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Restricting Access To Services

Linkerd policy resources can be used to restrict which clients may access a service. In this example, we’ll use Emojivoto to show how to restrict access to the Voting service so that it may only be called from the Web service.

For a more comprehensive description of the policy resources, see the Policy reference docs.

Prerequisites

To use this guide, you’ll need to have Linkerd installed on your cluster, along with its Viz extension. Follow the Installing Linkerd Guide if you haven’t already done this.

Setup

Inject and install the Emojivoto application:

$ linkerd inject https://run.linkerd.io/emojivoto.yml | kubectl apply -f -
...
$ linkerd check -n emojivoto --proxy -o short
...

Creating a Server resource

We start by creating a Server resource for the Voting service. A Server is a Linkerd custom resource which describes a specific port of a workload. Once the Server resource has been created, only clients which have been authorized may access it (we’ll see how to authorize clients in a moment).

kubectl apply -f - <<EOF
---
apiVersion: policy.linkerd.io/v1beta1
kind: Server
metadata:
  namespace: emojivoto
  name: voting-grpc
  labels:
    app: voting-svc
spec:
  podSelector:
    matchLabels:
      app: voting-svc
  port: grpc
  proxyProtocol: gRPC
EOF

We see that this Server uses a podSelector to select the pods that it describes: in this case the voting service pods. It also specifies the named port (grpc) that it applies to. Finally, it specifies the protocol that is served on this port. This ensures that the proxy treats traffic correctly and allows it skip protocol detection.

At this point, no clients have been authorized to access this service and you will likely see a drop in success rate as requests from the Web service to Voting start to get rejected.

We can use the linkerd viz authz command to look at the authorization status of requests coming to the voting service and see that all incoming requests to the voting-grpc server are currently unauthorized:

> linkerd viz authz -n emojivoto deploy/voting
ROUTE    SERVER                       AUTHORIZATION                UNAUTHORIZED  SUCCESS     RPS  LATENCY_P50  LATENCY_P95  LATENCY_P99
default  default:all-unauthenticated  default/all-unauthenticated        0.0rps  100.00%  0.1rps          1ms          1ms          1ms
probe    default:all-unauthenticated  default/probe                      0.0rps  100.00%  0.2rps          1ms          1ms          1ms
default  voting-grpc                                                     1.0rps    0.00%  0.0rps          0ms          0ms          0ms

Creating a ServerAuthorization resource

A ServerAuthorization grants a set of clients access to a set of Servers. Here we will create a ServerAuthorization which grants the Web service access to the Voting Server we created above. Note that meshed mTLS uses ServiceAccounts as the basis for identity, thus our authorization will also be based on ServiceAccounts.

kubectl apply -f - <<EOF
---
apiVersion: policy.linkerd.io/v1beta1
kind: ServerAuthorization
metadata:
  namespace: emojivoto
  name: voting-grpc
  labels:
    app.kubernetes.io/part-of: emojivoto
    app.kubernetes.io/name: voting
    app.kubernetes.io/version: v11
spec:
  server:
    name: voting-grpc
  # The voting service only allows requests from the web service.
  client:
    meshTLS:
      serviceAccounts:
        - name: web
EOF

With this in place, we can now see that all of the requests to the Voting service are authorized by the voting-grpc ServerAuthorization. Note that since the linkerd viz auth command queries over a time-window, you may see some UNAUTHORIZED requests displayed for a short amount of time.

> linkerd viz authz -n emojivoto deploy/voting
ROUTE    SERVER                       AUTHORIZATION                    UNAUTHORIZED  SUCCESS     RPS  LATENCY_P50  LATENCY_P95  LATENCY_P99
default  default:all-unauthenticated  default/all-unauthenticated            0.0rps  100.00%  0.1rps          1ms          1ms          1ms
probe    default:all-unauthenticated  default/probe                          0.0rps  100.00%  0.2rps          1ms          1ms          1ms
default  voting-grpc                  serverauthorization/voting-grpc        0.0rps   83.87%  1.0rps          1ms          1ms          1ms

We can also test that request from other pods will be rejected by creating a grpcurl pod and attempting to access the Voting service from it:

> kubectl run grpcurl --rm -it --image=networld/grpcurl --restart=Never --command -- ./grpcurl -plaintext voting-svc.emojivoto:8080 emojivoto.v1.VotingService/VoteDog
Error invoking method "emojivoto.v1.VotingService/VoteDog": failed to query for service descriptor "emojivoto.v1.VotingService": rpc error: code = PermissionDenied desc =
pod "grpcurl" deleted
pod default/grpcurl terminated (Error)

Because this client has not been authorized, this request gets rejected with a PermissionDenied error.

You can create as many ServerAuthorization resources as you like to authorize many different clients. You can also specify whether to authorize unauthenticated (i.e. unmeshed) client, any authenticated client, or only authenticated clients with a particular identity. For more details, please see the Policy reference docs.

Setting a Default Policy

To further lock down a cluster, you can set a default policy which will apply to all ports which do not have a Server resource defined. Linkerd uses the following logic when deciding whether to allow a request:

  • If the port has a Server resource and the client matches a ServerAuthorization resource for it: ALLOW
  • If the port has a Server resource but the client does not match any ServerAuthorizations for it: DENY
  • If the port does not have a Server resource: use the default policy

We can set the default policy to deny using the linkerd upgrade command:

> linkerd upgrade --default-inbound-policy deny | kubectl apply -f -

Alternatively, default policies can be set on individual workloads or namespaces by setting the config.linkerd.io/default-inbound-policy annotation. See the Policy reference docs for more details.

If a port does not have a Server defined, Linkerd will automatically use a default Server which allows readiness and liveness probes. However, if you create a Server resource for a port which handles probes, you will need to explicitly create an authorization to allow those probe requests. For more information about adding route-scoped authorizations, see Configuring Per-Route Policy.

Further Considerations

You may have noticed that there was a period of time after we created the Server resource but before we created the ServerAuthorization where all requests were being rejected. To avoid this situation in live systems, we recommend you either create the policy resources before deploying your services or to create the ServiceAuthorizations BEFORE creating the Server so that clients will be authorized immediately.

Per-Route Policy

In addition to service-level authorization policy, authorization policy can also be configured for individual HTTP routes. To learn more about per-route policy, see the documentation on configuring per-route policy.