Which kubernetes version is supported in docker version 18.09

As mentioned above, the message is more of a warning that not all features were tested against that specific Docker release. On your own risk, you can try to temporarily ignore those errors by using the --ignore-preflight-errors flag. Eg : kubedam init --ignore-preflight-errors all

In Kubernetes there is nothing like supported. Instead of it they use validated - it means that all features were tested and validated with some Docker version.

And validated Docker versions are still the same from Kubernetes version 1.8 until 1.11: Docker 1.11.2 to 1.13.1 and 17.03.x. See here: https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG-1.8.md#external-dependencies and here https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG-1.11.md#external-dependencies

Starting from Kubernetes version 1.12 Docker 17.06, 17.09 and 18.06 started to be also validated. See here: https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG-1.12.md#external-dependencies

As I know final version of Docker 18.09 was released 4 days ago so here we cannot expect this version to be validated in Kubernetes now.

Update (9.4.2019): Docker 18.09 is validated against newly released Kubernetes 1.14: https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG-1.14.md#external-dependencies

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You can specify exact Docker version for the get.docker.com script by VERSION variable:

export VERSION=18.03 && curl -sSL get.docker.com | sh

Here's how I "fixed" mine:

$ sudo kubeadm init --ignore-preflight-errors=SystemVerification
...
        [WARNING SystemVerification]: unsupported docker version: 18.09.0

Here's what I saw when "preflight errors" were not ignored:

$ sudo kubeadm init
...
[preflight] Some fatal errors occurred:
        [ERROR SystemVerification]: unsupported docker version: 18.09.0
[preflight] If you know what you are doing, you can make a check non-fatal with `--ignore-preflight-errors=...`

Here's the version of kubeadm I used:

$ kubeadm version --output json
{
  "clientVersion": {
    "major": "1",
    "minor": "12",
    "gitVersion": "v1.12.2",
    "gitCommit": "17c77c7898218073f14c8d573582e8d2313dc740",
    "gitTreeState": "clean",
    "buildDate": "2018-10-24T06:51:33Z",
    "goVersion": "go1.10.4",
    "compiler": "gc",
    "platform": "linux/amd64"
  }
}