What Is Kubernetes DNS?
Kubernetes DNS is a built-in service within the Kubernetes platform, designed to provide name resolution for services within a Kubernetes cluster. It simplifies the communication process between different services and pods within the cluster by allowing the use of hostnames instead of IP addresses. This is essential because in a dynamic environment like Kubernetes, where pods are continuously created and destroyed, tracking and using IP addresses is very difficult.
The Kubernetes DNS service is automatically configured for each new Kubernetes cluster and assigns a DNS name to each service within the cluster. This DNS name is then used to resolve to the service’s ClusterIP, the stable IP address assigned to the service within the cluster. This mechanism allows applications running within the cluster to easily discover and communicate with each other.
This is part of a series of articles about Kubernetes management
What Is CoreDNS?
CoreDNS is the default DNS server for Kubernetes, replacing the previous default, kube-dns, as of Kubernetes 1.13. CoreDNS is a flexible, extensible authoritative DNS server that can serve as a cluster DNS. It is a plugin-based DNS server, which means it can be easily extended with custom functionality. Furthermore, it supports multiple DNS protocols including UDP/TCP, TLS, DNS over HTTP/2, and DNS over QUIC.
CoreDNS is designed to be lightweight and easy to configure. It uses a minimal configuration file that can be modified to add or remove plugins, change the behavior of existing plugins, or even write new plugins.
Kubernetes DNS Resolution
Default Domain Name: cluster.local
The default domain name in Kubernetes DNS is “cluster.local”. This name is automatically assigned to every service within a Kubernetes cluster, allowing pods to communicate with these services using their hostnames. This significantly simplifies service discovery within a cluster, as pods no longer need to know the IP addresses of the services they need to interact with.
This domain name is customizable, which is useful in larger clusters where multiple services might share the same hostname. By changing the domain name, administrators can ensure that each service has a unique address within the cluster, avoiding naming conflicts and facilitating smoother communication between different components.
Beyond just convenience, the use of a default domain name also enhances security within a Kubernetes cluster. By requiring pods to communicate with services using their hostnames, Kubernetes can better monitor and control the flow of information within the cluster, preventing unauthorized access to critical services.
DNS Name for Services
Kubernetes DNS provides a DNS name for each service within a cluster. These DNS names follow a specific format: <service-name>.<namespace-name>.svc.cluster.local. Here, <service-name> is the name of the service, <namespace-name> is the name of the namespace in which the service resides, and svc.cluster.local is a fixed suffix that distinguishes service DNS names from those of pods.
These DNS names are automatically resolved to the ClusterIP of the service, enabling pods to easily discover and communicate with services within the same cluster. This not only simplifies service discovery but also allows for load balancing, as the ClusterIP serves as a single entry point through which traffic is distributed among the pods of the service.
Moreover, these DNS names are also resolvable outside of the cluster, provided that the external system has been properly configured to recognize the cluster’s DNS server. This allows services within a Kubernetes cluster to be accessible from outside, facilitating integration with external systems and applications.
DNS Name for Pods
Much like services, each pod within a Kubernetes cluster also has a DNS name assigned by Kubernetes DNS. The format of these DNS names is <pod-name>.<namespace-name>.pod.cluster.local, where <pod-name> is the name of the pod and <namespace-name> is the name of the namespace in which the pod resides.
These DNS names are automatically resolved to the IP address of the pod, allowing other pods and services within the cluster to easily discover and communicate with the pod. This simplifies communication within the cluster and allows for more efficient resource allocation, as pods can directly address each other rather than having to route their traffic through services.
Pod and Service Interaction Using DNS Names
There are two primary ways pods and services can use DNS names to interact:
Pods and Services in the Same Namespace
When pods and services reside in the same namespace, they can refer to each other using their short DNS names, which consist of just the name of the pod or service. This makes communication between pods and services in the same namespace very straightforward, as they don’t need to specify the namespace or the svc.cluster.local or pod.cluster.local suffix when addressing each other.
For example, a pod named pod1 in a namespace named namespace1 could communicate with a service named service1 in the same namespace by simply addressing it as service1. Kubernetes DNS would automatically resolve this to the ClusterIP of service1, allowing pod1 to send requests to or receive responses from service1.
Pods and Services in Different Namespaces
For pods and services to communicate with counterparts in other namespaces, they must specify the namespace of the pod or service they’re trying to address. This is because Kubernetes isolates namespaces from each other, requiring explicit references to objects in other namespaces.
For instance, a pod named pod1 in a namespace named namespace1 would need to address a service named service2 in a different namespace named namespace2 as service2.namespace2. Kubernetes DNS would then resolve this to the ClusterIP of service2, allowing pod1 to send requests to or receive responses from service2.
Kubernetes DNS Troubleshooting with Komodor
Kubernetes DNS can sometimes go wrong, creating complex issues involving multiple components which can be difficult to monitor, troubleshoot and resolve. Without the right tools and expertise in place, the troubleshooting process can become stressful, ineffective and time-consuming. Some best practices can help minimize the chances of things breaking down, but eventually something will go wrong – simply because it can.
This is where Komodor comes in – Komodor is the Continuous Kubernetes Reliability Platform, designed to democratize K8s expertise across the organization and enable engineering teams to leverage its full value.
Komodor’s platform empowers developers to confidently monitor and troubleshoot their workloads while allowing cluster operators to enforce standardization and optimize performance.
By leveraging Komodor, companies of all sizes significantly improve reliability, productivity, and velocity. Or, to put it simply – Komodor helps you spend less time and resources on managing Kubernetes, and more time on innovating at scale.
If you are interested in checking out Komodor, use this link to sign up for a Free Trial.
