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DNS round robin can distribute new connection attempts across two or more servers by publishing multiple A or AAAA records for one hostname. That alone is not high availability: ordinary DNS does not know whether an application is healthy. For dependable failover, combine the records with endpoint or application health checks, a defined fallback, and a TTL that matches your recovery objective.
What DNS round robin does—and does not do
Suppose app.example.com has three IPv4 addresses:
| Hostname | Type | Address |
|---|---|---|
| app.example.com | A | 192.0.2.11 |
| app.example.com | A | 192.0.2.12 |
| app.example.com | A | 192.0.2.13 |
A DNS resolver can return the three addresses in varying order. The client then attempts one address, normally choosing according to its own connection logic and address-preference rules. The next client may try a different address, so connection attempts are spread across the endpoints. The same design works with multiple AAAA records for IPv6. If you publish both record types, make sure every advertised address is actually reachable; an unusable IPv6 address can make dual-stack clients appear to fail even when IPv4 is healthy.
Round robin is a distribution technique, not a load balancer with a view of your servers. It does not measure CPU, active connections, response time, connection duration, session state, or current application load. A long-lived WebSocket or database connection remains on the address it selected, and DNS does not rebalance it. Recursive resolvers, operating systems, browsers, and client libraries cache answers, so the pattern will not be perfectly even.
Most importantly, plain round robin does not perform application health checks. If one server stops responding while its DNS record remains published, some clients will continue receiving and trying that address. High availability requires a health-aware DNS service or a load-balancing layer that suppresses unhealthy endpoints.
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Design the service before changing DNS
Use independent failure domains
Choose at least two application endpoints that do not depend on the same host, power circuit, network path, availability zone, or single database instance. Two virtual machines in one failed rack are not meaningful redundancy. Document each endpoint’s location, private dependencies, deployment version, and emergency owner.
Give every endpoint the same service identity
All endpoints must serve the same application behavior at the shared hostname. Install a certificate whose name includes the service hostname, such as app.example.com, on every HTTPS endpoint. Confirm that redirects, authentication cookies, API versions, static assets, and security headers are consistent. A client that is sent to a different server should not receive a different contract or a certificate-name error.
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Create a lightweight endpoint such as /health or /ready. A process-only check can return success while the application cannot reach its database, queue, identity provider, or required storage. The readiness check should verify the dependencies needed for real requests, while avoiding expensive operations that could overload a degraded service. Return a predictable HTTP status, commonly 200 for ready and a non-success status for unavailable. Protect the endpoint from accidental data exposure and allow the provider’s probe source addresses through firewalls and security groups.
Decide whether DNS alone is sufficient
Simple records may be adequate for a stateless, low-connection service where occasional retries are acceptable. Use a health-aware DNS service when an unhealthy endpoint must be removed automatically. Use an application-layer load balancer instead when you need per-request balancing, connection draining, sticky sessions, instant local failover, WebSocket control, or routing based on URL, headers, or live load.
Step-by-step configuration
1. Inventory and validate the endpoints
- List the production addresses, regions or zones, operating systems, application versions, and dependencies for each endpoint.
- From a network location representative of users, test the service hostname or each endpoint directly. Verify DNS resolution, TCP connection, TLS negotiation, and an ordinary application request.
- Confirm that firewalls permit user traffic and health-probe traffic on the intended ports. Ensure the certificate and virtual-host configuration respond correctly when the shared hostname is used.
Do not publish an address until it can serve the complete application path. A server that answers a TCP handshake but cannot complete authentication or retrieve required data is not healthy for this purpose.
2. Publish one record per endpoint
In your authoritative DNS zone, create one A record for every IPv4 endpoint or one AAAA record for every IPv6 endpoint, all using the identical service name. A typical illustrative set is:
app.example.com. 300 IN A 192.0.2.11
app.example.com. 300 IN A 192.0.2.12
app.example.com. 300 IN A 192.0.2.13The addresses above use documentation-only space; replace them with your routable service addresses. Keep the record name, TLS name, and application configuration aligned. If your provider offers proxying, acceleration, or CDN mode, note whether each record is proxied or DNS-only. Proxied and unproxied records can follow different routing and health behavior, and mixing modes without a deliberate design can produce unexpected answers.
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For a first deployment, query the authoritative nameservers directly and confirm that every expected address is present:
dig @<authoritative-nameserver> app.example.com A
dig @<authoritative-nameserver> app.example.com AAAAThen query through the recursive resolvers your users actually use:
dig +short app.example.com A
nslookup app.example.com3. Add health-aware routing for automatic removal
If an endpoint fails, a health-aware service should stop returning it while it is unhealthy. Configure checks against the endpoint or application pool rather than merely checking whether the DNS record exists. Depending on the provider, you may select simple health-checked answers, priority or failover routing, weighted routing, latency routing, geographic routing, or a pool-based policy.
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4. Define primary and fallback destinations
Create a secondary or fallback pool when the service supports one. A fallback is the destination used when every primary endpoint is considered unhealthy; it might be a static maintenance site, a read-only service, or endpoints in another region. Decide whether the fallback should preserve authentication and API semantics or clearly present a maintenance response. Test the all-unhealthy condition rather than assuming the provider will choose a safe result.
5. Set a TTL that matches your recovery objective
TTL is the maximum caching lifetime advertised for an answer, not a promise that every client will refresh at that exact moment. A resolver can continue returning a cached address until its TTL expires, and operating systems, browsers, service meshes, and intermediate DNS components may cache independently. Existing connections are unaffected by a DNS change.
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Microsoft’s high-availability example uses a 300-second TTL, while its Traffic Manager guidance recommends less than 60 seconds when faster downstream updates are needed. Those are provider recommendations, not universal guarantees. Shortening the TTL increases query volume and may increase service charges or authoritative-server load. Choose a value by balancing the maximum acceptable stale-answer period against operational cost. Before a planned migration, lower the TTL in advance and wait for the old value to age out; changing it at the same time as an outage does not retroactively clear caches.
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6. Verify the published behavior
- Query the authoritative server and confirm the complete record set, TTL, and record type.
- Query at least two unrelated recursive resolvers. Repeat after the TTL has elapsed to observe ordering and caching.
- Request the health endpoint directly from each endpoint and through the production hostname. Check status code, response content, TLS certificate, and latency.
- Review provider health status, probe logs, DNS query logs, application logs, and monitoring alerts.
Do not treat seeing a different address on one dig command as proof of equal traffic distribution. Resolver caching and client selection make DNS observations probabilistic.
7. Run failure and recovery exercises
- Stop the application on one endpoint or block its service port in a controlled window.
- Wait for the configured probe interval and failure threshold. Record when the provider marks the endpoint unhealthy.
- Query the authoritative service, then several recursive resolvers, and make real test requests from more than one network.
- Confirm that new answers omit the failed address, while documenting how long cached clients continue to see it.
- Restore the endpoint, verify that its health check passes, and observe whether it is reintroduced automatically. This is failback; it may need a recovery threshold or manual approval.
- Repeat with a dependency failure, such as an unavailable database, to ensure the readiness check detects an application that is reachable but unusable.
Record observed detection and propagation times. Never promise instant failover merely because the configured TTL is short.
Choosing a managed DNS failover approach
The major managed options differ in how they check health and select answers. Product names and plan capabilities change, so verify current documentation and limits before committing.
| Option | Health and routing model | Fallback behavior | Important considerations |
|---|---|---|---|
| Amazon Route 53 | Health checks can be associated with resources; simple and more complex health-checked configurations can return only healthy resources. Policies include simple, weighted, latency, geolocation, and failover patterns. | Failover and policy-specific behavior can direct queries to a secondary resource when the primary is unhealthy. | Check health-check scope, regional coverage, evaluation rules, resolver behavior, and current pricing or quotas. |
| Cloudflare Load Balancing | Uses pools, monitors, and health checks. Monitors can test responsiveness, timeout, expected HTTP status, and response content. | A fallback pool can serve as the last-resort destination when primary pools are unavailable. | Understand the difference between basic round-robin DNS and pool-based load balancing, and between proxied and DNS-only records. Session affinity and geographic features depend on the selected plan. |
| Azure Traffic Manager | Provides DNS-based routing with health probes and policies such as priority, weighted, performance, geographic, and multivalue routing. | Priority or policy configuration can direct users to another endpoint when probes fail. | DNS caching still controls client-visible timing. Microsoft cites 300 seconds in one high-availability example and recommends less than 60 seconds for faster updates in guidance. |
Compare services on more than the word “round robin.” Evaluate whether checks can test application dependencies, how many failures trigger removal, whether probes originate from multiple locations, how fallback is selected, whether session affinity is available, what logs and metrics are exposed, which regions are covered, and what plan limits apply.
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With health-aware routing
Prefer putting the endpoint into maintenance or draining state, then let the health check mark it unavailable. If the provider supports administrative enable/disable controls, disable the endpoint before shutting it down. Confirm that new DNS answers omit it and that monitoring shows the intended pool state. Keep the endpoint isolated until logs and dependency checks show that it is safe to re-enable.
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With plain DNS round robin
Delete the failed endpoint’s A and/or AAAA record from the authoritative zone. Removing the record affects only newly refreshed answers. Resolvers and clients that cached the old set can continue trying the failed address until their cached TTL expires, and an already-open connection will not move. If the outage is severe, serve a controlled response at the endpoint or use a separate load-balancing layer rather than relying on DNS deletion alone.
After removal, query both authoritative and recursive servers, inspect the zone’s serial or change history, and watch connection attempts for the retired address. Keep a change record so the endpoint can be restored without accidentally publishing an obsolete address or certificate.
TTL planning and realistic failover timing
A useful planning model is:
user-visible recovery time ≈ health-detection time + DNS cache remaining time + client retry time
Health-detection time comes from the probe interval, timeout, and number of consecutive failures. DNS cache remaining time can range from nearly zero to the full TTL for an answer that was just cached. Client retry time depends on connection and application behavior. A 60-second TTL therefore does not mean every user fails over in 60 seconds; it means a resolver should not retain that answer beyond the advertised lifetime under normal DNS behavior.
Long TTLs reduce DNS queries but prolong stale addresses after a failure. Very short TTLs improve update frequency but increase query traffic and do not move existing connections. Test the actual behavior with the recursive resolvers, operating systems, and client libraries used by your service.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common failure modes and fixes
All addresses are returned even though one server is down
This is expected for unmonitored round robin. Add provider health checks or place an application-aware load balancer in front of the servers. Deleting a record manually is only a temporary operational process and remains subject to caching.
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Users still reach the retired server
Check the authoritative answer first. If the address is gone there, inspect the resolver’s cached TTL, local DNS cache, browser or application cache, and any service-mesh DNS cache. Existing TCP or TLS connections also remain on the old endpoint. Do not repeatedly edit records while caches are still valid; verify the configured TTL and wait for the documented window.
Health checks pass but real requests fail
The probe may test only a process or port. Change it to a readiness endpoint that checks the dependencies required for production traffic. Validate the expected HTTP status and response content, and ensure the probe uses the same hostname, TLS mode, and authentication assumptions as real requests.
Traffic is badly unbalanced
DNS answers are cached at different times, clients may prefer one address family, and some clients retry the same address. Long-lived connections amplify the imbalance. Use weighted or latency policies where appropriate, or move balancing to an application-layer service that sees each connection or request. Do not infer server load from the number of DNS records.
HTTPS fails on one endpoint
Install a certificate covering the shared hostname on every server, configure the same virtual host, and test with the hostname rather than an IP address. An endpoint certificate for only its private name will fail when selected through the common service name.
IPv6 clients fail while IPv4 works
Remove an unusable AAAA record or fix IPv6 routing, firewall, and application binding. Publishing both address families advertises both as valid paths; DNS will not test whether a particular client can reach one of them.
Mixing proxied and unproxied records creates unexpected results
Review the provider’s proxy mode, health-check scope, and answer behavior for each record. Keep the records in one deliberate model unless you understand how the provider separates proxied traffic from direct DNS answers.
When DNS round robin is the wrong tool
Use a different architecture when the service requires any of the following:
- Connection draining before maintenance or deployment.
- Sticky sessions or stateful protocols that cannot move between servers.
- Per-request balancing based on current connection count, response time, URL, headers, or geography.
- Immediate removal and retry behavior without waiting for recursive DNS caches.
- Central TLS termination, WebSocket controls, request filtering, or a single point for observability.
An application-layer load balancer can still use multiple independent backends and health checks, while DNS provides a separate regional or disaster-recovery layer. In that design, publish the load balancer addresses rather than every individual server and keep the backend health policy in the layer that can observe requests.
Quick Recap
Production checklist
- At least two endpoints exist in independent failure domains.
- Every endpoint serves identical application behavior and the shared TLS name.
- Each advertised
AorAAAAaddress is reachable from intended client networks. - A readiness health path checks the dependencies needed for real traffic.
- Probe protocol, port, timeout, interval, expected status, content, and failure threshold are documented.
- Primary and all-unhealthy fallback behavior are explicitly configured and tested.
- TTL reflects the recovery objective, query load, and provider limits.
- Authoritative and recursive DNS queries are monitored during changes.
- Failure, recovery, and failback exercises have been completed from multiple networks.
- Operators know how to disable an endpoint, remove its records, restore it, and verify cache expiry.
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