Linkerd 2 Integration

Linkerd 2 is a zero-config and ultra-lightweight service mesh. Ambassador Edge Stack 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.

Ambassador Edge Stack 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 Ambassador to initiate mTLS connections to upstream services by grabbing a certificate from a Kubernetes Secret. However, Linkerd 2 does not work this way, so Ambassador must rely on Linkerd 2 for mTLS connections to upstream services. This means we want Linkerd 2 to inject its sidecar into Ambassador's pods, but not Istio and Consul.

Through that setup, Ambassador Edge Stack 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 Ambassador Edge Stack to dynamically route requests to that service based on Linkerd 2's service discovery data. If you already have Ambassador Edge Stack 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.

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:
bash
```
# 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
```
Now it is time to install Linkerd 2 itself. To do so execute the following command:
bash
```
linkerd install --ha | kubectl apply -f -
```
This will install Linkerd 2 in high-availability mode for the control plane. This means the controller and other components are started multiple times. Since Linkerd 2.5 it is also made sure the components are split across different nodes, if possible.
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!
Deploy Ambassador Edge Stack.
Note: If this is your first time deploying Ambassador Edge Stack, reviewing the Ambassador Edge Stack getting started is strongly recommended.
Terminal
```
kubectl apply -f https://www.getambassador.io/yaml/ambassador/ambassador-rbac.yaml
```
If you're on GKE, or haven't previously created the Ambassador Edge Stack service, please see the quick start guide.
Configure Ambassador Edge Stack to add Linkerd 2 Headers to requests.
yaml
```
---
apiVersion: getambassador.io/v2
kind: Module
metadata:
  name: ambassador
spec:
  config:
    add_linkerd_headers: true
```
This will tell Ambassador Edge Stack 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 Ambassador Edge Stack can route to this application using endpoint data from Linkerd 2.

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

Deploy the QOTM demo application.

---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: qotm
spec:
  replicas: 1
  strategy:
    type: RollingUpdate
  template:
    metadata:
      labels:
        app: qotm
    spec:
      containers:
      - name: qotm
        image: docker.io/datawire/qotm:1.7
        ports:
        - name: http-api
          containerPort: 5000
        env:
        - name: POD_IP
          valueFrom:
            fieldRef:
              fieldPath: status.podIP
        readinessProbe:
          httpGet:
            path: /health
            port: 5000
          initialDelaySeconds: 30
          periodSeconds: 3
        resources:
          limits:
            cpu: "0.1"
            memory: 100Mi

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

kubectl apply -f qotm.yaml

Watch via kubectl get pod -w as the Pod is created. Note that it starts with 0/2 containers automatically, as it has been auto-injected by the Linkerd 2 Webhook.

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.
shell
```
linkerd 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.

Create a Mapping for the qotm-Linkerd2 service.

---
apiVersion: getambassador.io/v2
kind: Mapping
metadata:
  name: linkerd2-qotm
spec:
  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 Ambassador Edge Stack already adds Linkerd Headers when forwarding requests to the service mesh.

Send a request to the qotm-Linkerd2 API.

curl -L http://$AMBASSADOR_IP/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 Ambassador Edge Stack is not TLS secured, but from Ambassador Edge Stack to the QOTM an automatic mTLS connection is being used.

If you now configure TLS termination in Ambassador Edge Stack, 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 the Ambassador 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 Ambassador as the ingress gateway between clusters, you eliminate the need to deploy and manage an additional ingress gateway.

Initial Multicluster Setup

Install Ambassador and the Linkerd multicluster control plane. Make sure you've also linked the clusters.
Inject the Ambassador deployment with Linkerd (even if you have AutoInject enabled):
Terminal
```
kubectl -n ambassador get deploy ambassador -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 Ambassador handle its own ports. AutoInject can't handle this on its own.)
Configure Ambassador as normal for your application. Don't forget to set add_linkerd_headers: true!

At this point, your Ambassador 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 Ambassador 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 the Ambassador as the target gateway.

Configure the target cluster Ambassador to allow insecure routing.
When Ambassador 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 Ambassador. 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
yaml
```
apiVersion: getambassador.io/v2
kind: Host
metadata:
  name: linkerd-host
spec:
  hostname: host.example.com
  acmeProvider:
    authority: none
  requestPolicy:
    insecure:
      action: Route
```
Configure the target cluster Ambassador 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:
yaml
```
apiVersion: getambassador.io/v2
kind: Mapping
metadata:
  name: public-health-check
  namespace: ambassador
spec:
  prefix: /-/ambassador/ready
  rewrite: /ambassador/v0/check_ready
  service: localhost:8877
  bypass_auth: true
```
When configuring Ambassador, 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.)

Configure the target cluster Ambassador for the service mirror controller.

This requires changes to the Ambassador'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 ambassador patch deploy ambassador -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 Ambassador is listening)

kubectl -n ambassador patch svc ambassador --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 ambassador patch svc ambassador -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.)

Configure individual exported services. Adding the following annotations to a service will tell the service to use Ambassador as the gateway:
bash
```
kubectl -n $namespace patch svc $service -p='
metadata:
    annotations:
        mirror.linkerd.io/gateway-name: ambassador
        mirror.linkerd.io/gateway-ns: ambassador
'
```
This annotation will tell Linkerd that the given service can be reached via the Ambassador in the ambassador namespace.
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:
bash
```
linkerd check --multicluster
```
Next, make sure that the Ambassador gateway shows up when listing active gateways:
bash
```
linkerd multicluster gateways
```
At this point, all should be well!

More information

For more about Ambassador Edge Stack'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.

Questions?

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