本文有参考：阿里云的文档：https://developer.aliyun.com/article/721569。然后参考对照实际部署的k8s集群。梳理部署在k8s的不同应用如何在公网被不同的域名访问到。

理解：
实际nginx-ingress-controller 就是在k8s集群中创建一个具有类似nginx作用的应用。此nginx 应用负责接受所有来自不同域名的请求，然后根据不同ingress 定义的规则转发到对应的service。

---

1. 看一下nginx-ingress service

apiVersion: v1
kind: Service
metadata:
  annotations:
    kubectl.kubernetes.io/last-applied-configuration: >
      {"apiVersion":"v1","kind":"Service","metadata":{"annotations":{},"labels":{"app":"nginx-ingress-lb"},"name":"nginx-ingress-lb","namespace":"kube-system"},"spec":{"ports":[{"name":"http","port":80,"targetPort":80},{"name":"https","port":443,"targetPort":443}],"selector":{"app":"ingress-nginx"},"type":"LoadBalancer"}}
  creationTimestamp: '2019-06-04T11:49:59Z'
  labels:
    app: nginx-ingress-lb
  name: nginx-ingress-lb
  namespace: kube-system
  resourceVersion: '147529875'
  selfLink: /api/v1/namespaces/kube-system/services/nginx-ingress-lb
  uid: 6106e164-7f80-11e8-b128-00163e10ac41
spec:
  clusterIP: 10.1.1.135
  externalTrafficPolicy: Local
  healthCheckNodePort: 31999
  ports:
    - name: http
      nodePort: 30613
      port: 80
      protocol: TCP
      targetPort: 80
    - name: https
      nodePort: 31070
      port: 443
      protocol: TCP
      targetPort: 443
  selector:
    app: ingress-nginx
  sessionAffinity: None
  type: LoadBalancer
status:
  loadBalancer:
    ingress:
      - ip: xx.xx.xxx.xxx 

解释： 该service选择暴露服务的type 为LoadBalancer，这样就可以被外网访问到。LoadBalancer的实际ip地址为：xx.xx.xxx.xxx。假设集群中有A 、B两个应用，分别需要使用 a.com、b.com 访问。那么将这两个域名的的A记录的解析值都设置为：LoadBalancer的实际ip地址为：xx.xx.xxx.xxx。

---

2. 上文1中只是创建了一个service，指定了service对应的应用。在运行的集群中使用命令可以看到对应的deploy。

kubectl get  deploy  -n kube-system -l app=ingress-nginx
NAME                       READY   UP-TO-DATE   AVAILABLE   AGE
nginx-ingress-controller   1/1     1            1           2y24d

如下是deploy的yaml：

apiVersion: apps/v1
kind: Deployment
metadata:
  annotations:
    component.revision: v5
    component.version: v0.22.0
    deployment.kubernetes.io/revision: '2'
    kubectl.kubernetes.io/last-applied-configuration: >
      {"apiVersion":"apps/v1","kind":"Deployment","metadata":{"annotations":{"component.revision":"v5","component.version":"v0.22.0"},"labels":{"app":"ingress-nginx"},"name":"nginx-ingress-controller","namespace":"kube-system"},"spec":{"replicas":1,"selector":{"matchLabels":{"app":"ingress-nginx"}},"template":{"metadata":{"annotations":{"prometheus.io/port":"10254","prometheus.io/scrape":"true","scheduler.alpha.kubernetes.io/critical-pod":""},"labels":{"app":"ingress-nginx"}},"spec":{"affinity":{"podAntiAffinity":{"preferredDuringSchedulingIgnoredDuringExecution":[{"podAffinityTerm":{"labelSelector":{"matchExpressions":[{"key":"app","operator":"In","values":["ingress-nginx"]}]},"topologyKey":"kubernetes.io/hostname"},"weight":100}]}},"containers":[{"args":["/nginx-ingress-controller","--configmap=$(POD_NAMESPACE)/nginx-configuration","--tcp-services-configmap=$(POD_NAMESPACE)/tcp-services","--udp-services-configmap=$(POD_NAMESPACE)/udp-services","--annotations-prefix=nginx.ingress.kubernetes.io","--publish-service=$(POD_NAMESPACE)/nginx-ingress-lb","--v=2"],"env":[{"name":"POD_NAME","valueFrom":{"fieldRef":{"fieldPath":"metadata.name"}}},{"name":"POD_NAMESPACE","valueFrom":{"fieldRef":{"fieldPath":"metadata.namespace"}}}],"image":"registry-vpc.cn-beijing.aliyuncs.com/acs/aliyun-ingress-controller:v0.22.0.5-552e0db-aliyun","livenessProbe":{"failureThreshold":3,"httpGet":{"path":"/healthz","port":10254,"scheme":"HTTP"},"initialDelaySeconds":10,"periodSeconds":10,"successThreshold":1,"timeoutSeconds":10},"name":"nginx-ingress-controller","ports":[{"containerPort":80,"name":"http"},{"containerPort":443,"name":"https"}],"readinessProbe":{"failureThreshold":3,"httpGet":{"path":"/healthz","port":10254,"scheme":"HTTP"},"periodSeconds":10,"successThreshold":1,"timeoutSeconds":10},"resources":{"requests":{"cpu":"100m","memory":"70Mi"}},"securityContext":{"capabilities":{"add":["NET_BIND_SERVICE"],"drop":["ALL"]},"runAsUser":33},"volumeMounts":[{"mountPath":"/etc/localtime","name":"localtime","readOnly":true}]}],"initContainers":[{"command":["/bin/sh","-c","mount
      -o remount rw /proc/sys\nsysctl -w net.core.somaxconn=65535\nsysctl -w
      net.ipv4.ip_local_port_range=\"1024 65535\"\nsysctl -w
      fs.file-max=1048576\nsysctl -w fs.inotify.max_user_instances=16384\nsysctl
      -w fs.inotify.max_user_watches=524288\nsysctl -w
      fs.inotify.max_queued_events=16384\n"],"image":"registry-vpc.cn-beijing.aliyuncs.com/acs/busybox:v1.29.2","name":"init-sysctl","securityContext":{"capabilities":{"add":["SYS_ADMIN"],"drop":["ALL"]}}}],"nodeSelector":{"beta.kubernetes.io/os":"linux"},"serviceAccountName":"nginx-ingress-controller","volumes":[{"hostPath":{"path":"/etc/localtime","type":"File"},"name":"localtime"}]}}}}
  creationTimestamp: '2018-07-04T11:49:59Z'
  generation: 2
  labels:
    app: ingress-nginx
  name: nginx-ingress-controller
  namespace: kube-system
  resourceVersion: '169291104'
  selfLink: /apis/apps/v1/namespaces/kube-system/deployments/nginx-ingress-controller
  uid: 610c6dac-7f80-11e8-b128-00163e10ac41
spec:
  progressDeadlineSeconds: 600
  replicas: 1
  revisionHistoryLimit: 10
  selector:
    matchLabels:
      app: ingress-nginx
  strategy:
    rollingUpdate:
      maxSurge: 1
      maxUnavailable: 1
    type: RollingUpdate
  template:
    metadata:
      annotations:
        prometheus.io/port: '10254'
        prometheus.io/scrape: 'true'
        scheduler.alpha.kubernetes.io/critical-pod: ''
      labels:
        app: ingress-nginx
    spec:
      affinity:
        podAntiAffinity:
          preferredDuringSchedulingIgnoredDuringExecution:
            - podAffinityTerm:
                labelSelector:
                  matchExpressions:
                    - key: app
                      operator: In
                      values:
                        - ingress-nginx
                topologyKey: kubernetes.io/hostname
              weight: 100
      containers:
        - args:
            - /nginx-ingress-controller
            - '--configmap=$(POD_NAMESPACE)/nginx-configuration'
            - '--tcp-services-configmap=$(POD_NAMESPACE)/tcp-services'
            - '--udp-services-configmap=$(POD_NAMESPACE)/udp-services'
            - '--annotations-prefix=nginx.ingress.kubernetes.io'
            - '--publish-service=$(POD_NAMESPACE)/nginx-ingress-lb'
            - '--v=2'
          env:
            - name: POD_NAME
              valueFrom:
                fieldRef:
                  apiVersion: v1
                  fieldPath: metadata.name
            - name: POD_NAMESPACE
              valueFrom:
                fieldRef:
                  apiVersion: v1
                  fieldPath: metadata.namespace
          image: >-
            registry-vpc.cn-beijing.aliyuncs.com/acs/aliyun-ingress-controller:v0.22.0.5-552e0db-aliyun
          imagePullPolicy: IfNotPresent
          livenessProbe:
            failureThreshold: 3
            httpGet:
              path: /healthz
              port: 10254
              scheme: HTTP
            initialDelaySeconds: 10
            periodSeconds: 10
            successThreshold: 1
            timeoutSeconds: 10
          name: nginx-ingress-controller
          ports:
            - containerPort: 80
              name: http
              protocol: TCP
            - containerPort: 443
              name: https
              protocol: TCP
          readinessProbe:
            failureThreshold: 3
            httpGet:
              path: /healthz
              port: 10254
              scheme: HTTP
            periodSeconds: 10
            successThreshold: 1
            timeoutSeconds: 10
          resources:
            requests:
              cpu: 100m
              memory: 70Mi
          securityContext:
            capabilities:
              add:
                - NET_BIND_SERVICE
              drop:
                - ALL
            runAsUser: 33
          terminationMessagePath: /dev/termination-log
          terminationMessagePolicy: File
          volumeMounts:
            - mountPath: /etc/localtime
              name: localtime
              readOnly: true
      dnsPolicy: ClusterFirst
      initContainers:
        - command:
            - /bin/sh
            - '-c'
            - |
              mount -o remount rw /proc/sys
              sysctl -w net.core.somaxconn=65535
              sysctl -w net.ipv4.ip_local_port_range="1024 65535"
              sysctl -w fs.file-max=1048576
              sysctl -w fs.inotify.max_user_instances=16384
              sysctl -w fs.inotify.max_user_watches=524288
              sysctl -w fs.inotify.max_queued_events=16384
          image: 'registry-vpc.cn-beijing.aliyuncs.com/acs/busybox:v1.29.2'
          imagePullPolicy: IfNotPresent
          name: init-sysctl
          resources: {}
          securityContext:
            capabilities:
              add:
                - SYS_ADMIN
              drop:
                - ALL
          terminationMessagePath: /dev/termination-log
          terminationMessagePolicy: File
      nodeSelector:
        beta.kubernetes.io/os: linux
      restartPolicy: Always
      schedulerName: default-scheduler
      securityContext: {}
      serviceAccount: nginx-ingress-controller
      serviceAccountName: nginx-ingress-controller
      terminationGracePeriodSeconds: 30
      volumes:
        - hostPath:
            path: /etc/localtime
            type: File
          name: localtime
status:
  availableReplicas: 1
  conditions:
    - lastTransitionTime: '2018-07-04T11:51:09Z'
      lastUpdateTime: '2018-07-04T11:51:09Z'
      message: Deployment has minimum availability.
      reason: MinimumReplicasAvailable
      status: 'True'
      type: Available
    - lastTransitionTime: '2018-07-04T11:51:09Z'
      lastUpdateTime: '2020-04-01T22:02:46Z'
      message: >-
        ReplicaSet "nginx-ingress-controller-5fc8b968d6" has successfully
        progressed.
      reason: NewReplicaSetAvailable
      status: 'True'
      type: Progressing
  observedGeneration: 2
  readyReplicas: 1
  replicas: 1
  updatedReplicas: 1

---

3. 继续使用命令可以看到deploy对应的pod

 kubectl get  pod  -n kube-system -l app=ingress-nginx
NAME                                        READY   STATUS    RESTARTS   AGE
nginx-ingress-controller-5fc8b968d6-s8gmc   1/1     Running   1          78d

使用命令进入到pod中查看/etc/nginx/nginx.conf ，我们可以看到在ingress配置的域名在配置nginx.conf中出现。看到这里就应该明白了nginx-ingress-controller 实际类似nginx 的意思。

---

4. 查看一个ingress的yaml

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  annotations:
    kubectl.kubernetes.io/last-applied-configuration: >
      {"apiVersion":"extensions/v1beta1","kind":"Ingress","metadata":{"annotations":{},"name":"ssyl-admin-ingress","namespace":"default"},"spec":{"rules":[{"host":"admin.com","http":{"paths":[{"backend":{"serviceName":"service-admin","servicePort":80},"path":"/"}]}}],"tls":[{"hosts":["admin.com"],"secretName":"admin.com"}]}}
    nginx.ingress.kubernetes.io/service-weight: ''
  creationTimestamp: '2019-09-25T07:01:52Z'
  generation: 3
  name: admin-ingress
  namespace: default
  resourceVersion: '169280718'
  selfLink: /apis/extensions/v1beta1/namespaces/default/ingresses/admin-ingress
  uid: e15d1ea1-c090-11e8-b6f2-00163e109a9e
spec:
  rules:
    - host: admin.com
      http:
        paths:
          - backend:
              serviceName: service-admin
              servicePort: 80
            path: /
  tls:
    - hosts:
        - admin.com
      secretName: admin.com
status:
  loadBalancer:
    ingress:
      - ip: xx.xx.xxx.xxx

解释：该ingress的作用就是将域名为admin.com的请求转发到名称为service-admin的服务上。域名admin.com 可以在3中的配置nginx.conf上看到。

---

好了，到这里就梳理完毕。在总结一下吧：不同域名解析到同一个LoadBalancer的 IP，假设为ip-a；然后nginx-ingress-controller 的service 的type 为LoadBalancer，ip为前面说的ip-a。这样不同域名的请求就被转发到了nginx-ingress-controller 的service。而nginx-ingress-controller 的service所对应的pod会根据 ingress定义的规则，再将不同域名的请求转发到不同的service上。