---
layout: docs
page_title: How does Consul Service Mesh Work on Kubernetes?
description: >-
  An injection annotation allows Consul to automatically deploy sidecar proxies on Kubernetes pods, enabling Consul's service mesh for containers running on k8s. Learn how to configure sidecars, enable services with multiple ports, change default injection settings.
---

# How does Consul Service Mesh Work on Kubernetes?

[Consul service mesh](/consul/docs/connect) is a feature built into to Consul that enables
automatic service-to-service authorization and connection encryption across
your Consul services. Consul Service Mesh can be used with Kubernetes to secure pod
communication with other pods and external Kubernetes services.

The noun _connect_ is used throughout this documentation to refer to the connect
subsystem that provides Consul's service mesh capabilities.
Where you encounter the _noun_ connect, it is usually functionality specific to
service mesh.

Consul can automatically inject the sidecar running Envoy into pods in
your cluster, making configuration for Kubernetes automatic.
This functionality is provided by the
[consul-k8s project](https://github.com/hashicorp/consul-k8s) and can be
automatically installed and configured using the
[Consul Helm chart](/consul/docs/k8s/installation/install#helm-chart-installation).

## Usage

-> **Important:** As of consul-k8s `v0.26.0` and Consul Helm `v0.32.0`, a Kubernetes
service is required to run services on the Consul service mesh.

Installing Consul on Kubernetes with [`connect-inject` enabled](/consul/docs/k8s/connect#installation-and-configuration) adds a sidecar to all pods. By default, it enables service mesh functionality with Consul Dataplane by injecting an Envoy proxy. You can also configure Consul to inject a client agent sidecar to connect to your service mesh. Refer to [Simplified Service Mesh with Consul Dataplane](/consul/docs/connect/dataplane) for more information.


```yaml
apiVersion: v1
kind: Service
metadata:
  # This name will be the service name in Consul.
  name: static-server
spec:
  selector:
    app: static-server
  ports:
    - protocol: TCP
      port: 80
      targetPort: 8080
---
apiVersion: v1
kind: ServiceAccount
metadata:
  name: static-server
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: static-server
spec:
  replicas: 1
  selector:
    matchLabels:
      app: static-server
  template:
    metadata:
      name: static-server
      labels:
        app: 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
      # If ACLs are enabled, the serviceAccountName must match the Consul service name.
      serviceAccountName: static-server
```

The only change for service mesh is the addition of the
`consul.hashicorp.com/connect-inject` annotation. This enables injection
for the Pod in this Deployment. The injector can also be
[configured](/consul/docs/k8s/connect#installation-and-configuration)
to automatically inject unless explicitly disabled, but the default
installation requires opt-in using the annotation shown above.

~> **A common mistake** is to set the annotation on the Deployment or
other resource. Ensure that the injector annotations are specified on
the _pod specification template_ as shown above.

This will start a sidecar proxy that listens on port `20000` registered
with Consul and proxies valid inbound connections to port 8080 in the pod.
To establish a connection to the pod using service mesh, a client must use another mesh
proxy. The client mesh 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`.
By default, the Consul service mesh runs in [transparent proxy](/consul/docs/connect/transparent-proxy) mode.
This means that even though the server binds to all interfaces,
the inbound and outbound connections will automatically go through to the sidecar proxy.
It also allows you to use Kubernetes DNS like you normally would without the
Consul Service Mesh.

-> **Note:** As of consul `v1.10.0`, consul-k8s `v0.26.0` and Consul Helm `v0.32.0`,
all Consul Service Mesh services will run with transparent proxy enabled by default. Running with transparent
proxy will enforce all inbound and outbound traffic to go through the Envoy proxy.

The service name registered in Consul will be set to the name of the Kubernetes service
associated with the Pod. This can be customized with the `consul.hashicorp.com/connect-service`
annotation. If using ACLs, this name must be the same as the Pod's `ServiceAccount` name.

### Connecting to Mesh-Enabled Services

The example Deployment specification below configures a Deployment 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 service mesh.

-> **Note:** As of consul-k8s `v0.26.0` and Consul Helm `v0.32.0`, having a Kubernetes
Service is **required** to run services on the Consul Service Mesh.

```yaml
apiVersion: v1
kind: Service
metadata:
  # This name will be the service name in Consul.
  name: static-client
spec:
  selector:
    app: static-client
  ports:
    - port: 80
---
apiVersion: v1
kind: ServiceAccount
metadata:
  name: static-client
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: static-client
spec:
  replicas: 1
  selector:
    matchLabels:
      app: static-client
  template:
    metadata:
      name: static-client
      labels:
        app: static-client
      annotations:
        'consul.hashicorp.com/connect-inject': 'true'
    spec:
      containers:
        - name: static-client
          image: curlimages/curl:latest
          # Just spin & wait forever, we'll use `kubectl exec` to demo
          command: ['/bin/sh', '-c', '--']
          args: ['while true; do sleep 30; done;']
      # If ACLs are enabled, the serviceAccountName must match the Consul service name.
      serviceAccountName: static-client
```

By default when ACLs are enabled or when ACLs default policy is `allow`,
Consul will automatically configure proxies with all upstreams from the same datacenter.
When ACLs are enabled with default `deny` policy,
you must supply an [intention](/consul/docs/connect/intentions) to tell Consul which upstream you need to talk to.

When upstreams are specified explicitly with the
[`consul.hashicorp.com/connect-service-upstreams` annotation](/consul/docs/k8s/annotations-and-labels#consul-hashicorp-com-connect-service-upstreams),
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
service mesh.

You can verify access to the static text server using `kubectl exec`.
Because transparent proxy is enabled by default,
use Kubernetes DNS to connect to your desired upstream.

```shell-session
$ kubectl exec deploy/static-client -- curl --silent http://static-server/
"hello world"
```

You can control access to the server using [intentions](/consul/docs/connect/intentions).
If you use the Consul UI or [CLI](/consul/commands/intention/create) to
deny communication 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.

```shell-session
$ kubectl exec deploy/static-client -- curl --silent http://static-server/
command terminated with exit code 52
```

### Kubernetes Pods with Multiple ports
To configure a pod with multiple ports to be a part of the service mesh and receive and send service mesh traffic, you
will need to add configuration so that a Consul service can be registered per port. This is because services in Consul
currently support a single port per service instance.

In the following example, suppose we have a pod which exposes 2 ports, `8080` and `9090`, both of which will need to
receive service mesh traffic.

First, decide on the names for the two Consul services that will correspond to those ports. In this example, the user
chooses the names `web` for `8080` and `web-admin` for `9090`.

Create two service accounts for `web` and `web-admin`:
```yaml
apiVersion: v1
kind: ServiceAccount
metadata:
  name: web
---
apiVersion: v1
kind: ServiceAccount
metadata:
  name: web-admin
```
Create two Service objects for `web` and `web-admin`:
```yaml
apiVersion: v1
kind: Service
metadata:
  name: web
spec:
  selector:
    app: web
  ports:
    - protocol: TCP
      port: 80
      targetPort: 8080
---
apiVersion: v1
kind: Service
metadata:
  name: web-admin
spec:
  selector:
    app: web
  ports:
    - protocol: TCP
      port: 80
      targetPort: 9090
```
`web` will target `containerPort` `8080` and select pods labeled `app: web`. `web-admin` will target `containerPort`
`9090` and will also select the same pods.

~> Kubernetes 1.24+ only
In Kubernetes 1.24+ you need to [create a Kubernetes secret](https://kubernetes.io/docs/concepts/configuration/secret/#service-account-token-secrets) for each multi-port service that references the ServiceAccount, and the Kubernetes secret must have the same name as the ServiceAccount:

```yaml
apiVersion: v1
kind: Secret
metadata:
  name: web
  annotations:
    kubernetes.io/service-account.name: web
  type: kubernetes.io/service-account-token
---
apiVersion: v1
kind: Secret
metadata:
  name: web-admin
  annotations:
    kubernetes.io/service-account.name: web-admin
  type: kubernetes.io/service-account-token
```

Create a Deployment with any chosen name, and use the following annotations:
```yaml
consul.hashicorp.com/connect-inject: true
consul.hashicorp.com/transparent-proxy: false
consul.hashicorp.com/connect-service: web,web-admin
consul.hashicorp.com/connect-service-port: 8080,9090
```
Note that the order the ports are listed in the same order as the service names, i.e. the first service name `web`
corresponds to the first port, `8080`, and the second service name `web-admin` corresponds to the second port, `9090`.

The service account on the pod spec for the deployment should be set to the first service name `web`:
```yaml
serviceAccountName: web
```

For reference, the full deployment example could look something like the following:
```yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: web
spec:
  replicas: 1
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      name: web
      labels:
        app: web
      annotations:
        'consul.hashicorp.com/connect-inject': 'true'
        'consul.hashicorp.com/transparent-proxy': 'false'
        'consul.hashicorp.com/connect-service': 'web,web-admin'
        'consul.hashicorp.com/connect-service-port': '8080,9090'
    spec:
      containers:
        - name: web
          image: hashicorp/http-echo:latest
          args:
            - -text="hello world"
            - -listen=:8080
          ports:
            - containerPort: 8080
              name: http
        - name: web-admin
          image: hashicorp/http-echo:latest
          args:
            - -text="hello world from 9090"
            - -listen=:9090
          ports:
            - containerPort: 9090
              name: http
      serviceAccountName: web
```

After deploying the `web` application, you can test service mesh connections by deploying the `static-client`
application with the configuration in the [previous section](#connecting-to-mesh-enabled-services) and add the
following annotation to the pod template on `static-client`:
```yaml
consul.hashicorp.com/connect-service-upstreams: "web:1234,web-admin:2234"
```

If you exec on to a static-client pod, using a command like:
```shell-session
$ kubectl exec -it static-client-5bd667fbd6-kk6xs -- /bin/sh
```
you can then run:
```shell-session
$ curl localhost:1234
```
to see the output `hello world` and run:
```shell-session
$ curl localhost:2234
```
to see the output `hello world from 9090`.

The way this works is that a Consul service instance is being registered per port on the Pod, so there are 2 Consul
services in this case. An additional Envoy sidecar proxy and `connect-init` init container are also deployed per port in
the Pod. So the upstream configuration can use the individual service names to reach each port as seen in the example.


#### Caveats for Multi-port Pods
* Transparent proxy is not supported for multi-port Pods.
* Metrics and metrics merging is not supported for multi-port Pods.
* Upstreams will only be set on the first service's Envoy sidecar proxy for the pod.
  * This means that ServiceIntentions from a multi-port pod to elsewhere, will need to use the first service's name,
    `web` in the example above to accept connections from either `web` or `web-admin`. ServiceIntentions from elsewhere
    to a multi-port pod can use the individual service names within the multi-port Pod.
* Health checking is done on a per-Pod basis, so if any Kubernetes health checks (like readiness, liveness, etc) are
  failing for any container on the Pod, the entire Pod is marked unhealthy, and any Consul service referencing that Pod
  will also be marked as unhealthy. So, if `web` has a failing health check, `web-admin` would also be marked as
  unhealthy for service mesh traffic.

## Installation and Configuration

The service mesh sidecar proxy is injected via a
[mutating admission webhook](https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/#admission-webhooks)
call the connect injector provided by the
[consul-k8s project](https://github.com/hashicorp/consul-k8s).
This enables the automatic pod mutation shown in the usage section above.
Installation of the mutating admission webhook is automated using the
[Helm chart](/consul/docs/k8s/installation/install).

To install the connect injector, enable the connect injection feature using
[Helm values](/consul/docs/k8s/helm#configuration-values) and
upgrade the installation using `helm upgrade` for existing installs or
`helm install` for a fresh install.

```yaml
connectInject:
  enabled: true
```

This will configure the injector to inject when the
[injection annotation](#consul-hashicorp-com-connect-inject)
is set to `true`. Other values in the Helm chart can be used to limit the namespaces
the injector runs in, enable injection by default, and more.

### Verifying the Installation

To verify the installation, run the
["Accepting Inbound Connections"](/consul/docs/k8s/connect#accepting-inbound-connections)
example from the "Usage" section above. After running this example, run
`kubectl get pod static-server --output yaml`. In the raw YAML output, you should
see connect injected 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.

### Controlling Injection Via Annotation

By default, the injector will inject only when the
[injection annotation](#consul-hashicorp-com-connect-inject)
on the pod (not the deployment) is set to `true`:

```yaml
annotations:
  'consul.hashicorp.com/connect-inject': 'true'
```

### Injection Defaults

If you wish for the injector to always inject, you can set the default to `true`
in the Helm chart:

```yaml
connectInject:
  enabled: true
  default: true
```

You can then exclude specific pods via annotation:

```yaml
annotations:
  'consul.hashicorp.com/connect-inject': 'false'
```

### Controlling Injection Via Namespace

You can control which Kubernetes namespaces are allowed to be injected via
the `k8sAllowNamespaces` and `k8sDenyNamespaces` keys:

```yaml
connectInject:
  enabled: true
  k8sAllowNamespaces: ['*']
  k8sDenyNamespaces: []
```

In the default configuration (shown above), services from all namespaces are allowed
to be injected. Whether or not they're injected depends on the value of `connectInject.default`
and the `consul.hashicorp.com/connect-inject` annotation.

If you wish to only enable injection in specific namespaces, you can list only those
namespaces in the `k8sAllowNamespaces` key. In the configuration below
only the `my-ns-1` and `my-ns-2` namespaces will be enabled for injection.
All other namespaces will be ignored, even if the connect inject [annotation](#consul-hashicorp-com-connect-inject)
is set.

```yaml
connectInject:
  enabled: true
  k8sAllowNamespaces: ['my-ns-1', 'my-ns-2']
  k8sDenyNamespaces: []
```

If you wish to enable injection in every namespace _except_ specific namespaces, you can
use `*` in the allow list to allow all namespaces and then specify the namespaces to exclude in the deny list:

```yaml
connectInject:
  enabled: true
  k8sAllowNamespaces: ['*']
  k8sDenyNamespaces: ['no-inject-ns-1', 'no-inject-ns-2']
```

-> **NOTE:** The deny list takes precedence over the allow list. If a namespace
is listed in both lists, it will **not** be synced.

~> **NOTE:** The `kube-system` and `kube-public` namespaces will never be injected.

### Consul Enterprise Namespaces

Consul Enterprise 1.7+ supports Consul namespaces. When Kubernetes pods are registered
into Consul, you can control which Consul namespace they are registered into.

There are three options available:

1.  **Single Destination Namespace** – Register all Kubernetes pods, regardless of namespace,
    into the same Consul namespace.

    This can be configured with:

    ```yaml
    global:
      enableConsulNamespaces: true

    connectInject:
      enabled: true
      consulNamespaces:
        consulDestinationNamespace: 'my-consul-ns'
    ```

    -> **NOTE:** If the destination namespace does not exist we will create it.

1.  **Mirror Namespaces** - Register each Kubernetes pod into a Consul namespace with the same name as its Kubernetes namespace.
    For example, pod `foo` in Kubernetes namespace `ns-1` will be synced to the Consul namespace `ns-1`.
    If a mirrored namespace does not exist in Consul, it will be created.

    This can be configured with:

    ```yaml
    global:
      enableConsulNamespaces: true

    connectInject:
      enabled: true
      consulNamespaces:
        mirroringK8S: true
    ```

1.  **Mirror Namespaces With Prefix** - Register each Kubernetes pod into a Consul namespace with the same name as its Kubernetes
    namespace **with a prefix**.
    For example, given a prefix `k8s-`, pod `foo` in Kubernetes namespace `ns-1` will be synced to the Consul namespace `k8s-ns-1`.

    This can be configured with:

    ```yaml
    global:
      enableConsulNamespaces: true

    connectInject:
      enabled: true
      consulNamespaces:
        mirroringK8S: true
        mirroringK8SPrefix: 'k8s-'
    ```

### Consul Enterprise Namespace Upstreams

When [transparent proxy](/consul/docs/connect/transparent-proxy) is enabled and ACLs are disabled,
the upstreams will be configured automatically across Consul namespaces.
When ACLs are enabled, you must configure it by specifying an [intention](/consul/docs/connect/intentions),
allowing services across Consul namespaces to talk to each other.

If you wish to specify an upstream explicitly via the `consul.hashicorp.com/connect-service-upstreams` annotation,
use the format `[service-name].[namespace]:[port]:[optional datacenter]`:

```yaml
annotations:
  'consul.hashicorp.com/connect-inject': 'true'
  'consul.hashicorp.com/connect-service-upstreams': '[service-name].[namespace]:[port]:[optional datacenter]'
```

See [consul.hashicorp.com/connect-service-upstreams](#consul-hashicorp-com-connect-service-upstreams) for more details.

-> **Note:** When you specify upstreams via an upstreams annotation, you will need to use
`localhost:<port>` with the port from the upstreams annotation instead of KubeDNS to connect to your upstream
application.