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Concepts

Detailed explanations of Kubernetes system concepts and abstractions.

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Secrets

Objects of type secret are intended to hold sensitive information, such as passwords, OAuth tokens, and ssh keys. Putting this information in a secret is safer and more flexible than putting it verbatim in a pod definition or in a docker image. See Secrets design document for more information.

Overview of Secrets

A Secret is an object that contains a small amount of sensitive data such as a password, a token, or a key. Such information might otherwise be put in a Pod specification or in an image; putting it in a Secret object allows for more control over how it is used, and reduces the risk of accidental exposure.

Users can create secrets, and the system also creates some secrets.

To use a secret, a pod needs to reference the secret. A secret can be used with a pod in two ways: as files in a volume mounted on one or more of its containers, or used by kubelet when pulling images for the pod.

Built-in Secrets

Service Accounts Automatically Create and Attach Secrets with API Credentials

Kubernetes automatically creates secrets which contain credentials for accessing the API and it automatically modifies your pods to use this type of secret.

The automatic creation and use of API credentials can be disabled or overridden if desired. However, if all you need to do is securely access the apiserver, this is the recommended workflow.

See the Service Account documentation for more information on how Service Accounts work.

Creating your own Secrets

Creating a Secret Using kubectl create secret

Say that some pods need to access a database. The username and password that the pods should use is in the files ./username.txt and ./password.txt on your local machine.

# Create files needed for rest of example.
$ echo -n "admin" > ./username.txt
$ echo -n "1f2d1e2e67df" > ./password.txt

The kubectl create secret command packages these files into a Secret and creates the object on the Apiserver.

$ kubectl create secret generic db-user-pass --from-file=./username.txt --from-file=./password.txt
secret "db-user-pass" created

You can check that the secret was created like this:

$ kubectl get secrets
NAME                  TYPE                                  DATA      AGE
db-user-pass          Opaque                                2         51s

$ kubectl describe secrets/db-user-pass
Name:            db-user-pass
Namespace:       default
Labels:          <none>
Annotations:     <none>

Type:            Opaque

Data
====
password.txt:    12 bytes
username.txt:    5 bytes

Note that neither get nor describe shows the contents of the file by default. This is to protect the secret from being exposed accidentally to someone looking or from being stored in a terminal log.

See decoding a secret for how to see the contents.

Creating a Secret Manually

You can also create a secret object in a file first, in json or yaml format, and then create that object.

Each item must be base64 encoded:

$ echo -n "admin" | base64
YWRtaW4=
$ echo -n "1f2d1e2e67df" | base64
MWYyZDFlMmU2N2Rm

Now write a secret object that looks like this:

apiVersion: v1
kind: Secret
metadata:
  name: mysecret
type: Opaque
data:
  username: YWRtaW4=
  password: MWYyZDFlMmU2N2Rm

The data field is a map. Its keys must match DNS_SUBDOMAIN, except that leading dots are also allowed. The values are arbitrary data, encoded using base64.

Create the secret using kubectl create:

$ kubectl create -f ./secret.yaml
secret "mysecret" created

Encoding Note: The serialized JSON and YAML values of secret data are encoded as base64 strings. Newlines are not valid within these strings and must be omitted. When using the base64 utility on Darwin/OS X users should avoid using the -b option to split long lines. Conversely Linux users should add the option -w 0 to base64 commands or the pipeline base64 | tr -d '\n' if -w option is not available.

Decoding a Secret

Get back the secret created in the previous section:

$ kubectl get secret mysecret -o yaml
apiVersion: v1
data:
  username: YWRtaW4=
  password: MWYyZDFlMmU2N2Rm
kind: Secret
metadata:
  creationTimestamp: 2016-01-22T18:41:56Z
  name: mysecret
  namespace: default
  resourceVersion: "164619"
  selfLink: /api/v1/namespaces/default/secrets/mysecret
  uid: cfee02d6-c137-11e5-8d73-42010af00002
type: Opaque

Decode the password field:

$ echo "MWYyZDFlMmU2N2Rm" | base64 --decode
1f2d1e2e67df

Using Secrets

Secrets can be mounted as data volumes or be exposed as environment variables to be used by a container in a pod. They can also be used by other parts of the system, without being directly exposed to the pod. For example, they can hold credentials that other parts of the system should use to interact with external systems on your behalf.

Using Secrets as Files from a Pod

To consume a Secret in a volume in a Pod:

  1. Create a secret or use an existing one. Multiple pods can reference the same secret.
  2. Modify your Pod definition to add a volume under spec.volumes[]. Name the volume anything, and have a spec.volumes[].secret.secretName field equal to the name of the secret object.
  3. Add a spec.containers[].volumeMounts[] to each container that needs the secret. Specify spec.containers[].volumeMounts[].readOnly = true and spec.containers[].volumeMounts[].mountPath to an unused directory name where you would like the secrets to appear.
  4. Modify your image and/or command line so that the program looks for files in that directory. Each key in the secret data map becomes the filename under mountPath.

This is an example of a pod that mounts a secret in a volume:

{
 "apiVersion": "v1",
 "kind": "Pod",
  "metadata": {
    "name": "mypod",
    "namespace": "myns"
  },
  "spec": {
    "containers": [{
      "name": "mypod",
      "image": "redis",
      "volumeMounts": [{
        "name": "foo",
        "mountPath": "/etc/foo",
        "readOnly": true
      }]
    }],
    "volumes": [{
      "name": "foo",
      "secret": {
        "secretName": "mysecret"
      }
    }]
  }
}

Each secret you want to use needs to be referred to in spec.volumes.

If there are multiple containers in the pod, then each container needs its own volumeMounts block, but only one spec.volumes is needed per secret.

You can package many files into one secret, or use many secrets, whichever is convenient.

Projection of secret keys to specific paths

We can also control the paths within the volume where Secret keys are projected. You can use spec.volumes[].secret.items field to change target path of each key:

{
 "apiVersion": "v1",
 "kind": "Pod",
  "metadata": {
    "name": "mypod",
    "namespace": "myns"
  },
  "spec": {
    "containers": [{
      "name": "mypod",
      "image": "redis",
      "volumeMounts": [{
        "name": "foo",
        "mountPath": "/etc/foo",
        "readOnly": true
      }]
    }],
    "volumes": [{
      "name": "foo",
      "secret": {
        "secretName": "mysecret",
        "items": [{
          "key": "username",
          "path": "my-group/my-username"
        }]
      }
    }]
  }
}

What will happen:

If spec.volumes[].secret.items is used, only keys specified in items are projected. To consume all keys from the secret, all of them must be listed in the items field. All listed keys must exist in the corresponding secret. Otherwise, the volume is not created.

Secret files permissions

You can also specify the permission mode bits files part of a secret will have. If you don’t specify any, 0644 is used by default. You can specify a default mode for the whole secret volume and override per key if needed.

For example, you can specify a default mode like this:

{
 "apiVersion": "v1",
 "kind": "Pod",
  "metadata": {
    "name": "mypod",
    "namespace": "myns"
  },
  "spec": {
    "containers": [{
      "name": "mypod",
      "image": "redis",
      "volumeMounts": [{
        "name": "foo",
        "mountPath": "/etc/foo"
      }]
    }],
    "volumes": [{
      "name": "foo",
      "secret": {
        "secretName": "mysecret",
        "defaultMode": 256
      }
    }]
  }
}

Then, the secret will be mounted on /etc/foo and all the files created by the secret volume mount will have permission 0400.

Note that the JSON spec doesn’t support octal notation, so use the value 256 for 0400 permissions. If you use yaml instead of json for the pod, you can use octal notation to specify permissions in a more natural way.

You can also use mapping, as in the previous example, and specify different permission for different files like this:

{
 "apiVersion": "v1",
 "kind": "Pod",
  "metadata": {
    "name": "mypod",
    "namespace": "myns"
  },
  "spec": {
    "containers": [{
      "name": "mypod",
      "image": "redis",
      "volumeMounts": [{
        "name": "foo",
        "mountPath": "/etc/foo"
      }]
    }],
    "volumes": [{
      "name": "foo",
      "secret": {
        "secretName": "mysecret",
        "items": [{
          "key": "username",
          "path": "my-group/my-username",
          "mode": 511
        }]
      }
    }]
  }
}

In this case, the file resulting in /etc/foo/my-group/my-username will have permission value of 0777. Owing to JSON limitations, you must specify the mode in decimal notation.

Note that this permission value might be displayed in decimal notation if you read it later.

Consuming Secret Values from Volumes

Inside the container that mounts a secret volume, the secret keys appear as files and the secret values are base-64 decoded and stored inside these files. This is the result of commands executed inside the container from the example above:

$ ls /etc/foo/
username
password
$ cat /etc/foo/username
admin
$ cat /etc/foo/password
1f2d1e2e67df

The program in a container is responsible for reading the secrets from the files.

Mounted Secrets are updated automatically

When a secret being already consumed in a volume is updated, projected keys are eventually updated as well. Kubelet is checking whether the mounted secret is fresh on every periodic sync. However, it is using its local ttl-based cache for getting the current value of the secret. As a result, the total delay from the moment when the secret is updated to the moment when new keys are projected to the pod can be as long as kubelet sync period + ttl of secrets cache in kubelet.

Using Secrets as Environment Variables

To use a secret in an environment variable in a pod:

  1. Create a secret or use an existing one. Multiple pods can reference the same secret.
  2. Modify your Pod definition in each container that you wish to consume the value of a secret key to add an environment variable for each secret key you wish to consume. The environment variable that consumes the secret key should populate the secret’s name and key in env[x].valueFrom.secretKeyRef.
  3. Modify your image and/or command line so that the program looks for values in the specified environment variables

This is an example of a pod that uses secrets from environment variables:

apiVersion: v1
kind: Pod
metadata:
  name: secret-env-pod
spec:
  containers:
    - name: mycontainer
      image: redis
      env:
        - name: SECRET_USERNAME
          valueFrom:
            secretKeyRef:
              name: mysecret
              key: username
        - name: SECRET_PASSWORD
          valueFrom:
            secretKeyRef:
              name: mysecret
              key: password
  restartPolicy: Never

Consuming Secret Values from Environment Variables

Inside a container that consumes a secret in an environment variables, the secret keys appear as normal environment variables containing the base-64 decoded values of the secret data. This is the result of commands executed inside the container from the example above:

$ echo $SECRET_USERNAME
admin
$ echo $SECRET_PASSWORD
1f2d1e2e67df

Using imagePullSecrets

An imagePullSecret is a way to pass a secret that contains a Docker (or other) image registry password to the Kubelet so it can pull a private image on behalf of your Pod.

Manually specifying an imagePullSecret

Use of imagePullSecrets is described in the images documentation

Arranging for imagePullSecrets to be Automatically Attached

You can manually create an imagePullSecret, and reference it from a serviceAccount. Any pods created with that serviceAccount or that default to use that serviceAccount, will get their imagePullSecret field set to that of the service account. See Adding ImagePullSecrets to a service account for a detailed explanation of that process.

Automatic Mounting of Manually Created Secrets

Manually created secrets (e.g. one containing a token for accessing a github account) can be automatically attached to pods based on their service account. See Injecting Information into Pods Using a PodPreset for a detailed explanation of that process.

Details

Restrictions

Secret volume sources are validated to ensure that the specified object reference actually points to an object of type Secret. Therefore, a secret needs to be created before any pods that depend on it.

Secret API objects reside in a namespace. They can only be referenced by pods in that same namespace.

Individual secrets are limited to 1MB in size. This is to discourage creation of very large secrets which would exhaust apiserver and kubelet memory. However, creation of many smaller secrets could also exhaust memory. More comprehensive limits on memory usage due to secrets is a planned feature.

Kubelet only supports use of secrets for Pods it gets from the API server. This includes any pods created using kubectl, or indirectly via a replication controller. It does not include pods created via the kubelets --manifest-url flag, its --config flag, or its REST API (these are not common ways to create pods.)

Secrets must be created before they are consumed in pods as environment variables unless they are marked as optional. References to Secrets that do not exist will prevent the pod from starting.

References via secretKeyRef to keys that do not exist in a named Secret will prevent the pod from starting.

Secrets used to populate environment variables via envFrom that have keys that are considered invalid environment variable names will have those keys skipped. The pod will be allowed to start. There will be an event whose reason is InvalidVariableNames and the message will contain the list of invalid keys that were skipped. The example shows a pod which refers to the default/mysecret ConfigMap that contains 2 invalid keys, 1badkey and 2alsobad.

$ kubectl get events
LASTSEEN   FIRSTSEEN   COUNT     NAME            KIND      SUBOBJECT                         TYPE      REASON
0s         0s          1         dapi-test-pod   Pod                                         Warning   InvalidEnvironmentVariableNames   kubelet, 127.0.0.1      Keys [1badkey, 2alsobad] from the EnvFrom secret default/mysecret were skipped since they are considered invalid environment variable names.

Secret and Pod Lifetime interaction

When a pod is created via the API, there is no check whether a referenced secret exists. Once a pod is scheduled, the kubelet will try to fetch the secret value. If the secret cannot be fetched because it does not exist or because of a temporary lack of connection to the API server, kubelet will periodically retry. It will report an event about the pod explaining the reason it is not started yet. Once the secret is fetched, the kubelet will create and mount a volume containing it. None of the pod’s containers will start until all the pod’s volumes are mounted.

Use cases

Use-Case: Pod with ssh keys

Create a secret containing some ssh keys:

$ kubectl create secret generic ssh-key-secret --from-file=ssh-privatekey=/path/to/.ssh/id_rsa --from-file=ssh-publickey=/path/to/.ssh/id_rsa.pub

Security Note: think carefully before sending your own ssh keys: other users of the cluster may have access to the secret. Use a service account which you want to have accessible to all the users with whom you share the Kubernetes cluster, and can revoke if they are compromised.

Now we can create a pod which references the secret with the ssh key and consumes it in a volume:

{
  "kind": "Pod",
  "apiVersion": "v1",
  "metadata": {
    "name": "secret-test-pod",
    "labels": {
      "name": "secret-test"
    }
  },
  "spec": {
    "volumes": [
      {
        "name": "secret-volume",
        "secret": {
          "secretName": "ssh-key-secret"
        }
      }
    ],
    "containers": [
      {
        "name": "ssh-test-container",
        "image": "mySshImage",
        "volumeMounts": [
          {
            "name": "secret-volume",
            "readOnly": true,
            "mountPath": "/etc/secret-volume"
          }
        ]
      }
    ]
  }
}

When the container’s command runs, the pieces of the key will be available in:

/etc/secret-volume/ssh-publickey
/etc/secret-volume/ssh-privatekey

The container is then free to use the secret data to establish an ssh connection.

Use-Case: Pods with prod / test credentials

This example illustrates a pod which consumes a secret containing prod credentials and another pod which consumes a secret with test environment credentials.

Make the secrets:

$ kubectl create secret generic prod-db-secret --from-literal=username=produser --from-literal=password=Y4nys7f11
secret "prod-db-secret" created
$ kubectl create secret generic test-db-secret --from-literal=username=testuser --from-literal=password=iluvtests
secret "test-db-secret" created

Now make the pods:

{
  "apiVersion": "v1",
  "kind": "List",
  "items":
  [{
    "kind": "Pod",
    "apiVersion": "v1",
    "metadata": {
      "name": "prod-db-client-pod",
      "labels": {
        "name": "prod-db-client"
      }
    },
    "spec": {
      "volumes": [
        {
          "name": "secret-volume",
          "secret": {
            "secretName": "prod-db-secret"
          }
        }
      ],
      "containers": [
        {
          "name": "db-client-container",
          "image": "myClientImage",
          "volumeMounts": [
            {
              "name": "secret-volume",
              "readOnly": true,
              "mountPath": "/etc/secret-volume"
            }
          ]
        }
      ]
    }
  },
  {
    "kind": "Pod",
    "apiVersion": "v1",
    "metadata": {
      "name": "test-db-client-pod",
      "labels": {
        "name": "test-db-client"
      }
    },
    "spec": {
      "volumes": [
        {
          "name": "secret-volume",
          "secret": {
            "secretName": "test-db-secret"
          }
        }
      ],
      "containers": [
        {
          "name": "db-client-container",
          "image": "myClientImage",
          "volumeMounts": [
            {
              "name": "secret-volume",
              "readOnly": true,
              "mountPath": "/etc/secret-volume"
            }
          ]
        }
      ]
    }
  }]
}

Both containers will have the following files present on their filesystems:

/etc/secret-volume/username
/etc/secret-volume/password

Note how the specs for the two pods differ only in one field; this facilitates creating pods with different capabilities from a common pod config template.

You could further simplify the base pod specification by using two Service Accounts: one called, say, prod-user with the prod-db-secret, and one called, say, test-user with the test-db-secret. Then, the pod spec can be shortened to, for example:

{
  "kind": "Pod",
  "apiVersion": "v1",
  "metadata": {
    "name": "prod-db-client-pod",
    "labels": {
      "name": "prod-db-client"
    }
  },
  "spec": {
    "serviceAccount": "prod-db-client",
    "containers": [
      {
        "name": "db-client-container",
        "image": "myClientImage"
      }
    ]
  }
}

Use-case: Dotfiles in secret volume

In order to make piece of data ‘hidden’ (i.e., in a file whose name begins with a dot character), simply make that key begin with a dot. For example, when the following secret is mounted into a volume:

{
  "kind": "Secret",
  "apiVersion": "v1",
  "metadata": {
    "name": "dotfile-secret"
  },
  "data": {
    ".secret-file": "dmFsdWUtMg0KDQo="
  }
}

{
  "kind": "Pod",
  "apiVersion": "v1",
  "metadata": {
    "name": "secret-dotfiles-pod"
  },
  "spec": {
    "volumes": [
      {
        "name": "secret-volume",
        "secret": {
          "secretName": "dotfile-secret"
        }
      }
    ],
    "containers": [
      {
        "name": "dotfile-test-container",
        "image": "gcr.io/google_containers/busybox",
        "command": [ "ls", "-l", "/etc/secret-volume" ],
        "volumeMounts": [
          {
            "name": "secret-volume",
            "readOnly": true,
            "mountPath": "/etc/secret-volume"
          }
        ]
      }
    ]
  }
}

The secret-volume will contain a single file, called .secret-file, and the dotfile-test-container will have this file present at the path /etc/secret-volume/.secret-file.

NOTE

Files beginning with dot characters are hidden from the output of ls -l; you must use ls -la to see them when listing directory contents.

Use-case: Secret visible to one container in a pod

Consider a program that needs to handle HTTP requests, do some complex business logic, and then sign some messages with an HMAC. Because it has complex application logic, there might be an unnoticed remote file reading exploit in the server, which could expose the private key to an attacker.

This could be divided into two processes in two containers: a frontend container which handles user interaction and business logic, but which cannot see the private key; and a signer container that can see the private key, and responds to simple signing requests from the frontend (e.g. over localhost networking).

With this partitioned approach, an attacker now has to trick the application server into doing something rather arbitrary, which may be harder than getting it to read a file.

Best practices

Clients that use the secrets API

When deploying applications that interact with the secrets API, access should be limited using authorization policies such as RBAC.

Secrets often hold values that span a spectrum of importance, many of which can cause escalations within Kubernetes (e.g. service account tokens) and to external systems. Even if an individual app can reason about the power of the secrets it expects to interact with, other apps within the same namespace can render those assumptions invalid.

For these reasons watch and list requests for secrets within a namespace are extremely powerful capabilities and should be avoided, since listing secrets allows the clients to inspect the values if all secrets are in that namespace. The ability to watch and list all secrets in a cluster should be reserved for only the most privileged, system-level components.

Applications that need to access the secrets API should perform get requests on the secrets they need. This lets administrators restrict access to all secrets while white-listing access to individual instances that the app needs.

For improved performance over a looping get, clients can design resources that reference a secret then watch the resource, re-requesting the secret when the reference changes. Additionally, a “bulk watch” API to let clients watch individual resources has also been proposed, and will likely be available in future releases of Kubernetes.

Security Properties

Protections

Because secret objects can be created independently of the pods that use them, there is less risk of the secret being exposed during the workflow of creating, viewing, and editing pods. The system can also take additional precautions with secret objects, such as avoiding writing them to disk where possible.

A secret is only sent to a node if a pod on that node requires it. It is not written to disk. It is stored in a tmpfs. It is deleted once the pod that depends on it is deleted.

On most Kubernetes-project-maintained distributions, communication between user to the apiserver, and from apiserver to the kubelets, is protected by SSL/TLS. Secrets are protected when transmitted over these channels.

Secret data on nodes is stored in tmpfs volumes and thus does not come to rest on the node.

There may be secrets for several pods on the same node. However, only the secrets that a pod requests are potentially visible within its containers. Therefore, one Pod does not have access to the secrets of another pod.

There may be several containers in a pod. However, each container in a pod has to request the secret volume in its volumeMounts for it to be visible within the container. This can be used to construct useful security partitions at the Pod level.

Risks

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