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Peeking under the hood of Kubernetes on AWS

This document provides high-level insight into how Kubernetes works on AWS and maps to AWS objects. We assume that you are familiar with AWS.

We encourage you to use kube-up to create clusters on AWS. We recommend that you avoid manual configuration but are aware that sometimes it's the only option.

Tip: You should open an issue and let us know what enhancements can be made to the scripts to better suit your needs.

That said, it's also useful to know what's happening under the hood when Kubernetes clusters are created on AWS. This can be particularly useful if problems arise or in circumstances where the provided scripts are lacking and you manually created or configured your cluster.

Table of contents:

Architecture overview

Kubernetes is a cluster of several machines that consists of a Kubernetes master and a set number of nodes (previously known as 'nodes') for which the master which is responsible. See the Architecture topic for more details.

By default on AWS:

  • Instances run Ubuntu 15.04 (the official AMI). It includes a sufficiently modern kernel that pairs well with Docker and doesn't require a reboot. (The default SSH user is ubuntu for this and other ubuntu images.)
  • Nodes use aufs instead of ext4 as the filesystem / container storage (mostly because this is what Google Compute Engine uses).

You can override these defaults by passing different environment variables to kube-up.

Storage

AWS supports persistent volumes by using Elastic Block Store (EBS). These can then be attached to pods that should store persistent data (e.g. if you're running a database).

By default, nodes in AWS use instance storage unless you create pods with persistent volumes (EBS). In general, Kubernetes containers do not have persistent storage unless you attach a persistent volume, and so nodes on AWS use instance storage. Instance storage is cheaper, often faster, and historically more reliable. Unless you can make do with whatever space is left on your root partition, you must choose an instance type that provides you with sufficient instance storage for your needs.

To configure Kubernetes to use EBS storage, pass the environment variable KUBE_AWS_STORAGE=ebs to kube-up.

Note: The master uses a persistent volume (etcd) to track its state. Similar to nodes, containers are mostly run against instance storage, except that we repoint some important data onto the persistent volume.

The default storage driver for Docker images is aufs. Specifying btrfs (by passing the environment variable DOCKER_STORAGE=btrfs to kube-up) is also a good choice for a filesystem. btrfs is relatively reliable with Docker and has improved its reliability with modern kernels. It can easily span multiple volumes, which is particularly useful when we are using an instance type with multiple ephemeral instance disks.

Auto Scaling group

Nodes (but not the master) are run in an Auto Scaling group on AWS. Currently auto-scaling (e.g. based on CPU) is not actually enabled (#11935). Instead, the Auto Scaling group means that AWS will relaunch any nodes that are terminated.

We do not currently run the master in an AutoScalingGroup, but we should (#11934).

Networking

Kubernetes uses an IP-per-pod model. This means that a node, which runs many pods, must have many IPs. AWS uses virtual private clouds (VPCs) and advanced routing support so each pod is assigned a /24 CIDR. The assigned CIDR is then configured to route to an instance in the VPC routing table.

It is also possible to use overlay networking on AWS, but that is not the default configuration of the kube-up script.

NodePort and LoadBalancer services

Kubernetes on AWS integrates with Elastic Load Balancing (ELB). When you create a service with Type=LoadBalancer, Kubernetes (the kube-controller-manager) will create an ELB, create a security group for the ELB which allows access on the service ports, attach all the nodes to the ELB, and modify the security group for the nodes to allow traffic from the ELB to the nodes. This traffic reaches kube-proxy where it is then forwarded to the pods.

ELB has some restrictions:

  • ELB requires that all nodes listen on a single port,
  • ELB acts as a forwarding proxy (i.e. the source IP is not preserved, but see below on ELB annotations for pods speaking HTTP).

To work with these restrictions, in Kubernetes, LoadBalancer services are exposed as NodePort services. Then kube-proxy listens externally on the cluster-wide port that's assigned to NodePort services and forwards traffic to the corresponding pods.

For example, if we configure a service of Type LoadBalancer with a public port of 80:

  • Kubernetes will assign a NodePort to the service (e.g. port 31234)
  • ELB is configured to proxy traffic on the public port 80 to the NodePort assigned to the service (in this example port 31234).
  • Then any in-coming traffic that ELB forwards to the NodePort (31234) is recognized by kube-proxy and sent to the correct pods for that service.

Note that we do not automatically open NodePort services in the AWS firewall (although we do open LoadBalancer services). This is because we expect that NodePort services are more of a building block for things like inter-cluster services or for LoadBalancer. To consume a NodePort service externally, you will likely have to open the port in the node security group (kubernetes-node-<clusterid>).

For SSL support, starting with 1.3 two annotations can be added to a service:

service.beta.kubernetes.io/aws-load-balancer-ssl-cert=arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012

The first specifies which certificate to use. It can be either a certificate from a third party issuer that was uploaded to IAM or one created within AWS Certificate Manager.

service.beta.kubernetes.io/aws-load-balancer-backend-protocol=(https|http|ssl|tcp)

The second annotation specifies which protocol a pod speaks. For HTTPS and SSL, the ELB will expect the pod to authenticate itself over the encrypted connection.

HTTP and HTTPS will select layer 7 proxying: the ELB will terminate the connection with the user, parse headers and inject the X-Forwarded-For header with the user's IP address (pods will only see the IP address of the ELB at the other end of its connection) when forwarding requests.

TCP and SSL will select layer 4 proxying: the ELB will forward traffic without modifying the headers.

Identity and Access Management (IAM)

kube-proxy sets up two IAM roles, one for the master called kubernetes-master and one for the nodes called kubernetes-node.

The master is responsible for creating ELBs and configuring them, as well as setting up advanced VPC routing. Currently it has blanket permissions on EC2, along with rights to create and destroy ELBs.

The nodes do not need a lot of access to the AWS APIs. They need to download a distribution file, and then are responsible for attaching and detaching EBS volumes from itself.

The node policy is relatively minimal. In 1.2 and later, nodes can retrieve ECR authorization tokens, refresh them every 12 hours if needed, and fetch Docker images from it, as long as the appropriate permissions are enabled. Those in AmazonEC2ContainerRegistryReadOnly, without write access, should suffice. The master policy is probably overly permissive. The security conscious may want to lock-down the IAM policies further (#11936).

We should make it easier to extend IAM permissions and also ensure that they are correctly configured (#14226).

Tagging

All AWS resources are tagged with a tag named "KubernetesCluster", with a value that is the unique cluster-id. This tag is used to identify a particular 'instance' of Kubernetes, even if two clusters are deployed into the same VPC. Resources are considered to belong to the same cluster if and only if they have the same value in the tag named "KubernetesCluster". (The kube-up script is not configured to create multiple clusters in the same VPC by default, but it is possible to create another cluster in the same VPC.)

Within the AWS cloud provider logic, we filter requests to the AWS APIs to match resources with our cluster tag. By filtering the requests, we ensure that we see only our own AWS objects.

** Important: ** If you choose not to use kube-up, you must pick a unique cluster-id value, and ensure that all AWS resources have a tag with Name=KubernetesCluster,Value=<clusterid>.

AWS objects

The kube-up script does a number of things in AWS:

  • Creates an S3 bucket (AWS_S3_BUCKET) and then copies the Kubernetes distribution and the salt scripts into it. They are made world-readable and the HTTP URLs are passed to instances; this is how Kubernetes code gets onto the machines.
  • Creates two IAM profiles based on templates in cluster/aws/templates/iam:
    • kubernetes-master is used by the master.
    • kubernetes-node is used by nodes.
  • Creates an AWS SSH key named kubernetes-<fingerprint>. Fingerprint here is the OpenSSH key fingerprint, so that multiple users can run the script with different keys and their keys will not collide (with near-certainty). It will use an existing key if one is found at AWS_SSH_KEY, otherwise it will create one there. (With the default Ubuntu images, if you have to SSH in: the user is ubuntu and that user can sudo).
  • Creates a VPC for use with the cluster (with a CIDR of 172.20.0.0/16) and enables the dns-support and dns-hostnames options.
  • Creates an internet gateway for the VPC.
  • Creates a route table for the VPC, with the internet gateway as the default route.
  • Creates a subnet (with a CIDR of 172.20.0.0/24) in the AZ KUBE_AWS_ZONE (defaults to us-west-2a). Currently, each Kubernetes cluster runs in a single AZ on AWS. Although, there are two philosophies in discussion on how to achieve High Availability (HA):
    • cluster-per-AZ: An independent cluster for each AZ, where each cluster is entirely separate.
    • cross-AZ-clusters: A single cluster spans multiple AZs. The debate is open here, where cluster-per-AZ is discussed as more robust but cross-AZ-clusters are more convenient.
  • Associates the subnet to the route table
  • Creates security groups for the master (kubernetes-master-<clusterid>) and the nodes (kubernetes-node-<clusterid>).
  • Configures security groups so that masters and nodes can communicate. This includes intercommunication between masters and nodes, opening SSH publicly for both masters and nodes, and opening port 443 on the master for the HTTPS API endpoints.
  • Creates an EBS volume for the master of size MASTER_DISK_SIZE and type MASTER_DISK_TYPE.
  • Launches a master with a fixed IP address (172.20.0.9) that is also configured for the security group and all the necessary IAM credentials. An instance script is used to pass vital configuration information to Salt. Note: The hope is that over time we can reduce the amount of configuration information that must be passed in this way.
  • Once the instance is up, it attaches the EBS volume and sets up a manual routing rule for the internal network range (MASTER_IP_RANGE, defaults to 10.246.0.0/24).
  • For auto-scaling, on each nodes it creates a launch configuration and group. The name for both is <KUBE_AWS_INSTANCE_PREFIX>-node-group. The default name is kubernetes-node-group. The auto-scaling group has a min and max size that are both set to NUM_NODES. You can change the size of the auto-scaling group to add or remove the total number of nodes from within the AWS API or Console. Each nodes self-configures, meaning that they come up; run Salt with the stored configuration; connect to the master; are assigned an internal CIDR; and then the master configures the route-table with the assigned CIDR. The kube-up script performs a health-check on the nodes but it's a self-check that is not required.

If attempting this configuration manually, it is recommend to follow along with the kube-up script, and being sure to tag everything with a tag with name KubernetesCluster and value set to a unique cluster-id. Also, passing the right configuration options to Salt when not using the script is tricky: the plan here is to simplify this by having Kubernetes take on more node configuration, and even potentially remove Salt altogether.

Manual infrastructure creation

While this work is not yet complete, advanced users might choose to manually create certain AWS objects while still making use of the kube-up script (to configure Salt, for example). These objects can currently be manually created:

  • Set the AWS_S3_BUCKET environment variable to use an existing S3 bucket.
  • Set the VPC_ID environment variable to reuse an existing VPC.
  • Set the SUBNET_ID environment variable to reuse an existing subnet.
  • If your route table has a matching KubernetesCluster tag, it will be reused.
  • If your security groups are appropriately named, they will be reused.

Currently there is no way to do the following with kube-up:

  • Use an existing AWS SSH key with an arbitrary name.
  • Override the IAM credentials in a sensible way (#14226).
  • Use different security group permissions.
  • Configure your own auto-scaling groups.

If any of the above items apply to your situation, open an issue to request an enhancement to the kube-up script. You should provide a complete description of the use-case, including all the details around what you want to accomplish.

Instance boot

The instance boot procedure is currently pretty complicated, primarily because we must marshal configuration from Bash to Salt via the AWS instance script. As we move more post-boot configuration out of Salt and into Kubernetes, we will hopefully be able to simplify this.

When the kube-up script launches instances, it builds an instance startup script which includes some configuration options passed to kube-up, and concatenates some of the scripts found in the cluster/aws/templates directory. These scripts are responsible for mounting and formatting volumes, downloading Salt and Kubernetes from the S3 bucket, and then triggering Salt to actually install Kubernetes.

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