How to access kind control plane port from another docker container?

Question

I'm creating a kind cluster with kind create cluster --name kind and I want to access it from another docker container but when I try to apply a Kubernetes file from a container (kubectl apply -f deployment.yml) I got this error:

The connection to the server 127.0.0.1:6445 was refused - did you specify the right host or port?

Indeed when I try to curl kind control-plane from a container, it's unreachable.

> docker run --entrypoint curl curlimages/curl:latest 127.0.0.1:6445
curl: (7) Failed to connect to 127.0.0.1 port 6445 after 0 ms: Connection refused

However kind control-plane is publishing to the right port but only to the localhost.

> docker ps --format "table {{.Image}}\t{{.Ports}}"
IMAGE                  PORTS
kindest/node:v1.23.4   127.0.0.1:6445->6443/tcp

Currently the only solution I found is to set the host network mode.

> docker run --network host --entrypoint curl curlimages/curl:latest 127.0.0.1:6445
Client sent an HTTP request to an HTTPS server.

This solution don't look to be the most secure. Is there another way like connecting the kind network to my container or something like that that I missed ?

Answer 1

I don't know exactly why you want to do this. but no problem I think this could help you:

first, lets pull your docker image:

? docker pull curlimages/curl

In my kind cluster I got 3 control plane nodes and 3 worker nodes. Here are the pod of my kind cluster:

? docker ps
CONTAINER ID   IMAGE                                COMMAND                  CREATED          STATUS          PORTS                       NAMES
39dbbb8ca320   kindest/node:v1.23.5                 "/usr/local/bin/entr…"   7 days ago       Up 7 days       127.0.0.1:35327->6443/tcp   so-cluster-1-control-plane
62b5538275e9   kindest/haproxy:v20220207-ca68f7d4   "haproxy -sf 7 -W -d…"   7 days ago       Up 7 days       127.0.0.1:35625->6443/tcp   so-cluster-1-external-load-balancer
9f189a1b6c52   kindest/node:v1.23.5                 "/usr/local/bin/entr…"   7 days ago       Up 7 days       127.0.0.1:40845->6443/tcp   so-cluster-1-control-plane3
4c53f745a6ce   kindest/node:v1.23.5                 "/usr/local/bin/entr…"   7 days ago       Up 7 days       127.0.0.1:36153->6443/tcp   so-cluster-1-control-plane2
97e5613d2080   kindest/node:v1.23.5                 "/usr/local/bin/entr…"   7 days ago       Up 7 days       0.0.0.0:30081->30080/tcp    so-cluster-1-worker2
0ca64a907707   kindest/node:v1.23.5                 "/usr/local/bin/entr…"   7 days ago       Up 7 days       0.0.0.0:30080->30080/tcp    so-cluster-1-worker
9c5d26caee86   kindest/node:v1.23.5                 "/usr/local/bin/entr…"   7 days ago       Up 7 days       0.0.0.0:30082->30080/tcp    so-cluster-1-worker3

The container that is interesting for us here is the haproxy one (kindest/haproxy:v20220207-ca68f7d4) which have the role of loadbalancing the enterring traffic to the nodes (and, in our example, especially the control plane nodes.) we can see that the port 35625 of our host machine is mapped to the port 6443 of the haproxy container. (127.0.0.1:35625->6443/tcp)

so, our cluster endpoint is https://127.0.0.1:35625, we can confirm this in our kubeconfig file (~/.kube/config):

? cat .kube/config
apiVersion: v1
kind: Config
preferences: {}
users:
  - name: kind-so-cluster-1
    user:
        client-certificate-data: <base64data>
        client-key-data: <base64data>
clusters:
  - cluster:
        certificate-authority-data: <certificate-authority-dataBase64data>
        server: https://127.0.0.1:35625
    name: kind-so-cluster-1
contexts:
  - context:
        cluster: kind-so-cluster-1
        user: kind-so-cluster-1
        namespace: so-tests
    name: kind-so-cluster-1
current-context: kind-so-cluster-1

let's run the curl container in background:

? docker run -d --network host curlimages/curl sleep 3600
ba183fe2bb8d715ed1e503a9fe8096dba377f7482635eb12ce1322776b7e2366

as expected, we cant HTTP request the endpoint that listen on an HTTPS port:

?  docker exec -it ba curl 127.0.0.1:35625
Client sent an HTTP request to an HTTPS server.

we can try to use the certificate that is in the field "certificate-authority-data" in our kubeconfig to check if that change something (it should): Lets create a file named my-ca.crt that contain the stringData of the certificate:

base64 -d <<<  <certificate-authority-dataBase64dataFromKubeConfig> > my-ca.crt 

since the working directory of the curl docker image is "/" lets copy our cert to this location in the container and verify that it is actually there:

docker cp my-ca.crt ba183fe:/

? docker exec -it ba sh
/ $ ls my-ca.crt
my-ca.crt

Let's try again our curl request but with the certificate:

? docker exec -it ba curl --cacert my-ca.crt https://127.0.0.1:35625
{
  "kind": "Status",
  "apiVersion": "v1",
  "metadata": {},
  "status": "Failure",
  "message": "forbidden: User \"system:anonymous\" cannot get path \"/\"",
  "reason": "Forbidden",
  "details": {},
  "code": 403
}

YOU, can get the same result by adding the "--insecure" flag to your curl request:

?  docker exec -it ba curl https://127.0.0.1:35625 --insecure
{
  "kind": "Status",
  "apiVersion": "v1",
  "metadata": {},
  "status": "Failure",
  "message": "forbidden: User \"system:anonymous\" cannot get path \"/\"",
  "reason": "Forbidden",
  "details": {},
  "code": 403
}

However, we can't access our cluster with anonymous user ! So lets get a token from kubernetes (cf https://kubernetes.io/docs/tasks/administer-cluster/access-cluster-api/):

# Create a secret to hold a token for the default service account
kubectl apply -f - <<EOF
apiVersion: v1
kind: Secret
metadata:
  name: default-token
  annotations:
    kubernetes.io/service-account.name: default
type: kubernetes.io/service-account-token
EOF

Once the token controller has populated the secret with a token:

# Get the token value
? kubectl get secret default-token -o jsonpath='{.data.token}' | base64 --decode
eyJhbGciOiJSUzI1NiIsImtpZCI6InFSTThZZ05lWHFXMWExQlVSb1hTcHNxQ3F6Z2Z2aWpUaUYwd2F2TGdVZ0EifQ.eyJpc3MiOiJrdWJlcm5ldGVzL3NlcnZpY2VhY2NvdW50Iiwia3ViZXJuZXRlcy5pby9zZXJ2aWNlYWNjb3VudC9uYW1lc3BhY2UiOiJzby10ZXN0cyIsImt1YmVybmV0ZXMuaW8vc2VydmljZWFjY291bnQvc2VjcmV0Lm5hbWUiOiJkZWZhdWx0LXRva2VuIiwia3ViZXJuZXRlcy5pby9zZXJ2aWNlYWNjb3VudC9zZXJ2aWNlLWFjY291bnQubmFtZSI6ImRlZmF1bHQiLCJrdWJlcm5ldGVzLmlvL3NlcnZpY2VhY2NvdW50L3NlcnZpY2UtYWNjb3VudC51aWQiOiIzYzY0OTg1OS0xNzkyLTQzYTQtOGJjOC0zMDEzZDgxNjRmY2IiLCJzdWIiOiJzeXN0ZW06c2VydmljZWFjY291bnQ6c28tdGVzdHM6ZGVmYXVsdCJ9.VLfjuym0fohYTT_uoLPwM0A6u7dUt2ciWZF2K9LM_YvQ0UZT4VgkM8UBVOQpWjTmf9s2B5ZxaOkPu4cz_B4xyDLiiCgqiHCbUbjxE9mphtXGKQwAeKLvBlhbjYnHb9fCTRW19mL7VhqRgfz5qC_Tae7ysD3uf91FvqjjxsCyzqSKlsq0T7zXnzQ_YQYoUplGa79-LS_xDwG-2YFXe0RfS9hkpCILpGDqhLXci_gwP9DW0a6FM-L1R732OdGnb9eCPI6ReuTXQz7naQ4RQxZSIiNd_S7Vt0AYEg-HGvSkWDl0_DYIyHShMeFHu1CtfTZS5xExoY4-_LJD8mi

Now lets execute the curl command directly with the token !

? docker exec -it ba curl -X GET https://127.0.0.1:35625/api --header "Authorization: Bearer eyJhbGciOiJSUzI1NiIsImtpZCI6InFSTThZZ05lWHFXMWExQlVSb1hTcHNxQ3F6Z2Z2aWpUaUYwd2F2TGdVZ0EifQ.eyJpc3MiOiJrdWJlcm5ldGVzL3NlcnZpY2VhY2NvdW50Iiwia3ViZXJuZXRlcy5pby9zZXJ2aWNlYWNjb3VudC9uYW1lc3BhY2UiOiJzby10ZXN0cyIsImt1YmVybmV0ZXMuaW8vc2VydmljZWFjY291bnQvc2VjcmV0Lm5hbWUiOiJkZWZhdWx0LXRva2VuIiwia3ViZXJuZXRlcy5pby9zZXJ2aWNlYWNjb3VudC9zZXJ2aWNlLWFjY291bnQubmFtZSI6ImRlZmF1bHQiLCJrdWJlcm5ldGVzLmlvL3NlcnZpY2VhY2NvdW50L3NlcnZpY2UtYWNjb3VudC51aWQiOiIzYzY0OTg1OS0xNzkyLTQzYTQtOGJjOC0zMDEzZDgxNjRmY2IiLCJzdWIiOiJzeXN0ZW06c2VydmljZWFjY291bnQ6c28tdGVzdHM6ZGVmYXVsdCJ9.VLfjuym0fohYTT_uoLPwM0A6u7dUt2ciWZF2K9LM_YvQ0UZT4VgkM8UBVOQpWjTmf9s2B5ZxaOkPu4cz_B4xyDLiiCgqiHCbUbjxE9mphtXGKQwAeKLvBlhbjYnHb9fCTRW19mL7VhqRgfz5qC_Tae7ysD3uf91FvqjjxsCyzqSKlsq0T7zXnzQ_YQYoUplGa79-LS_xDwG-2YFXe0RfS9hkpCILpGDqhLXci_gwP9DW0a6FM-L1R732OdGnb9eCPI6ReuTXQz7naQ4RQxZSIiNd_S7Vt0AYEg-HGvSkWDl0_DYIyHShMeFHu1CtfTZS5xExoY4-_LJD8mi" --insecure
{
  "kind": "APIVersions",
  "versions": [
    "v1"
  ],
  "serverAddressByClientCIDRs": [
    {
      "clientCIDR": "0.0.0.0/0",
      "serverAddress": "172.18.0.5:6443"
    }
  ]
}

It works ! I still don't know why you want to do this but I hope that this helped you.

Since It's not what you wanted because here I use host network, You can use this : How to communicate between Docker containers via "hostname" as proposed @SergioSantiago thanks for your comment !

bguess