The official Helm repo https://charts.helm.sh/stable is unreachable from some machines in China, so you need to configure a mirror.

1. Official Repo

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helm repo add stable https://charts.helm.sh/stable

2. Git Pages Mirror

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helm repo add stable https://burdenbear.github.io/kube-charts-mirror/

See kube-charts-mirror for a reference on building a self-hosted, self-controlled mirror.

3. Aliyun Mirror

Not updated for a long time; versions are old

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helm repo add stable  https://kubernetes.oss-cn-hangzhou.aliyuncs.com/charts/

4. Azure Mirror

No longer available, updated 2021.07.08.

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helm repo add stable http://mirror.azure.cn/kubernetes/charts/
helm repo add incubator http://mirror.azure.cn/kubernetes/charts-incubator/