Clustering in Kubernetes

Overview

The term "clustering" refers to the process of having all members of the Kubernetes cluster find and trust each other. There are multiple different ways to achieve clustering with different security and usability profiles. This document attempts to lay out the user experiences for clustering that Kubernetes aims to address.

Once a cluster is established, the following is true:

Master -> Node The master needs to know which nodes can take work and what their current status is wrt capacity.
1. Location The master knows the name and location of all of the nodes in the cluster.
2. For the purposes of this doc, location and name should be enough information so that the master can open a TCP connection to the Node. Most probably we will make this either an IP address or a DNS name. It is going to be important to be consistent here (master must be able to reach kubelet on that DNS name) so that we can verify certificates appropriately.
3. Target AuthN A way to securely talk to the kubelet on that node. Currently we call out to the kubelet over HTTP. This should be over HTTPS and the master should know what CA to trust for that node.
4. Caller AuthN/Z This would be the master verifying itself (and permissions) when calling the node. Currently, this is only used to collect statistics as authorization isn't critical. This may change in the future though.
Node -> Master The nodes currently talk to the master to know which pods have been assigned to them and to publish events.
1. Location The nodes must know where the master is at.
2. Target AuthN Since the master is assigning work to the nodes, it is critical that they verify whom they are talking to.
3. Caller AuthN/Z The nodes publish events and so must be authenticated to the master. Ideally this authentication is specific to each node so that authorization can be narrowly scoped. The details of the work to run (including things like environment variables) might be considered sensitive and should be locked down also.

Note: While the description here refers to a singular Master, in the future we should enable multiple Masters operating in an HA mode. While the "Master" is currently the combination of the API Server, Scheduler and Controller Manager, we will restrict ourselves to thinking about the main API and policy engine -- the API Server.

Current Implementation

A central authority (generally the master) is responsible for determining the set of machines which are members of the cluster. Calls to create and remove worker nodes in the cluster are restricted to this single authority, and any other requests to add or remove worker nodes are rejected. (1.i.)

Communication from the master to nodes is currently over HTTP and is not secured or authenticated in any way. (1.ii, 1.iii.)

The location of the master is communicated out of band to the nodes. For GCE, this is done via Salt. Other cluster instructions/scripts use other methods. (2.i.)

Currently most communication from the node to the master is over HTTP. When it is done over HTTPS there is currently no verification of the cert of the master (2.ii.)

Currently, the node/kubelet is authenticated to the master via a token shared across all nodes. This token is distributed out of band (using Salt for GCE) and is optional. If it is not present then the kubelet is unable to publish events to the master. (2.iii.)

Our current mix of out of band communication doesn't meet all of our needs from a security point of view and is difficult to set up and configure.

Proposed Solution

The proposed solution will provide a range of options for setting up and maintaining a secure Kubernetes cluster. We want to both allow for centrally controlled systems (leveraging pre-existing trust and configuration systems) or more ad-hoc automagic systems that are incredibly easy to set up.

The building blocks of an easier solution:

Move to TLS We will move to using TLS for all intra-cluster communication. We will explicitly identify the trust chain (the set of trusted CAs) as opposed to trusting the system CAs. We will also use client certificates for all AuthN.
[optional] API driven CA Optionally, we will run a CA in the master that will mint certificates for the nodes/kubelets. There will be pluggable policies that will automatically approve certificate requests here as appropriate.
- CA approval policy This is a pluggable policy object that can automatically approve CA signing requests. Stock policies will include always-reject, queue and insecure-always-approve. With queue there would be an API for evaluating and accepting/rejecting requests. Cloud providers could implement a policy here that verifies other out of band information and automatically approves/rejects based on other external factors.
Scoped Kubelet Accounts These accounts are per-node and (optionally) give a node permission to register itself.
- To start with, we'd have the kubelets generate a cert/account in the form of kubelet:<host>. To start we would then hard code policy such that we give that particular account appropriate permissions. Over time, we can make the policy engine more generic.
[optional] Bootstrap API endpoint This is a helper service hosted outside of the Kubernetes cluster that helps with initial discovery of the master.

Static Clustering

In this sequence diagram there is out of band admin entity that is creating all certificates and distributing them. It is also making sure that the kubelets know where to find the master. This provides for a lot of control but is more difficult to set up as lots of information must be communicated outside of Kubernetes.

Dynamic Clustering

This diagram shows dynamic clustering using the bootstrap API endpoint. This endpoint is used to both find the location of the master and communicate the root CA for the master.

This flow has the admin manually approving the kubelet signing requests. This is the queue policy defined above. This manual intervention could be replaced by code that can verify the signing requests via other means.

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