主机网络（hostNetwork）

背景信息

Kubernetes支持Pod直接使用主机（节点）的网络，当Pod配置为hostNetwork: true时，在此Pod中运行的应用程序可以直接看到Pod所在主机的网络接口。

配置说明

Pod使用主机网络只需要在配置中添加hostNetwork: true即可，如下所示。

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx
spec:
  replicas: 1
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      hostNetwork: true
      containers:
      - image: nginx:alpine
        name: nginx
      imagePullSecrets:
      - name: default-secret

部署后可以看到Pod的IP与节点的IP相同，说明Pod直接使用了主机网络。

$ kubectl get pod -owide
NAME                    READY   STATUS    RESTARTS   AGE     IP          NODE        NOMINATED NODE   READINESS GATES
nginx-6fdf99c8b-6wwft   1/1     Running   0          3m41s   10.1.0.55   10.1.0.55   <none>           <none>

hostNetwork使用注意事项

Pod直接使用主机的网络会占用宿主机的端口，Pod的IP就是宿主机的IP，使用时需要考虑是否与主机上的端口冲突，因此一般情况下除非您知道某个特定应用需要占用宿主机上的特定端口，否则不建议使用主机网络。

由于Pod使用主机网络，访问Pod需要直接通过节点端口，因此要注意放通节点安全组端口，否则会出现访问不通的情况。

另外由于占用主机端口，使用Deployment部署hostNetwork类型Pod时，要注意Pod的副本数不要超过节点数量，否则会导致一个节点上调度了多个Pod，Pod启动时端口冲突无法创建。例如上面例子中的nginx，如果服务数为2，并部署在只有1个节点的集群上，就会有一个Pod无法创建，查询Pod日志会发现是由于端口占用导致nginx无法启动。

请避免在同一个节点上调度多个使用主机网络的Pod，否则在创建ClusterIP类型的Service访问Pod时，会出现访问ClusterIP不通的情况。

$ kubectl get deploy
NAME    READY   UP-TO-DATE   AVAILABLE   AGE
nginx   1/2     2            1           67m
$ kubectl get pod
NAME                    READY   STATUS             RESTARTS   AGE
nginx-6fdf99c8b-6wwft   1/1     Running            0          67m
nginx-6fdf99c8b-rglm7   0/1     CrashLoopBackOff   13         44m
$ kubectl logs nginx-6fdf99c8b-rglm7
/docker-entrypoint.sh: /docker-entrypoint.d/ is not empty, will attempt to perform configuration
/docker-entrypoint.sh: Looking for shell scripts in /docker-entrypoint.d/
/docker-entrypoint.sh: Launching /docker-entrypoint.d/10-listen-on-ipv6-by-default.sh
10-listen-on-ipv6-by-default.sh: info: Getting the checksum of /etc/nginx/conf.d/default.conf
10-listen-on-ipv6-by-default.sh: info: Enabled listen on IPv6 in /etc/nginx/conf.d/default.conf
/docker-entrypoint.sh: Launching /docker-entrypoint.d/20-envsubst-on-templates.sh
/docker-entrypoint.sh: Launching /docker-entrypoint.d/30-tune-worker-processes.sh
/docker-entrypoint.sh: Configuration complete; ready for start up
2022/05/11 07:18:11 [emerg] 1#1: bind() to 0.0.0.0:80 failed (98: Address in use)
nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address in use)
2022/05/11 07:18:11 [emerg] 1#1: bind() to [::]:80 failed (98: Address in use)
nginx: [emerg] bind() to [::]:80 failed (98: Address in use)
2022/05/11 07:18:11 [emerg] 1#1: bind() to 0.0.0.0:80 failed (98: Address in use)
nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address in use)
2022/05/11 07:18:11 [emerg] 1#1: bind() to [::]:80 failed (98: Address in use)
nginx: [emerg] bind() to [::]:80 failed (98: Address in use)
2022/05/11 07:18:11 [emerg] 1#1: bind() to 0.0.0.0:80 failed (98: Address in use)
nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address in use)
2022/05/11 07:18:11 [emerg] 1#1: bind() to [::]:80 failed (98: Address in use)
nginx: [emerg] bind() to [::]:80 failed (98: Address in use)
2022/05/11 07:18:11 [emerg] 1#1: bind() to 0.0.0.0:80 failed (98: Address in use)
nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address in use)
2022/05/11 07:18:11 [emerg] 1#1: bind() to [::]:80 failed (98: Address in use)
nginx: [emerg] bind() to [::]:80 failed (98: Address in use)
2022/05/11 07:18:11 [emerg] 1#1: bind() to 0.0.0.0:80 failed (98: Address in use)
nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address in use)
2022/05/11 07:18:11 [emerg] 1#1: bind() to [::]:80 failed (98: Address in use)
nginx: [emerg] bind() to [::]:80 failed (98: Address in use)
2022/05/11 07:18:11 [emerg] 1#1: still could not bind()
nginx: [emerg] still could not bind()