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Authenticate Across Clusters with kubeconfig

Authentication in Kubernetes can differ for different individuals.

A running kubelet might have one way of authenticating (i.e. certificates).
Users might have a different way of authenticating (i.e. tokens).
Administrators might have a list of certificates which they provide individual users.
There may be multiple clusters, and we may want to define them all in one place - giving users the ability to use their own certificates and reusing the same global configuration.

So in order to easily switch between multiple clusters, for multiple users, a kubeconfig file was defined.

This file contains a series of authentication mechanisms and cluster connection information associated with nicknames. It also introduces the concept of a tuple of authentication information (user) and cluster connection information called a context that is also associated with a nickname.

Multiple kubeconfig files are allowed, if specified explicitly. At runtime they are loaded and merged along with override options specified from the command line (see rules below).

http://issue.k8s.io/1755

Components of a kubeconfig file

Example kubeconfig file

current-context: federal-context
apiVersion: v1
clusters:
- cluster:
    api-version: v1
    server: http://cow.org:8080
  name: cow-cluster
- cluster:
    certificate-authority: path/to/my/cafile
    server: https://horse.org:4443
  name: horse-cluster
- cluster:
    insecure-skip-tls-verify: true
    server: https://pig.org:443
  name: pig-cluster
contexts:
- context:
    cluster: horse-cluster
    namespace: chisel-ns
    user: green-user
  name: federal-context
- context:
    cluster: pig-cluster
    namespace: saw-ns
    user: black-user
  name: queen-anne-context
kind: Config
preferences:
  colors: true
users:
- name: blue-user
  user:
    token: blue-token
- name: green-user
  user:
    client-certificate: path/to/my/client/cert
    client-key: path/to/my/client/key

Breakdown/explanation of components

cluster

clusters:
- cluster:
    certificate-authority: path/to/my/cafile
    server: https://horse.org:4443
  name: horse-cluster
- cluster:
    insecure-skip-tls-verify: true
    server: https://pig.org:443
  name: pig-cluster

A cluster contains endpoint data for a kubernetes cluster. This includes the fully qualified url for the kubernetes apiserver, as well as the cluster’s certificate authority or insecure-skip-tls-verify: true, if the cluster’s serving certificate is not signed by a system trusted certificate authority. A cluster has a name (nickname) which acts as a dictionary key for the cluster within this kubeconfig file. You can add or modify cluster entries using kubectl config set-cluster.

user

users:
- name: blue-user
  user:
    token: blue-token
- name: green-user
  user:
    client-certificate: path/to/my/client/cert
    client-key: path/to/my/client/key

A user defines client credentials for authenticating to a kubernetes cluster. A user has a name (nickname) which acts as its key within the list of user entries after kubeconfig is loaded/merged. Available credentials are client-certificate, client-key, token, and username/password. username/password and token are mutually exclusive, but client certs and keys can be combined with them. You can add or modify user entries using kubectl config set-credentials.

context

contexts:
- context:
    cluster: horse-cluster
    namespace: chisel-ns
    user: green-user
  name: federal-context

A context defines a named cluster,user,namespace tuple which is used to send requests to the specified cluster using the provided authentication info and namespace. Each of the three is optional; it is valid to specify a context with only one of cluster, user,namespace, or to specify none. Unspecified values, or named values that don’t have corresponding entries in the loaded kubeconfig (e.g. if the context specified a pink-user for the above kubeconfig file) will be replaced with the default. See Loading and merging rules below for override/merge behavior. You can add or modify context entries with kubectl config set-context.

current-context

current-context: federal-context

current-context is the nickname or ‘key’ for the cluster,user,namespace tuple that kubectl will use by default when loading config from this file. You can override any of the values in kubectl from the commandline, by passing --context=CONTEXT, --cluster=CLUSTER, --user=USER, and/or --namespace=NAMESPACE respectively. You can change the current-context with kubectl config use-context.

miscellaneous

apiVersion: v1
kind: Config
preferences:
  colors: true

apiVersion and kind identify the version and schema for the client parser and should not be edited manually.

preferences specify optional (and currently unused) kubectl preferences.

Viewing kubeconfig files

kubectl config view will display the current kubeconfig settings. By default it will show you all loaded kubeconfig settings; you can filter the view to just the settings relevant to the current-context by passing --minify. See kubectl config view for other options.

Building your own kubeconfig file

NOTE, that if you are deploying k8s via kube-up.sh, you do not need to create your own kubeconfig files, the script will do it for you.

In any case, you can easily use this file as a template to create your own kubeconfig files.

So, lets do a quick walk through the basics of the above file so you can easily modify it as needed…

The above file would likely correspond to an api-server which was launched using the --token-auth-file=tokens.csv option, where the tokens.csv file looked something like this:

blue-user,blue-user,1
mister-red,mister-red,2

Also, since we have other users who validate using other mechanisms, the api-server would have probably been launched with other authentication options (there are many such options, make sure you understand which ones YOU care about before crafting a kubeconfig file, as nobody needs to implement all the different permutations of possible authentication schemes).

Since the user for the current context is “green-user”, any client of the api-server using this kubeconfig file would naturally be able to log in successfully, because we are providing the green-user’s client credentials.
Similarly, we can operate as the “blue-user” if we choose to change the value of current-context.

In the above scenario, green-user would have to log in by providing certificates, whereas blue-user would just provide the token. All this information would be handled for us by the

Loading and merging rules

The rules for loading and merging the kubeconfig files are straightforward, but there are a lot of them. The final config is built in this order:

Get the kubeconfig from disk. This is done with the following hierarchy and merge rules:

If the CommandLineLocation (the value of the kubeconfig command line option) is set, use this file only. No merging. Only one instance of this flag is allowed.

Else, if EnvVarLocation (the value of $KUBECONFIG) is available, use it as a list of files that should be merged. Merge files together based on the following rules. Empty filenames are ignored. Files with non-deserializable content produced errors. The first file to set a particular value or map key wins and the value or map key is never changed. This means that the first file to set CurrentContext will have its context preserved. It also means that if two files specify a “red-user”, only values from the first file’s red-user are used. Even non-conflicting entries from the second file’s “red-user” are discarded.

Otherwise, use HomeDirectoryLocation (~/.kube/config) with no merging.
Determine the context to use based on the first hit in this chain
1. command line argument - the value of the context command line option
2. current-context from the merged kubeconfig file
3. Empty is allowed at this stage
Determine the cluster info and user to use. At this point, we may or may not have a context. They are built based on the first hit in this chain. (run it twice, once for user, once for cluster)
1. command line argument - user for user name and cluster for cluster name
2. If context is present, then use the context’s value
3. Empty is allowed
Determine the actual cluster info to use. At this point, we may or may not have a cluster info. Build each piece of the cluster info based on the chain (first hit wins):
1. command line arguments - server, api-version, certificate-authority, and insecure-skip-tls-verify
2. If cluster info is present and a value for the attribute is present, use it.
3. If you don’t have a server location, error.
Determine the actual user info to use. User is built using the same rules as cluster info, EXCEPT that you can only have one authentication technique per user.
1. Load precedence is 1) command line flag, 2) user fields from kubeconfig
2. The command line flags are: client-certificate, client-key, username, password, and token.
3. If there are two conflicting techniques, fail.
For any information still missing, use default values and potentially prompt for authentication information
All file references inside of a kubeconfig file are resolved relative to the location of the kubeconfig file itself. When file references are presented on the command line they are resolved relative to the current working directory. When paths are saved in the ~/.kube/config, relative paths are stored relatively while absolute paths are stored absolutely.

Any path in a kubeconfig file is resolved relative to the location of the kubeconfig file itself.

Manipulation of kubeconfig via `kubectl config <subcommand>`

In order to more easily manipulate kubeconfig files, there are a series of subcommands to kubectl config to help. See kubectl/kubectl_config for help.

Example

$ kubectl config set-credentials myself --username=admin --password=secret
$ kubectl config set-cluster local-server --server=http://localhost:8080
$ kubectl config set-context default-context --cluster=local-server --user=myself
$ kubectl config use-context default-context
$ kubectl config set contexts.default-context.namespace the-right-prefix
$ kubectl config view

produces this output

apiVersion: v1
clusters:
- cluster:
    server: http://localhost:8080
  name: local-server
contexts:
- context:
    cluster: local-server
    namespace: the-right-prefix
    user: myself
  name: default-context
current-context: default-context
kind: Config
preferences: {}
users:
- name: myself
  user:
    password: secret
    username: admin

and a kubeconfig file that looks like this

apiVersion: v1
clusters:
- cluster:
    server: http://localhost:8080
  name: local-server
contexts:
- context:
    cluster: local-server
    namespace: the-right-prefix
    user: myself
  name: default-context
current-context: default-context
kind: Config
preferences: {}
users:
- name: myself
  user:
    password: secret
    username: admin

Commands for the example file

$ kubectl config set preferences.colors true
$ kubectl config set-cluster cow-cluster --server=http://cow.org:8080 --api-version=v1
$ kubectl config set-cluster horse-cluster --server=https://horse.org:4443 --certificate-authority=path/to/my/cafile
$ kubectl config set-cluster pig-cluster --server=https://pig.org:443 --insecure-skip-tls-verify=true
$ kubectl config set-credentials blue-user --token=blue-token
$ kubectl config set-credentials green-user --client-certificate=path/to/my/client/cert --client-key=path/to/my/client/key
$ kubectl config set-context queen-anne-context --cluster=pig-cluster --user=black-user --namespace=saw-ns
$ kubectl config set-context federal-context --cluster=horse-cluster --user=green-user --namespace=chisel-ns
$ kubectl config use-context federal-context

Final notes for tying it all together

So, tying this all together, a quick start to create your own kubeconfig file:

Take a good look and understand how your api-server is being launched: You need to know YOUR security requirements and policies before you can design a kubeconfig file for convenient authentication.
Replace the snippet above with information for your cluster’s api-server endpoint.
Make sure your api-server is launched in such a way that at least one user (i.e. green-user) credentials are provided to it. You will of course have to look at api-server documentation in order to determine the current state-of-the-art in terms of providing authentication details.

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