://proyecto26.com/system-design-skills/

Install and usage instructions live in the source repository linked above.