Evaluating fallback behaviors and troubleshooting Domain Name System (DNS) resolution across hybrid networks ensures continuous traffic flow when endpoints fail. Cloud DNS routing policies work alongside health checks and hybrid connections to direct queries away from failed infrastructure. Configuring resilient resolution requires verifying route reachability between Google Cloud virtual private clouds (VPCs) and on-premises environments, as well as confirming consistent zone configurations.
A Cloud DNS failover policy routes traffic to a designated primary endpoint pool and switches to a backup pool only when the primary targets fail. Network administrators configure a failover policy by defining primary and backup record sets inside a managed DNS zone and attaching a health check to the primary record set. The health checker continuously monitors the availability of primary endpoints using specified protocols and ports. For hybrid environments, health check probes require network paths across Cloud VPN or Cloud Interconnect to reach on-premises or external cloud backends.
The default fallback behavior in a Cloud DNS failover routing policy triggers only when every endpoint in the primary health-checked record set is marked unhealthy. If at least one primary endpoint remains healthy, Cloud DNS continues resolving queries using the primary record set. When all primary endpoints fail, Cloud DNS immediately returns the IP addresses defined in the backup record set. Fallback stops at the backup record set because the policy does not support automatic cascading to tertiary targets. Ingress firewall rules must allow Google health check probe IP ranges (35.191.0.0/16 and 130.211.0.0/22) to reach the target endpoints.
Troubleshooting hybrid DNS resolution requires tracing query and response paths across network boundaries and verifying transport permissions. A private Cloud DNS zone only resolves names for the specific VPC networks authorized in its configuration. When Google Cloud resources must resolve on-premises hostnames, administrators configure forwarding zones or outbound server policies pointing to on-premises DNS servers. Hybrid interconnects and VPN tunnels must maintain valid routes and permit bidirectional UDP and TCP traffic on port 53. Responses from external DNS servers must return to the exact Google Cloud IP address that sent the original query, or the resolver drops the packet.
Cloud DNS evaluates routing policies differently depending on whether a managed zone is public or private. Public managed zones evaluate routing policies for clients querying from the public internet, whereas private managed zones evaluate policies exclusively for queries originating inside attached VPC networks. Inconsistent name resolution occurs when private zone attachments omit peered VPCs or hybrid networks that require access. Maintaining parity across environments requires binding private zones to all relevant VPC networks and keeping policy definitions synchronized between public and private zones.
Configuring routing policies and supporting components in Cloud DNS allows network administrators to manage traffic paths across Google Cloud and hybrid networks. Establishing a functional routing infrastructure involves creating private DNS zones, setting up hybrid connectivity, configuring network endpoint groups, and deploying proxy-based routing components.
A Cloud DNS managed private zone provides internal domain name resolution restricted to specified VPC networks. Administrators create private zones using the command-line interface by defining a zone name, the DNS domain name, and setting network visibility to private. Adding DNS records to the managed zone uses a transaction process where records are staged, defined with specific record types, time-to-live (TTL) values, and target IP addresses, and then committed. This structured transaction model prevents partial updates and ensures consistent DNS state across the network.
Hybrid connectivity provides the physical and logical network links needed for Google Cloud services to communicate with on-premises and third-party cloud environments. Tunnels established through Cloud VPN or dedicated circuits through Cloud Interconnect carry traffic between environments. VPC networks require proper routing configurations and firewall rules to permit bidirectional traffic between client proxies and remote service endpoints. Enabling global dynamic routing across the VPC allows Border Gateway Protocol (BGP) routes learned from on-premises routers to propagate across all Google Cloud regions automatically.
A network endpoint group (NEG) defines a collection of individual backend endpoints, such as IP addresses or fully qualified domain names (FQDNs), for routing systems. An internet FQDN NEG uses a specific endpoint type to reference external services by their domain names and ports. Backend services configured with internal load-balancing schemes bind these NEGs into logical pools. Attaching health checks directly to backend services ensures that the routing layer only forwards traffic to operational backends.
A routing rule map directs incoming service requests to specific backend services by combining a URL map, a target HTTP proxy, and a forwarding rule. The URL map parses incoming request attributes, matching hostnames and URL path patterns to determine the target backend service. Forwarding rules can use a placeholder IP address of 0.0.0.0, instructing the data plane to route traffic solely based on the parameters evaluated in the URL map. Each forwarding rule in a VPC network must maintain a unique IP address and port combination to avoid routing collisions.
Distributed health checking in hybrid service mesh architectures delegates probing responsibilities to local proxy instances rather than using centralized health checkers. When hybrid NEGs use private IP and port endpoints, Envoy proxy instances independently evaluate backend availability using data-plane health checking. This client-side approach prevents proxies from sending traffic to degraded remote backends. Because every proxy instance sends its own health checks to every backend endpoint, probe traffic volume scales quadratically as the number of clients and endpoints increases.
Health-checked failover routing policies provide active-passive redundancy by redirecting client DNS queries between active primary targets and standby backup targets. In normal operations, Cloud DNS resolves queries exclusively to the primary load balancer. When health check failures signal an outage on the primary infrastructure, Cloud DNS executes an automated failover to the backup load balancer, followed by an automated failback once the primary recovers.
An active-passive routing architecture pairs an active primary load balancer with one or more standby backup load balancers to maintain service continuity. Only a regional external Application Load Balancer can serve as the backup target in a Cloud DNS failover policy. Regional external Application Load Balancers operate on isolated infrastructure separate from global external Application Load Balancers and classic Application Load Balancers within the same region. Supported failover combinations include routing from a global external Application Load Balancer to a regional external Application Load Balancer, between two regional external Application Load Balancers, or from a classic Application Load Balancer to a regional external Application Load Balancer.
Failure detection in Cloud DNS failover policies uses global health checks that continuously test the operational health of the primary load balancer. Administrators configure a global health check that originates probe traffic from exactly three source regions aligned with client traffic distribution. Probes originate from internet Points of Presence (PoPs) located near the selected source regions and test target endpoints using HTTP, HTTPS, or TCP protocols. A failover event triggers when probes originating from two or more of the three configured source regions report that the primary endpoint is unhealthy.
The failback workflow automatically restores traffic routing to the primary load balancer once health checks confirm that the primary infrastructure has recovered. When health check probes from the required source regions resume reporting healthy responses, Cloud DNS returns primary IP addresses to client queries without administrative intervention. Because both primary and backup load balancers remain active and capable of serving traffic simultaneously, the transition causes zero service disruption. Operational teams validate these workflows by simulating outages to observe both failover redirection and subsequent failback recovery.
Backup load balancers must maintain feature parity with primary load balancers to ensure identical application behavior during a failover event. Deployments running on Google Kubernetes Engine (GKE) must use the GKE Gateway controller, which supports both global and regional external Application Load Balancers, whereas the older GKE Ingress controller only supports classic Application Load Balancers. Regional external Application Load Balancers do not support Cloud CDN, which means caching workflows require third-party CDN integrations for standby environments. Furthermore, security policies configured in Google Cloud Armor must be applied across regional scopes to match the protections enforced on global endpoints.
Only a regional external Application Load Balancer can serve as the backup load balancer in a Cloud DNS failover policy. It can back up a global external Application Load Balancer, a classic Application Load Balancer, or another regional external Application Load Balancer.
Cloud DNS uses a global health check configured with three distinct source regions. A failover triggers automatically when health check probes from two or more of those three source regions report unhealthy responses.
Distributed health checking assigns the probing task to individual Envoy proxy instances rather than Google's centralized health checking service. Because every proxy instance independently probes every backend endpoint, total probe traffic multiplies rapidly as the number of clients and backends increases.
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