Linkerd 2

Linkerd 2 is a zero-config and ultra-lightweight service mesh. Emissary natively supports Linkerd 2 for service discovery, end-to-end TLS (including mTLS between services), and (with Linkerd 2.8) multicluster operation.

Architecture

Linkerd 2 is designed for simplicity, security, and performance. In the cluster, it runs a control plane in its own namespace and then injects sidecar proxy containers in every Pod that should be meshed.

Emissary itself also needs to be interwoven or “meshed” with Linkerd 2, and then configured to add special Linkerd headers to requests that tell Linkerd 2 where to forward them. This ie because mTLS between services is automatically handled by the control plane and the proxies. Istio and Consul allow Emissary to initiate mTLS connections to upstream services by grabbing a certificate from a Kubernetes Secret. However, Linkerd 2 does not work this way, so Emissary must rely on Linkerd 2 for mTLS connections to upstream services. This means we want Linkerd 2 to inject its sidecar into Emissary’s pods, but not Istio and Consul.

Through that setup, Emissary terminates external TLS as the gateway and traffic is then immediately wrapped into mTLS by Linkerd 2 again. Thus we have a full end-to-end TLS encryption chain.

Getting started

In this guide, you will use Linkerd 2 Auto-Inject to mesh a service and use Emissary to dynamically route requests to that service based on Linkerd 2’s service discovery data. If you already have Emissary installed, you will just need to install Linkerd 2 and deploy your service.

Setting up Linkerd 2 requires to install three components. The first is the CLI on your local machine, the second is the actual Linkerd 2 control plane in your Kubernetes Cluster. Finally, you have to inject your services’ Pods with Linkerd Sidecars to mesh them.

1. Install and configure Linkerd 2 instructions. Follow the guide until Step 3. That should give you the CLI on your machine and all required pre-flight checks.

   In a nutshell, these steps boil down to the following:

   # install linkerd cli tool
   curl -sL https://run.linkerd.io/install | sh
   # add linkerd to your path
   export PATH=$PATH:$HOME/.linkerd2/bin
   # verify installation
   linkerd version
   

2. Now it is time to install Linkerd 2 itself. To do so execute the following command:

   # install the Linkerd control plane
   
   linkerd install | kubectl apply -f -
   
   linkerd check
   
   # install the Linkerd dashboard component
   
   linkerd viz install | kubectl apply -f -
   
   linkerd viz check
   

   This will install Linkerd 2 in your cluster. For more details on installing Linkerd visit their docs.

   Note that this simple command automatically enables mTLS by default and registers the AutoInject Webhook with your Kubernetes API Server. You now have a production-ready Linkerd 2 setup rolled out into your cluster!

3. Deploy Emissary if you have not already, by following the Emissary Getting Started guide.

4. Configure Emissary to add it to the Linkerd 2 service mesh.

   kubectl -n $namespace get deploy $deployment -o yaml | \
   linkerd inject \
   --skip-inbound-ports 80,443 - | \
   kubectl apply -f -
   

   This will tell Emissary to add additional headers to each request forwarded to Linkerd 2 with information about where to route this request to. This is a general setting. You can also set add_linkerd_headers per Mapping.

Routing to Linkerd 2 services

You’ll now register a demo application with Linkerd 2, and show how Emissary can route to this application using endpoint data from Linkerd 2.

1. Enable AutoInjection on the Namespace you are about to deploy to:

   apiVersion: v1
   kind: Namespace
   metadata:
     name: default # change this to your namespace if you're not using 'default'
     annotations:
       linkerd.io/inject: enabled
   

   Save the above to a file called namespace.yaml and run kubectl apply -f namespace.yaml. This will enable the namespace to be handled by the AutoInjection Webhook of Linkerd 2. Every time something is deployed to that namespace, the deployment is passed to the AutoInject Controller and injected with the Linkerd 2 proxy sidecar automatically.

2. Deploy the QOTM demo application.

   ---
   apiVersion: apps/v1
   kind: Deployment
   metadata:
     name: qotm
     namespace: default
   spec:
     replicas: 1
     selector:
       matchLabels:
         app: qotm
     template:
       metadata:
         labels:
           app: qotm
       spec:
         containers:
         - name: qotm
           image: docker.io/datawire/qotm:$qotmVersion$
           ports:
           - name: http-api
             containerPort: 5000
           env:
           - name: POD_IP
             valueFrom:
               fieldRef:
                 fieldPath: status.podIP
           readinessProbe:
             httpGet:
               path: /health
               port: 5000
             initialDelaySeconds: 60
             periodSeconds: 3
           resources:
             limits:
               cpu: "0.1"
               memory: 100Mi
   ---
   apiVersion: v1
   kind: Service
   metadata:
     name: qotm-linkerd2
     namespace: default
   spec:
     ports:
     - name: http
       port: 80
       targetPort: 5000
     selector:
       app: qotm
   ---
   

   Save the above to a file called qotm.yaml and deploy it with

   kubectl apply -f qotm.yaml
   

3. Verify the QOTM pod has been registered with Linkerd 2. You can verify the QOTM pod is registered correctly by accessing the Linkerd 2 Dashboard.

   linkerd viz dashboard
   

   Your browser should automatically open the correct URL. Otherwise, note the output from the above command and open that in a browser of your choice.

4. Create a Mapping for the qotm-Linkerd2 service.

   ---
   apiVersion: getambassador.io/v3alpha1
   kind: Mapping
   metadata:
     name: linkerd2-qotm
   spec:
     hostname: "*"
     prefix: /qotm-linkerd2/
     service: qotm-linkerd2
   

Save the above YAML to a file named qotm-mapping.yaml, and apply it with:

kubectl apply -f qotm-mapping.yaml

to apply this configuration to your Kubernetes cluster. Note that in the above config there is nothing special to make it work with Linkerd 2. The general config for Emissary already adds Linkerd Headers when forwarding requests to the service mesh.

1. Send a request to the qotm-Linkerd2 API.

   curl -Lki http://$LB_ENDPOINT/qotm-linkerd2/
   
   {"hostname":"qotm-749c675c6c-hq58f","ok":true,"quote":"The last sentence you read is often sensible nonsense.","time":"2019-03-29T22:21:42.197663","version":"1.7"}
   

Congratulations! You’re successfully routing traffic to the QOTM application, the location of which is registered in Linkerd 2. The traffic to Emissary is not TLS secured, but from Emissary to the QOTM an automatic mTLS connection is being used.

If you now configure TLS termination in Emissary, you have an end-to-end secured connection.

Multicluster operation

Linkerd 2.8 can support multicluster operation, where the Linkerd mesh transparently bridges from one cluster to another, allowing seamless access between the two. This works using the Linkerd “service mirror controller” to discover services in the target cluster, and expose (mirror) them in the source cluster. Requests to mirrored services in the source cluster are transparently proxied via Emissary in the target cluster to the appropriate target service, using Linkerd’s automatic mTLS to protect the requests in flight between clusters. By configuring Linkerd to use the existing Emissary as the ingress gateway between clusters, you eliminate the need to deploy and manage an additional ingress gateway.

Initial multicluster setup

1. Install Emissary and the Linkerd multicluster control plane. Make sure you’ve also linked the clusters.

2. Inject Emissary deployment with Linkerd (even if you have AutoInject enabled):

   kubectl -n emissary get deploy emissary-ingress -o yaml | \
     linkerd inject \
     --skip-inbound-ports 80,443 \
     --require-identity-on-inbound-ports 4183 - | \
     kubectl apply -f -
   

   (It’s important to require identity on the gateway port so that automatic mTLS works, but it’s also important to let Emissary handle its own ports. AutoInject can’t handle this on its own.)

3. Configure Emissary as normal for your application.

At this point, your Emissary installation should work fine with multicluster Linkerd as a source cluster: you can configure Linkerd to bridge to a target cluster, and all should be well.

Using the cluster as a target cluster

Allowing the Emissary installation to serve as a target cluster requires explicitly giving permission for Linkerd to mirror services from the cluster, and explicitly telling Linkerd to use Emissary as the target gateway.

1. Configure the target cluster Emissary to allow insecure routing.

   When Emissary is running in a Linkerd mesh, Linkerd provides transport security, so connections coming in from the Linkerd in the source cluster will always be HTTP when they reach Emissary. Therefore, the Host CRDs corresponding to services that you’ll be accessing from the source cluster must be configured to Route insecure requests. More information on this topic is available in the Host documentation; an example might be

   apiVersion: getambassador.io/v3alpha1
   kind: Host
   metadata:
     name: linkerd-host
   spec:
     hostname: host.example.com
     acmeProvider:
       authority: none
     requestPolicy:
       insecure:
         action: Route
   

2. Configure the target cluster Emissary to support Linkerd health checks.

   Multicluster Linkerd does its own health checks beyond what Kubernetes does, so a Mapping is needed to allow Linkerd’s health checks to succeed:

   apiVersion: getambassador.io/v3alpha1
   kind: Mapping
   metadata:
     name: public-health-check
     namespace: ambassador
   spec:
     hostname: "*"
     prefix: /-/ambassador/ready
     rewrite: /ambassador/v0/check_ready
     service: localhost:8877
     bypass_auth: true
   

   When configuring Emissary, Kubernetes is usually configured to run health checks directly against port 8877 – however, that port is not meant to be exposed outside the cluster. The Mapping permits accessing the health check endpoint without directly exposing the port.

   (The actual prefix in the Mapping is not terribly important, but it needs to match the metadata supplied to the service mirror controller, below.)

3. Configure the target cluster Emissary for the service mirror controller.

   This requires changes to the Emissary’s deployment and service. For all of these commands, you will need to make sure your Kubernetes context is set to talk to the target cluster.

   In the deployment, you need the config.linkerd.io/enable-gateway annotation:

   kubectl -n emissary patch deploy emissary-ingress -p='
   spec:
       template:
           metadata:
               annotations:
                   config.linkerd.io/enable-gateway: "true"
   '
   

   In the service, you need to provide appropriate named port definitions:

   - `mc-gateway` needs to be defined as `port` 4143
   - `mc-probe` needs to be defined as `port` 80, `targetPort` 8080 (or wherever Emissary is listening)
   

   kubectl -n emissary patch svc emissary-ingress --type='json' -p='[
           {"op":"add","path":"/spec/ports/-", "value":{"name": "mc-gateway", "port": 4143}},
           {"op":"replace","path":"/spec/ports/0", "value":{"name": "mc-probe", "port": 80, "targetPort": 8080}}
       ]'
   

   Finally, the service also needs its own set of annotations:

   kubectl -n emissary patch svc emissary-ingress -p='
   metadata:
       annotations:
           mirror.linkerd.io/gateway-identity: ambassador.ambassador.serviceaccount.identity.linkerd.cluster.local
           mirror.linkerd.io/multicluster-gateway: "true"
           mirror.linkerd.io/probe-path: -/ambassador/ready
           mirror.linkerd.io/probe-period: "3"
   '
   

   (Here, the value of mirror.linkerd.io/probe-path must match the prefix using for the probe Mapping above.)

4. Configure individual exported services. Adding the following annotations to a service will tell the service to use Emissary as the gateway:

   kubectl -n $namespace patch svc $service -p='
   metadata:
       annotations:
           mirror.linkerd.io/gateway-name: emissary-ingress
           mirror.linkerd.io/gateway-ns: emissary-ingress
   '
   

   This annotation will tell Linkerd that the given service can be reached via the Emissary in the emissary namespace.

5. Verify that all is well from the source cluster.

   For all of these commands, you’ll need to set your Kubernetes context for the source cluster.

   First, check to make that the clusters are correctly linked:

   linkerd check --multicluster
   

   Next, make sure that the Emissary gateway shows up when listing active gateways:

   linkerd multicluster gateways
   

   At this point, all should be well!

More information

For more about Emissary’s integration with Linkerd 2, read the service discovery configuration documentation. For further reading about Linkerd 2 multi-cluster, see the install documentation and introduction.