» Connect Sidecar on Kubernetes

Connect is a feature built into to Consul that enables automatic service-to-service authorization and connection encryption across your Consul services. Connect can be used with Kubernetes to secure pod communication with other pods and external Kubernetes services.

The Connect sidecar running Envoy can be automatically injected into pods in your cluster, making configuration for Kubernetes automatic. This functionality is provided by the consul-k8s project and can be automatically installed and configured using the Consul Helm chart.

» Usage

When the Connect injector is installed, the Connect sidecar is automatically added to all pods. This sidecar can both accept and establish connections using Connect, enabling the pod to communicate to clients and dependencies exclusively over authorized and encrypted connections.

» Accepting Inbound Connections

An example pod is shown below with Connect enabled to accept inbound connections. Notice that the pod would still be fully functional without Connect. Minimal to zero modifications are required to pod specifications to enable Connect in Kubernetes.

This pod specification starts a server that responds to any HTTP request with the static text "hello world".

apiVersion: v1
kind: Pod
metadata:
  name: static-server
  annotations:
    "consul.hashicorp.com/connect-inject": "true"
spec:
  containers:
    - name: static-server
      image: hashicorp/http-echo:latest
      args:
        - -text="hello world"
        - -listen=:8080
      ports:
        - containerPort: 8080
          name: http

The only change for Connect is the addition of the consul.hashicorp.com/connect-inject annotation. This enables injection for this pod. The injector can also be configured to automatically inject unless explicitly disabled, but the default installation requires opt-in using the annotation shown above.

This will start a Connect sidecar that listens on a random port registered with Consul and proxies valid inbound connections to port 8080 in the pod. To establish a connection to the pod using Connect, a client must use another Connect proxy. The client Connect proxy will use Consul service discovery to find all available upstream proxies and their public ports.

In the example above, the server is listening on :8080. This means the server will still bind to the pod IP and allow external connections. This is useful to transition to Connect by allowing both Connect and non-Connect connections. To restrict access to only Connect-authorized clients, any listeners should bind to localhost only (such as 127.0.0.1).

» Connecting to Connect-Enabled Services

The example pod specification below configures a pod that is capable of establishing connections to our previous example "static-server" service. The connection to this static text service happens over an authorized and encrypted connection via Connect.

apiVersion: v1
kind: Pod
metadata:
  name: static-client
  annotations:
    "consul.hashicorp.com/connect-inject": "true"
    "consul.hashicorp.com/connect-service-upstreams": "static-server:1234"
spec:
  containers:
    - name: static-client
      image: tutum/curl:latest
      # Just spin & wait forever, we'll use `kubectl exec` to demo
      command: [ "/bin/sh", "-c", "--" ]
      args: [ "while true; do sleep 30; done;" ]

Pods must specify upstream dependencies with the consul.hashicorp.com/connect-service-upstreams annotation. This annotation declares the names of any upstream dependencies and a local port for the proxy to listen on. When a connection is established to that local port, the proxy establishes a connection to the target service ("static-server" in this example) using mutual TLS and identifying as the source service ("static-client" in this example).

The injector will also set environment variables <NAME>_CONNECT_SERVICE_HOST and <NAME>_CONNECT_SERVICE_PORT in every container in the pod for every defined upstream. This is analogous to the standard Kubernetes service environment variables, but point instead to the correct local proxy port to establish connections via Connect.

Any containers running in the pod that need to establish connections to dependencies must be reconfigured to use the local upstream address either directly or using the environment variables set by the injector (defined above). This means pods should not use Kubernetes service DNS or environment variables for these connections.

We can verify access to the static text server using kubectl exec. Notice that we use the local address and port from the upstream annotation (1234) for this verification.

$ kubectl exec static-client -- curl -s http://127.0.0.1:1234/
"hello world"

We can control access to the server using intentions. If you use the Consul UI or CLI to create a deny intention between "static-client" and "static-server", connections are immediately rejected without updating either of the running pods. You can then remove this intention to allow connections again.

$ kubectl exec static-client -- curl -s http://127.0.0.1:1234/
command terminated with exit code 52

» Available Annotations

Annotations can be used to configure the injection behavior.

  • consul.hashicorp.com/connect-inject - If this is "true" then injection is enabled. If this is "false" then injection is explicitly disabled. The default injector behavior requires pods to opt-in to injection by specifying this value as "true". This default can be changed in the injector's configuration if desired.

  • consul.hashicorp.com/connect-service - For pods that accept inbound connections, this specifies the name of the service that is being served. This defaults to the name of the first container in the pod.

  • consul.hashicorp.com/connect-service-port - For pods that accept inbound connections, this specifies the port to route inbound connections to. This is the port that the service is listening on. The service port defaults to the first exposed port on any container in the pod. If specified, the value can be the name of a configured port, such as "http" or it can be a direct port value such as "8080". This is the port of the service, the proxy public listener will listen on a dynamic port.

  • consul.hashicorp.com/connect-service-upstreams - The list of upstream services that this pod needs to connect to via Connect along with a static local port to listen for those connections.

    • Services

      The name of the service is the name of the service registered with Consul. You can optionally specify datacenters with this annotation.

      "consul.hashicorp.com/connect-service-upstreams":"[service-name]:[port]:[optional datacenter]"
      
    • Prepared Query

      "consul.hashicorp.com/connect-service-upstreams": "prepared_query:[query name]:[port]"
      
    • Multiple Upstreams

      If you would like to specify multiple services or upstreams, delimit them with commas

      "consul.hashicorp.com/connect-service-upstreams":"[service-name]:[port]:[optional datacenter],[service-name]:[port]:[optional datacenter]"
      
      "consul.hashicorp.com/connect-service-upstreams":"[service-name]:[port]:[optional datacenter],prepared_query:[query name]:[port]"
      
  • consul.hashicorp.com/connect-service-protocol - For pods that will be registered with Consul's central configuration feature, information about the protocol the service uses is required. Users can define the protocol directly using this annotation on the pod spec, or by defining a default value for all services using the Helm chart's defaultProtocol option. Specific annotations will always override the default value.

» Deployments, StatefulSets, etc.

The annotations for configuring Connect must be on the pod specification. Since higher level resources such as Deployments wrap pod specification templates, Connect can be used with all of these higher level constructs, too.

An example Deployment below shows how to enable Connect injection:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: consul-example-deployment
spec:
  replicas: 1
  selector:
    matchLabels:
      app: consul-example
  template:
    metadata:
      labels:
        app: consul-example
      annotations:
        "consul.hashicorp.com/connect-inject": "true"
    spec:
      containers:
        - name: example
          image: "nginx"

» Installation and Configuration

The Connect sidecar proxy is injected via a mutating admission webhook provided by the consul-k8s project. This enables the automatic pod mutation shown in the usage section above. Installation of the mutating admission webhook is automated using the Helm chart.

To install the Connect injector, enable the Connect injection feature using Helm values and upgrade the installation using helm upgrade for existing installs or helm install for a fresh install. The Connect injector also requires client agents are enabled on the node with pods that are using Connect and that gRPC is enabled.

connectInject:
  enabled: true

client:
  enabled: true
  grpc: true

This will configure the injector to inject when the injection annotation is present. Other values in the Helm chart can be used to limit the namespaces the injector runs in, enable injection by default, and more.

As noted above, the Connect auto-injection requires that local client agents are configured. These client agents must be successfully joined to a Consul cluster. The Consul server cluster can run either in or out of a Kubernetes cluster.

» Verifying the Installation

To verify the installation, run the "Accepting Inbound Connections" example from the "Usage" section above. After running this example, run kubectl get pod static-server -o yaml. In the raw YAML output, you should see injected Connect containers and an annotation consul.hashicorp.com/connect-inject-status set to injected. This confirms that injection is working properly.

If you do not see this, then use kubectl logs against the injector pod and note any errors.