Docker & Kubernetes for Developers · 课时

K8s 中的服务发现与 DNS

探索 Kubernetes 如何处理服务发现和 DNS 解析，以支持服务间通信。

K8s 中的服务发现与 DNS 是 CoddyKit 上的免费 Docker & Kubernetes for Developers 课时。这是第 3 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Docker & Kubernetes for Developers 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Docker & Kubernetes for Developers 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Intro to Service Discovery

Welcome to Service Discovery & DNS in K8s! In a dynamic Kubernetes cluster, Pods are constantly created, destroyed, and moved, leading to ever-changing IP addresses.

How do applications running in one Pod find and communicate with applications in another Pod or Service?

This is where Service Discovery comes in. It's how services find each other without needing to know their specific, ephemeral IP addresses.

The Role of DNS in K8s

You might already know about DNS (Domain Name System) on the internet. It translates human-readable names like google.com into IP addresses.

Kubernetes uses a similar concept internally. Instead of relying on unstable Pod IPs, K8s assigns stable DNS names to its Services.

This allows your applications to connect to other services using simple, consistent names.

CoreDNS: The K8s DNS Server

Every Kubernetes cluster comes with its own DNS server, typically CoreDNS (or sometimes kube-dns in older versions).

CoreDNS runs as a Pod within your cluster and is responsible for resolving all internal Kubernetes service names to their corresponding cluster IP addresses.

It's automatically configured for you when you set up your cluster.

How Pods Get DNS Config

When a Pod starts, Kubernetes automatically injects DNS configuration into it.

Each Pod's /etc/resolv.conf file is updated to point to the cluster's CoreDNS Service IP.
It also includes search paths, allowing you to use shorter service names.

This means any application inside a Pod can immediately use the cluster's DNS for name resolution.

Service DNS Names

When you create a Kubernetes Service (e.g., a ClusterIP Service), CoreDNS automatically creates a DNS record for it.

The most common format for a Service's DNS name within its own namespace is simply: <service-name>.

For example, a service named my-web-app in the default namespace can be reached by other Pods in the same namespace using just my-web-app.

Example: Same-Namespace Access

Let's say you have a deployment hello-app and a service hello-service in the default namespace. Any other Pod in default can reach it via hello-service.

Here's a simple example of a Deployment and Service:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: hello-app
spec:
  selector:
    matchLabels:
      app: hello
  replicas: 1
  template:
    metadata:
      labels:
        app: hello
    spec:
      containers:
      - name: hello-container
        image: busybox
        command: ["sh", "-c", "while true; do echo Hello K8s; sleep 5; done"]

---

apiVersion: v1
kind: Service
metadata:
  name: hello-service
spec:
  selector:
    app: hello
  ports:
    - protocol: TCP
      port: 80
      targetPort: 80
  type: ClusterIP

Testing Same-Namespace DNS

After applying the YAML from the previous scene, you can test service discovery. First, create a temporary Pod (e.g., debug-pod) in the same namespace.

Then, exec into debug-pod and try to ping the service:

kubectl run debug-pod --image=busybox --restart=Never --rm -it --command -- sh
Inside the pod: ping hello-service

You should see the hello-service ClusterIP being resolved!

Cross-Namespace DNS (FQDN)

What if your services are in different namespaces? Kubernetes uses a Fully Qualified Domain Name (FQDN) format:

<service-name>.<namespace-name>.svc.cluster.local

For convenience, if the Pod's namespace is in its DNS search path (which it usually is), you can often use a shorter form:

<service-name>.<namespace-name>

This allows clear and unambiguous communication across different parts of your application.

Example: Cross-Namespace Access

Let's imagine a backend-service in the prod namespace. From a Pod in the default namespace, you'd access it like this:

ping backend-service.prod

Kubernetes DNS handles the resolution, directing traffic to the correct service, even across namespaces.

This provides strong isolation while still enabling communication where needed.

Quick Check

Which of the following statements are TRUE regarding Kubernetes Service Discovery and DNS?

Recap: Service Discovery & DNS

Great job! In this lesson, you learned how Kubernetes handles service discovery and DNS resolution for internal communication:

Pods have ephemeral IPs, requiring a stable discovery mechanism.
CoreDNS is the cluster's internal DNS server.
Services get stable DNS names, resolvable by other Pods.
Pods are automatically configured to use the cluster's DNS.
You can access services in the same namespace by name or in other namespaces using FQDNs.

This powerful system ensures your applications can always find each other reliably!

常见问题解答

「K8s 中的服务发现与 DNS」课时是免费的吗？

是的 — 「K8s 中的服务发现与 DNS」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Docker & Kubernetes for Developers 课程的其余内容，请升级到 CoddyKit PRO。 Docker & Kubernetes for Developers 课程共包含 4 节课。

「K8s 中的服务发现与 DNS」这节课中我会学到什么？

探索 Kubernetes 如何处理服务发现和 DNS 解析，以支持服务间通信。你通过在浏览器中直接运行的动手代码来练习 Docker & Kubernetes for Developers，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Docker & Kubernetes for Developers 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Docker & Kubernetes for Developers 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 3 节课，共 4 节。

「K8s 中的服务发现与 DNS」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Docker & Kubernetes for Developers 课中编写并运行代码吗？

能。每节 Docker & Kubernetes for Developers 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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