Kubernetes 里，怎么让 Pod 有 DNS 记录？

很容易让一个 StatefulSet 中的 Pod 拥有 DNS 记录。如果一个 StatefulSet 的名字是 memcached, 而它指定了关联的 serviceName 叫 memcached-cluster，那 kube-dns 就会为它的每个 pod 解析如下的 DNS A 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.