[TOC]

Overview

TCR Hosts Updater, a component of TCR Addon, helps you deploy the hosts file on K8s cluster worker nodes in regions without VPC DNS service.

How It Works

Hosts Updater mounts a specific ConfigMaps in the K8s cluster as the Volume of a worker node, watches the changes of the config file via inotify (inode notify) of Linux OS, and updates the /etc/hosts file of the worker node accordingly.
As we need to edit the /etc/hosts file of the worker node, Daemonset workload is usually used for deployment, and it should be run on each node that you need to update hosts. To allow the DaemonSet workload runs on nodes in the cluster as many as possible, the following tolerations are set in the YAML file of the DaemonSet:

   tolerations:
     - key: node-role.kubernetes.io/master
       effect: NoSchedule
     - key: node.kubernetes.io/disk-pressure
       effect: NoSchedule
     - key: node.kubernetes.io/memory-pressure
       effect: NoSchedule
     - key: node.kubernetes.io/network-unavailable
       effect: NoSchedule
     - key: node.kubernetes.io/not-ready
       effect: NoExecute
     - key: node.kubernetes.io/pid-pressure
       effect: NoSchedule
     - key: node.kubernetes.io/unreachable
       effect: NoExecute
     - key: node.kubernetes.io/unschedulable
       effect: NoSchedule

Deployment and Usage

Before the deployment, please create a ConfigMaps resource named updater-config under Namespace of Host Updater (e.g. kube-system), and then create DaemonSet.

To add or delete entries in hosts, simply edit the updater-config. See below for examples:

apiVersion: v1
kind: ConfigMap
metadata:
name: updater-config
namespace: kube-system
data:
hosts.yaml: |
 - domain: demo.tencentcloudcr.com
   ip: 10.0.0.2
   disabled: false
 - domain: vpc-demo.tencentcloudcr.com
   ip: 10.0.0.2
   disabled: false

Note: As K8s ConfigMaps Volume is used, after the modification of updater-config, the effective time of the hosts update depends on the sync period of worker node kubelet (default to 1 min) and the Cache TTL of ConfigMaps (default to 1 min). For details, see https://kubernetes.io/docs/tasks/configure-pod-container/configure-pod-configmap/#mounted-configmaps-are-updated-automatically.