Kubernetes DNS Optimization // Support Tools

DNS plays a critical role in Kubernetes clusters by enabling service discovery and communication between Pods and services. However, poor DNS performance can lead to high latency, timeouts, and a degraded user experience. Optimizing DNS in Kubernetes is essential for maintaining high performance, especially in large-scale environments with dynamic workloads.

In this post, we will explore key strategies for optimizing DNS in Kubernetes clusters, focusing on improving both reliability and latency.

Why DNS Optimization Matters in Kubernetes

Kubernetes clusters rely heavily on DNS for service discovery. Every time a Pod wants to communicate with another service, a DNS lookup is required to resolve the service name into an IP address. In environments with thousands of Pods and frequent service changes, the DNS workload can become overwhelming, resulting in:

Increased Latency: Slow DNS lookups can delay communication between services.
DNS Timeouts: High DNS query rates can cause DNS queries to timeout, leading to failed service calls.
High CPU and Memory Usage: Inefficient DNS configurations can lead to resource overconsumption by DNS-related services like CoreDNS.

Step 1: Tune CoreDNS Configuration

CoreDNS is the default DNS service used by Kubernetes to resolve internal DNS names for services and Pods. Tuning the CoreDNS configuration can have a significant impact on DNS performance.

1.1 Increase Cache Size

By default, CoreDNS has a small cache size. Increasing the cache size can help reduce DNS query times by storing frequently accessed DNS records.

Edit the CoreDNS ConfigMap:

kubectl -n kube-system edit configmap coredns

Update the cache settings in the CoreDNS configuration:

apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        errors
        cache 10000 30
        forward . /etc/resolv.conf
        loop
        reload
        loadbalance
    }

In this example, the cache is set to store 10,000 records for 30 seconds.

1.2 Adjust Timeouts

CoreDNS has a default timeout setting for DNS queries. If the backend DNS server is slow, increasing the timeout can prevent query failures.

In the forward directive, you can adjust the timeout to 5 seconds (default is 2 seconds):

forward . /etc/resolv.conf {
    max_concurrent 1000
    timeout 5s
}

This will help handle DNS queries more efficiently under load.

Step 2: Reduce DNS Query Overhead

Kubernetes frequently performs DNS lookups for service communication. By reducing the DNS query overhead, you can improve performance across the cluster.

2.1 Use Headless Services

Headless services do not require DNS to resolve to a service IP, reducing the DNS overhead. If a service does not need load balancing, you can define it as headless:

apiVersion: v1
kind: Service
metadata:
  name: my-headless-service
spec:
  clusterIP: None
  selector:
    app: my-app

In this configuration, Pods communicate directly with each other using IPs, bypassing the DNS resolution step.

2.2 Use StatefulSets with DNS

StatefulSets provide DNS records for each Pod that are based on the Pod name, which reduces the need for additional DNS queries. Use StatefulSets in workloads where each Pod requires a stable identity and network identity.

For example, a StatefulSet with three replicas will result in the following DNS entries:

pod-0.my-statefulset.default.svc.cluster.local
pod-1.my-statefulset.default.svc.cluster.local
pod-2.my-statefulset.default.svc.cluster.local

This eliminates the need for dynamic DNS queries in some cases.

Step 3: Optimize DNS Caching in Pods

Each Pod has its own DNS resolver configuration, which can be optimized to reduce the number of external queries.

3.1 Modify the ndots Option

The ndots option in /etc/resolv.conf determines how many dots must appear in a DNS name before an absolute lookup is attempted. By default, Kubernetes sets ndots: 5, which can lead to unnecessary DNS queries.

You can reduce the number of retries by setting ndots: 1, which reduces the overhead for short names:

apiVersion: v1
kind: Pod
metadata:
  name: dns-optimized-pod
spec:
  containers:
  - name: my-app
    image: my-app:latest
  dnsConfig:
    options:
    - name: ndots
      value: "1"

This change ensures that queries are resolved more efficiently, particularly for services with simple names.

3.2 Enable DNS Caching at the Pod Level

For high-throughput services that make frequent DNS requests, you can enable DNS caching at the application level or use a DNS caching agent like dnsmasq. This avoids sending repetitive DNS queries to CoreDNS.

To set up dnsmasq as a sidecar container for DNS caching:

apiVersion: v1
kind: Pod
metadata:
  name: dns-caching-pod
spec:
  containers:
  - name: my-app
    image: my-app:latest
  - name: dnsmasq
    image: andyshinn/dnsmasq
    args: ["-k"]
    ports:
    - containerPort: 53

This approach improves DNS response times for applications that repeatedly query the same DNS names.

Step 4: Monitor and Scale CoreDNS

Once you’ve applied these optimizations, it’s crucial to monitor DNS performance. Use tools like Prometheus and Grafana to track DNS query rates, cache hit ratios, and CoreDNS resource usage.

4.1 Increase CoreDNS Replicas

If CoreDNS is under heavy load, increasing the number of replicas can help distribute the DNS query workload:

kubectl scale deployment coredns --replicas=3 -n kube-system

Scaling CoreDNS helps ensure that DNS queries are handled quickly without bottlenecks.

Conclusion

Optimizing DNS in Kubernetes is essential for improving the performance and reliability of your applications. By tuning CoreDNS configurations, reducing DNS query overhead, and caching DNS queries at the Pod level, you can significantly enhance the efficiency of service discovery in your cluster.

Whether you’re running a small Kubernetes deployment or managing a large-scale production environment, these DNS optimizations will help you avoid common pitfalls and improve overall cluster performance.