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Docs to clarify k8s auth options with short-lived tokens (#13275)
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* Rework 1.21 content into one heading and add note at top
* Add notes about extended k8s token duration
* Add example of ClusterRoleBinding for using client JWTs
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tomhjp committed Dec 8, 2021
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Expand Up @@ -253,6 +253,220 @@ vault write auth/oidc/role/your_default_role \
1. Save.
1. Visit Credentials. Select Client ID and Secret and note the generated secret.
## Kubernetes
Kubernetes can function as an OIDC provider such that Vault can validate its
service account tokens using JWT/OIDC auth.
-> **Note:** The JWT auth engine does **not** use Kubernetes' `TokenReview` API
during authentication, and instead uses public key cryptography to verify the
contents of JWTs. This means tokens that have been revoked by Kubernetes will
still be considered valid by Vault until their expiry time. To mitigate this
risk, use short TTLs for service account tokens or use
[Kubernetes auth](/docs/auth/kubernetes) which _does_ use the `TokenReview` API.
### Using service account issuer discovery
When using service account issuer discovery, you only need to provide the JWT
auth mount with an OIDC discovery URL, and sometimes a TLS certificate authority
to trust. This makes it the most straightforward method to configure if your
Kubernetes cluster meets the requirements.
Kubernetes cluster requirements:
* [`ServiceAccountIssuerDiscovery`][k8s-sa-issuer-discovery] feature enabled.
* Present from 1.18, defaults to enabled from 1.20.
* kube-apiserver's `--service-account-issuer` flag is set to a URL that is
reachable from Vault. Public by default for most managed Kubernetes solutions.
* Must use short-lived service account tokens when logging in.
* Tokens mounted into pods default to short-lived from 1.21.
Configuration steps:
1. Ensure OIDC discovery URLs do not require authentication, as detailed
[here][k8s-sa-issuer-discovery]:
```bash
kubectl create clusterrolebinding oidc-reviewer \
--clusterrole=system:service-account-issuer-discovery \
--group=system:unauthenticated
```
1. Find the issuer URL of the cluster.
```bash
kubectl proxy &
ISSUER="$(curl --fail --silent --show-error 127.0.0.1:8001/.well-known/openid-configuration | jq -r '.issuer')"
# Kill the background proxy process when you're done
kill %%
```
1. Enable and configure JWT auth in Vault.
1. If Vault is running in Kubernetes:
```bash
kubectl exec vault-0 -- vault auth enable jwt
kubectl exec vault-0 -- vault write auth/jwt/config \
oidc_discovery_url=https://kubernetes.default.svc.cluster.local \
oidc_discovery_ca_pem=@/var/run/secrets/kubernetes.io/serviceaccount/ca.crt
```
1. Alternatively, if Vault is _not_ running in Kubernetes:
-> **Note:** When Vault is outside the cluster, the `$ISSUER` endpoint below may
or may not be reachable. If not, you can configure JWT auth using
[`jwt_validation_pubkeys`](#using-jwt-validation-public-keys) instead.
```bash
vault auth enable jwt
vault write auth/jwt/config oidc_discovery_url="${ISSUER}"
```
1. Configure a role and log in as detailed [below](#creating-a-role-and-logging-in).
[k8s-sa-issuer-discovery]: https://kubernetes.io/docs/tasks/configure-pod-container/configure-service-account/#service-account-issuer-discovery
### Using JWT validation public keys
This method can be useful if Kubernetes' API is not reachable from Vault or if
you would like a single JWT auth mount to service multiple Kubernetes clusters
by chaining their public signing keys.
Kubernetes cluster requirements:
* [`ServiceAccountIssuerDiscovery`][k8s-sa-issuer-discovery] feature enabled.
* Present from 1.18, defaults to enabled from 1.20.
* This requirement can be avoided if you can access the Kubernetes master
nodes to read the public signing key directly from disk at
`/etc/kubernetes/pki/sa.pub`. In this case, you can skip the steps to
retrieve and then convert the key as it will already be in PEM format.
* Must use short-lived service account tokens when logging in.
* Tokens mounted into pods default to short-lived from 1.21.
Configuration steps:
1. Fetch the service account signing public key from your cluster's JWKS URI.
```bash
# 1. Find the issuer URL of the cluster.
kubectl proxy &
ISSUER="$(curl --fail --silent --show-error 127.0.0.1:8001/.well-known/openid-configuration | jq -r '.issuer')"
# 2. Query the jwks_uri specified in /.well-known/openid-configuration
# NB: You may need to run this from a pod within the cluster if the $ISSUER
# URL is not available outside the cluster.
curl "$(curl --fail --silent --show-error "${ISSUER}/.well-known/openid-configuration" | jq -r '.jwks_uri')"
# Kill the background proxy process when you're done
kill %%
```
1. Convert the keys from JWK format to PEM. You can use a CLI tool or an online
converter such as [this one][jwk-to-pem].
1. Configure the JWT auth mount with those public keys.
```bash
vault write auth/jwt/config \
jwt_validation_pubkeys="-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9...
-----END PUBLIC KEY-----","-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9...
-----END PUBLIC KEY-----"
```
1. Configure a role and log in as detailed [below](#creating-a-role-and-logging-in).
[jwk-to-pem]: https://8gwifi.org/jwkconvertfunctions.jsp
### Creating a role and logging in
Once your JWT auth mount is configured, you're ready to configure a role and
log in.
1. Create a role for JWT auth that the `default` service account from the
`default` namespace can use. The audience of tokens defaults to the same as
the issuer, but it is configurable.
```bash
vault write auth/jwt/role/my-role \
role_type="jwt" \
bound_audiences="${ISSUER}" \
user_claim="sub" \
bound_subject="system:serviceaccount:default:default" \
policies="default" \
ttl="1h"
```
1. Pods or other clients with access to a service account JWT can then log in.
```bash
vault write auth/jwt/login \
role=my-role \
jwt=@/var/run/secrets/kubernetes.io/serviceaccount/token
# OR equivalent to:
curl \
--fail \
--request PUT \
--header "X-Vault-Request: true" \
--data '{"jwt":"<JWT-TOKEN-HERE>","role":"my-role"}' \
"${VAULT_ADDR}/v1/auth/jwt/login"
```
### Specifying TTL and audience
If you would like to specify a custom TTL or audience for service account tokens,
the following pod spec illustrates a volume mount that overrides the default
admission injected token. This is especially relevant if you are unable to
disable the [--service-account-extend-token-expiration][k8s-extended-tokens]
flag for `kube-apiserver` and want to use short TTLs.
When using the resulting token, you will need to set `bound_audiences=vault`
when creating roles in Vault's JWT auth mount.
```yaml
apiVersion: v1
kind: Pod
metadata:
name: nginx
spec:
# automountServiceAccountToken is redundant in this example because the
# mountPath overlapping with the default path below will already stop the
# default admission injected token from being created. Use this option if you
# choose a different mount path.
automountServiceAccountToken: false
containers:
- name: nginx
image: nginx
volumeMounts:
- name: custom-token
mountPath: /var/run/secrets/kubernetes.io/serviceaccount
volumes:
- name: custom-token
projected:
defaultMode: 420
sources:
- serviceAccountToken:
path: token
expirationSeconds: 600 # 10 minutes is the minimum TTL
audience: vault
- configMap:
name: kube-root-ca.crt
items:
- key: ca.crt
path: ca.crt
- downwardAPI:
items:
- fieldRef:
apiVersion: v1
fieldPath: metadata.namespace
path: namespace
```
[k8s-extended-tokens]: https://kubernetes.io/docs/reference/command-line-tools-reference/kube-apiserver/#options
## Okta
1. Make sure an Authorization Server has been created. The "Issuer" field shown on the Setting page
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