Controlling Access to the Kubernetes API

  • API Server Ports and IPs By default the Kubernetes API server serves HTTP on 2 ports: Localhost Port:
    • is intended for testing and bootstrap, and for other components of the master node (scheduler, controller-manager) to talk to the API
    • no TLS
    • default is port 8080, change with --insecure-port flag.
    • defaults IP is localhost, change with --insecure-bind-address flag.
    • request bypasses authentication and authorization modules.
    • request handled by admission control module(s).
    • protected by need to have host access

    Secure Port:

    • use whenever possible
    • uses TLS. Set cert with --tls-cert-file and key with --tls-private-key-file flag.
    • default is port 6443, change with --secure-port flag.
    • default IP is first non-localhost network interface, change with --bind-address flag.
    • request handled by authentication and authorization modules.
    • request handled by admission control module(s).
    • authentication and authorisation modules run.

Users in Kubernetes

All Kubernetes clusters have two categories of users: service accounts managed by Kubernetes, and normal users.

  • Kubernetes does not have objects which represent normal user accounts. Regular users cannot be added to a cluster through an API call.
  • In contrast, service accounts are users managed by the Kubernetes API.
  • API requests are tied to either a normal user or a service account, or are treated as anonymous requests.

Kubernetes Authenticating

Authentication strategies

  • Authentication modules include Client Certificates, Password, and Plain Tokens, Bootstrap Tokens, and JWT Tokens (used for service accounts).
  • Multiple authentication modules can be specified, in which case each one is tried in sequence, until one of them succeeds.
  • The API server does not guarantee the order authenticators run in.
  • The system:authenticated group is included in the list of groups for all authenticated users.
  • authentication plugins attempt to associate the following attributes with the request:
    • Username
    • UID
    • Groups
    • Extra fields
  • X509 Client Certs
    • Client certificate authentication is enabled by passing the --client-ca-file=SOMEFILE option to API server.
    • --client-ca-file=/srv/kubernetes/ca.crt
    • --tls-cert-file=/srv/kubernetes/server.crt
    • --tls-private-key-file=/srv/kubernetes/server.key
    • the common name of the subject is used as the user name for the request. For example, using the openssl command line tool to generate a certificate signing request: openssl req -new -key jbeda.pem -out jbeda-csr.pem -subj "/CN=jbeda/O=app1/O=app2" This would create a CSR for the username “jbeda”, belonging to two groups, “app1” and “app2”.

    use openssl to manually generate certificates for your cluster.

    1. openssl genrsa -out ca.key 2048
    2. openssl req -x509 -new -nodes -key ca.key -subj "/CN=${MASTER_IP}" -days 10000 -out ca.crt
    3. openssl genrsa -out server.key 2048
    4. openssl req -new -key server.key -subj "/CN=${MASTER_IP}" -out server.csr
    5. openssl x509 -req -in server.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out server.crt -days 10000
    6. openssl x509 -noout -text -in ./server.crt
  • Static Token File
    • The API server reads bearer tokens from a file when given the --token-auth-file=SOMEFILE option on the command line.
    • The token file is a csv file with a minimum of 3 columns: token, user name, user uid, followed by optional group names. For example, token,user,uid,"group1,group2,group3"
    • Putting a Bearer Token in a Request: Authorization header with a value of Bearer THETOKEN. For example, Authorization: Bearer 31ada4fd-adec-460c-809a-9e56ceb75269
  • Bootstrap Tokens
    • This feature is currently in alpha.
    • --experimental-bootstrap-token-auth flag on the API Server.
    • You must enable the TokenCleaner controller via the --controllers=*,tokencleaner flag on the Controller Manager.
  • Static Password File
    • Basic authentication is enabled by passing the --basic-auth-file=SOMEFILE option to API server.
    • The basic auth file is a csv file with a minimum of 3 columns: password, user name, user id.
    • In Kubernetes version 1.6 and later, you can specify an optional fourth column containing comma-separated group names. For example: password,user,uid,"group1,group2,group3"
    • When using basic authentication from an http client, the API server expects an Authorization header with a value of Basic BASE64ENCODED(USER:PASSWORD).
  • Service Account Tokens
    • --service-account-key-file A file containing a PEM encoded key for signing bearer tokens. If unspecified, the API server’s TLS private key will be used.
    • --service-account-lookup If enabled, tokens which are deleted from the API will be revoked.
    • Service accounts authenticate with the username system:serviceaccount:(NAMESPACE):(SERVICEACCOUNT), and are assigned to the groups system:serviceaccounts and system:serviceaccounts:(NAMESPACE).
  • OpenID Connect Tokens
    • --oidc-issuer-url
    • --oidc-client-id
    • --oidc-username-claim
    • --oidc-groups-claim
    • --oidc-ca-file

Kubernetes Authorization

  • The request is authorized if an existing policy declares that the user has permissions to complete the requested action.
  • Review Your Request Attributes
    • user - The user string provided during authentication
    • group - The list of group names to which the authenticated user belongs
    • “extra” - A map of arbitrary string keys to string values, provided by the authentication layer
    • API - Indicates whether the request is for an API resource
    • Request path - Path to miscellaneous non-resource endpoints like /api or /healthz (see kubectl).
    • API request verb - API verbs get, list, create, update, patch, watch, proxy, redirect, delete, and deletecollection are used for resource requests. To determine the request verb for a resource API endpoint, see Determine the request verb below.
    • HTTP request verb - HTTP verbs get, post, put, and delete are used for non-resource requests
    • Resource - The ID or name of the resource that is being accessed (for resource requests only) –* For resource requests using get, update, patch, and delete verbs, you must provide the resource name.
    • Subresource - The subresource that is being accessed (for resource requests only)
    • Namespace - The namespace of the object that is being accessed (for namespaced resource requests only)
    • API group - The API group being accessed (for resource requests only). An empty string designates the core API group.
  • Authorization Modules
    • ABAC Mode --authorization-mode=ABAC --authorization-policy-file=SOME_FILENAME
    • RBAC Mode --authorization-mode=RBAC
    • Webhook Mode --authorization-mode=Webhook --authorization-webhook-config-file=SOME_FILENAME
    • AlwaysDeny --authorization-mode=AlwaysDeny
    • AlwaysAllow --authorization-mode=AlwaysAllow
    • Custom Modules

Using Admission Controllers

  • If any of the plug-ins in the sequence reject the request, the entire request is rejected immediately and an error is returned to the end-user.
  • All Admission Controllers:
    • AlwaysAdmit
    • AlwaysPullImages
    • AlwaysDeny
    • DenyEscalatingExec
    • ImagePolicyWebhook
    • ServiceAccount
    • SecurityContextDeny
    • ResourceQuota
    • LimitRanger
    • InitialResources (experimental)
    • NamespaceLifecycle
    • DefaultStorageClass
    • DefaultTolerationSeconds
    • PodSecurityPolicy
  • For Kubernetes >= 1.6.0, we strongly recommend running the following set of admission control plug-ins (order matters):
    • --admission-control=NamespaceLifecycle,LimitRanger,ServiceAccount,PersistentVolumeLabel,DefaultStorageClass,ResourceQuota,DefaultTolerationSeconds

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