Kubernetes 里，怎么让 Pod 有 DNS 记录？

• memcached-0.memcached-cluster.svc.cluster.local
• memcached-1.memcached-cluster.svc.cluster.local
• ...

这里假设 cluster domain 是默认的 cluster.local。关于 StatefulSet 的 pod DNS，官方文档中也有简单说明。

那除了由 StatefulSet 管理的 pod，其它的 pod 能不能有 DNS 记录呢？就在该专栏第27章，我们看到 etcd operator 在生成 etcd 的启动命令时，使用了 pod 的 DNS 记录而不是 IP，这说明答案是肯定的—— pod 是可以有自己的 DNS 记录的。

我们来部署一个官方文档中提到的 nginx deployment，其定义如下：

apiVersion: apps/v1 # for versions before 1.9.0 use apps/v1beta2 kind: Deployment metadata: name: nginx-deployment spec: selector: matchLabels: app: nginx replicas: 2 # tells deployment to run 2 pods matching the template template: metadata: labels: app: nginx spec: containers: - name: nginx image: nginx:1.7.9 ports: - containerPort: 80 

部署后创建了两个 pod：

$ kubectl apply -f nginx-deployment.yaml $ kubectl get pod -o wide NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE nginx-deployment-67594d6bf6-brfcw 1/1 Running 0 18m 10.32.0.6 k8s-0 <none> nginx-deployment-67594d6bf6-nnxg5 1/1 Running 0 18m 10.44.0.6 k8s-1 <none> 

然后定义如下的 headless service:

apiVersion: v1 kind: Service metadata: name: nginx spec: clusterIP: None ports: - name: http port: 80 protocol: TCP targetPort: 9330 selector: app: nginx type: ClusterIP 

创建该 service，并尝试解析 service DNS：

$ kubectl apply -f nginx-service.yaml service/nginx created $ kubectl get svc NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE kubernetes ClusterIP 10.96.0.1 <none> 443/TCP 47m nginx ClusterIP None <none> 80/TCP 21s $ dig @10.96.0.10 nginx.default.svc.cluster.local ; <<>> DiG 9.11.3-1ubuntu1.5-Ubuntu <<>> @10.96.0.10 nginx.default.svc.cluster.local ; (1 server found) ;; global options: +cmd ;; Got answer: ;; WARNING: .local is reserved for Multicast DNS ;; You are currently testing what happens when an mDNS query is leaked to DNS ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 13949 ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 4096 ; COOKIE: 095945e3dc8b19b8 (echoed) ;; QUESTION SECTION: ;nginx.default.svc.cluster.local. IN A ;; ANSWER SECTION: nginx.default.svc.cluster.local. 5 IN A 10.32.0.6 nginx.default.svc.cluster.local. 5 IN A 10.44.0.6 ;; Query time: 0 msec ;; SERVER: 10.96.0.10#53(10.96.0.10) ;; WHEN: Fri Apr 19 14:50:13 CST 2019 ;; MSG SIZE rcvd: 166 

跟预期的一样，kube-dns 为 service 的名字返回了多条 A 记录，每一条对应一个 pod。上面 dig 命令中使用的 10.96.0.10 就是 kube-dns 的 cluster IP，可以在 kube-system namespace 中查看：

$ kubectl -n kube-system get svc NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE kube-dns ClusterIP 10.96.0.10 <none> 53/UDP,53/TCP 52m 

现在试试在 service 名字前面加上 pod 名字交给 kube-dns 做解析：

$ dig @10.96.0.10 nginx-deployment-67594d6bf6-brfcw.nginx.default.svc.cluster.local ; <<>> DiG 9.11.3-1ubuntu1.5-Ubuntu <<>> @10.96.0.10 nginx-deployment-67594d6bf6-brfcw.nginx.default.svc.cluster.local ; (1 server found) ;; global options: +cmd ;; Got answer: ;; WARNING: .local is reserved for Multicast DNS ;; You are currently testing what happens when an mDNS query is leaked to DNS ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 10513 ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 4096 ; COOKIE: 2ddad0cd6f72dfd7 (echoed) ;; QUESTION SECTION: ;nginx-deployment-67594d6bf6-brfcw.nginx.default.svc.cluster.local. IN A ;; AUTHORITY SECTION: cluster.local. 30 IN SOA ns.dns.cluster.local. hostmaster.cluster.local. 1555656528 7200 1800 86400 30 ;; Query time: 0 msec ;; SERVER: 10.96.0.10#53(10.96.0.10) ;; WHEN: Fri Apr 19 14:51:01 CST 2019 ;; MSG SIZE rcvd: 199 

无法解析。官方文档中有一段 Pod’s hostname and subdomain fields 说：

The Pod spec also has an optional subdomain field which can be used to specify its subdomain. For example, a Pod with hostname set to “foo”, and subdomain set to “bar”, in namespace “my-namespace”, will have the fully qualified domain name (FQDN) “foo.bar.my-namespace.svc.cluster.local”.

If there exists a headless service in the same namespace as the pod and with the same name as the subdomain, the cluster’s KubeDNS Server also returns an A record for the Pod’s fully qualified hostname.

编辑一下 nginx-deployment.yaml 加上 subdomain：

apiVersion: apps/v1 # for versions before 1.9.0 use apps/v1beta2 kind: Deployment metadata: name: nginx-deployment spec: selector: matchLabels: app: nginx replicas: 2 # tells deployment to run 2 pods matching the template template: metadata: labels: app: nginx spec: subdomain: nginx containers: - name: nginx image: nginx:1.7.9 ports: - containerPort: 80 

更新部署再尝试解析 pod DNS （注意现在两个 pod 都是重新创建的，名字已变）：

$ kubectl apply -f nginx-deployment.yaml $ kubectl get pod -o wide NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE nginx-deployment-59d854d9c8-nbvdm 1/1 Running 0 11s 10.32.0.8 k8s-0 <none> nginx-deployment-59d854d9c8-thvkd 1/1 Running 0 13s 10.44.0.7 k8s-1 <none> $ dig @10.96.0.10 nginx-deployment-59d854d9c8-thvkd.nginx.default.svc.cluster.local ; <<>> DiG 9.11.3-1ubuntu1.5-Ubuntu <<>> @10.96.0.10 nginx-deployment-59d854d9c8-thvkd.nginx.default.svc.cluster.local ; (1 server found) ;; global options: +cmd ;; Got answer: ;; WARNING: .local is reserved for Multicast DNS ;; You are currently testing what happens when an mDNS query is leaked to DNS ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 4952 ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 4096 ; COOKIE: 0e2b8426f0e8fe33 (echoed) ;; QUESTION SECTION: ;nginx-deployment-59d854d9c8-thvkd.nginx.default.svc.cluster.local. IN A ;; AUTHORITY SECTION: cluster.local. 30 IN SOA ns.dns.cluster.local. hostmaster.cluster.local. 1555658111 7200 1800 86400 30 ;; Query time: 0 msec ;; SERVER: 10.96.0.10#53(10.96.0.10) ;; WHEN: Fri Apr 19 15:18:14 CST 2019 ;; MSG SIZE rcvd: 199 

还是不行！

那就试试官方文档中的例子 （还是之前链接中 Pod’s hostname and subdomain fields 那一段），不用 Deployment 直接创建 pod 吧。第一步先试试把 hostname 和 subdomain 注释掉：

apiVersion: v1 kind: Service metadata: name: default-subdomain spec: selector: name: busybox clusterIP: None ports: - name: foo # Actually, no port is needed. port: 1234 targetPort: 1234 --- apiVersion: v1 kind: Pod metadata: name: busybox1 labels: name: busybox spec: hostname: busybox-1 subdomain: default-subdomain containers: - image: busybox:1.28 command: - sleep - "3600" name: busybox --- apiVersion: v1 kind: Pod metadata: name: busybox2 labels: name: busybox spec: hostname: busybox-2 subdomain: default-subdomain containers: - image: busybox:1.28 command: - sleep - "3600" name: busybox 

部署然后尝试解析 pod DNS (注意这里 hostname 和 pod 的名字有区别，中间多了减号)：

$ kubectl apply -f individual-pods-example.yaml $ $ dig @10.96.0.10 busybox-1.default-subdomain.default.svc.cluster.local ; <<>> DiG 9.11.3-1ubuntu1.5-Ubuntu <<>> @10.96.0.10 busybox-1.default-subdomain.default.svc.cluster.local ; (1 server found) ;; global options: +cmd ;; Got answer: ;; WARNING: .local is reserved for Multicast DNS ;; You are currently testing what happens when an mDNS query is leaked to DNS ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 12636 ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 4096 ; COOKIE: 5499ded915cf1ff2 (echoed) ;; QUESTION SECTION: ;busybox-1.default-subdomain.default.svc.cluster.local. IN A ;; ANSWER SECTION: busybox-1.default-subdomain.default.svc.cluster.local. 5 IN A 10.44.0.6 ;; Query time: 0 msec ;; SERVER: 10.96.0.10#53(10.96.0.10) ;; WHEN: Fri Apr 19 15:27:38 CST 2019 ;; MSG SIZE rcvd: 163 

终于可以了。我实际测试中发现 hostname 和 subdomain 二者都必须显式指定，缺一不可。我修改了一下之前的 nginx deployment 加上 hostname，果然可以解析了：

$ dig @10.96.0.10 nginx.nginx.default.svc.cluster.local ; <<>> DiG 9.11.3-1ubuntu1.5-Ubuntu <<>> @10.96.0.10 nginx.nginx.default.svc.cluster.local ; (1 server found) ;; global options: +cmd ;; Got answer: ;; WARNING: .local is reserved for Multicast DNS ;; You are currently testing what happens when an mDNS query is leaked to DNS ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 16903 ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 4096 ; COOKIE: 1a664224b34c5f87 (echoed) ;; QUESTION SECTION: ;nginx.nginx.default.svc.cluster.local. IN A ;; ANSWER SECTION: nginx.nginx.default.svc.cluster.local. 5 IN A 10.32.0.9 nginx.nginx.default.svc.cluster.local. 5 IN A 10.44.0.8 ;; Query time: 2 msec ;; SERVER: 10.96.0.10#53(10.96.0.10) ;; WHEN: Fri Apr 19 15:41:25 CST 2019 ;; MSG SIZE rcvd: 184 

果然可以解析了。但是因为 deployment 中无法给每个 pod 指定不同的 hostname，所以两个 pod 有同样的 hostname，解析出来两个 IP，跟我们的本意就不符合了。

从这个角度也可以看出，etcd operator 是直接管理 pod，而不是通过 deployment。这可以理解，在这种场景中，没必要也不应该通过 deployment 去管理 pod。可以从源代码中看见 etcd operator 给 pod 设置 hostname 和 subdomain。在 changelog 里，Release 0.2.5 也记录了相应的变动。

其实社区也有人呼吁给让 deployment 创建的 pod 也支持 DNS 解析，但是呼声不够高，没有被采纳—— Make pod hostname/subdomain feature compatible with Deployments.