---
layout: docs
page_title: Service sync
description: >-
  Service sync is a Consul on Kubernetes feature that makes Kubernetes and Consul services available to each other. Learn how to configure Helm values so services can communicate and make Kubernetes services appear in the Consul UI.
---

# Service sync

The services in Kubernetes and Consul can be automatically synced so that Kubernetes
services are available to Consul agents and services in Consul can be available
as first-class Kubernetes services. 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 K8s Helm chart](/consul/docs/deploy/server/k8s/helm).

![screenshot of a Kubernetes service in the UI](/img/k8s-service.png)

**Why sync Kubernetes services to Consul?** Kubernetes services synced to the
Consul catalog enable Kubernetes services to be accessed by any node that
is part of the Consul cluster, including other distinct Kubernetes clusters.
For non-Kubernetes nodes, they can access services using the standard
[Consul DNS](/consul/docs/discover/dns) or HTTP API.

**Why sync Consul services to Kubernetes?** Syncing Consul services to
Kubernetes services enables non-Kubernetes services to be accessed using kube-dns and Kubernetes-specific
environment variables. This integration makes it very easy to automate external
service discovery, including hosted services like databases.

## Installation and configuration

~> Enabling both Service Mesh and Service Sync on the same Kubernetes services is not supported, as Service Mesh also registers Kubernetes service instances to Consul. Ensure that Service Sync is only enabled for namespaces and services that are not injected with the Consul sidecar for Service Mesh as described in [Sync Enable/Disable](/consul/docs/register/service/k8s/service-sync#enable-and-disable-sync).

The service sync feature deploys a long-running process which can run either inside or outside of a Kubernetes cluster. However, running this process within the Kubernetes cluster is generally easier since it is automated using the [Helm chart](/consul/docs/reference/k8s/helm).

The Consul server cluster can run either in or out of a Kubernetes cluster.
The Consul server cluster does not need to be running on the same machine
or same platform as the sync process. The sync process needs to be configured
with the address to a Consul agent as well as any additional access
information such as ACL tokens.

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

```yaml
syncCatalog:
  enabled: true
```

This value enables service syncing in both direction. You can also disable a direction so that only Kubernetes services sync to Consul, or only Consul services sync to Kubernetes.

To only enable syncing Consul services to Kubernetes use the config:

```yaml
syncCatalog:
  enabled: true
  toConsul: false
  toK8S: true
```

To only enable syncing Kubernetes services to Consul use:

```yaml
syncCatalog:
  enabled: true
  toConsul: true
  toK8S: false
```

Refer to the [Helm configuration](/consul/docs/reference/k8s/helm#v-synccatalog)
for more information.

### Authentication

The sync process must authenticate to both Kubernetes and Consul to read
and write services.

If running `consul-k8s` using the Helm chart, then this authentication is handled for you.

If running `consul-k8s` outside of Kubernetes, a valid kubeconfig file must be provided with cluster
and authentication information. The sync process looks into the default locations
for both in-cluster and out-of-cluster authentication. If `kubectl` works,
then the sync program should work.

If ACLs are configured on the Consul cluster, you need to provide a Consul
[ACL token](/consul/docs/secure/acl/token). Review the [ACL rules](/consul/docs/secure/acl/rule)
when creating this token so that it only allows the necessary privileges. The catalog
sync process accepts this token by using the [`CONSUL_HTTP_TOKEN`](/consul/commands#consul_http_token)
environment variable. This token should be set as a
[Kubernetes secret](https://kubernetes.io/docs/concepts/configuration/secret/#creating-your-own-secrets)
and referenced in the Helm chart.

## Kubernetes to Consul

This sync registers Kubernetes services to the Consul catalog automatically.

This sync enables discovery and connection to Kubernetes services using native
Consul service discovery protocols such as DNS or HTTP. This is particularly useful for
non-Kubernetes nodes. This also causes all discoverable services to be part of
a central service catalog in Consul for further syncing into alternate
Kubernetes clusters or other platforms.

Each synced service is registered onto a Consul node called `k8s-sync`. This node
is not a real node. Instead, the catalog sync process is monitoring Kubernetes
and syncing the services to Consul.

### Kubernetes service types

Not all Kubernetes services are externally accessible. The sync program by
default only syncs services of the following types or configurations.
If a service type is not listed below, then the sync program ignores that
service type.

#### NodePort

[NodePort services](https://kubernetes.io/docs/concepts/services-networking/service/#nodeport)
register a static port that every node in the K8S cluster listens on.

For NodePort services, a Consul service instance is created for each
node that has the representative pod running. While Kubernetes configures
a static port on all nodes in the cluster, this limits the number of service
instances to be equal to the nodes running the target pods.
By default it uses the external IP of the node but this can be configured through
the [`nodePortSyncType` helm option](/consul/docs/reference/k8s/helm#v-synccatalog-nodeportsynctype).

The service instance's port is set to the first defined node port of the service unless
set specifically in the `consul.hashicorp.com/service-port` annotation. Refer to [Service Ports](/consul/docs/register/service/k8s/service-sync#service-ports) for more information.

#### LoadBalancer

For LoadBalancer services, a single service instance is registered with
the external IP of the created load balancer. Because this is already a load
balancer, only one service instance is registered with Consul rather
than registering each individual pod endpoint.

The service instance's port is set to the first defined port of the
service unless set specifically in the `consul.hashicorp.com/service-port` annotation. Refer to [Service Ports](/consul/docs/register/service/k8s/service-sync#service-ports) for more information.

#### External IPs

Any service type may specify an
"[external IP](https://kubernetes.io/docs/concepts/services-networking/service/#external-ips)"
configuration. The external IP must be configured by some other system, but
any service discovery resolves to this set of IP addresses rather than a
virtual IP.

If an external IP list is present, a service instance in Consul is created
for each external IP. It is assumed that if an external IP is present that it
is routable and configured by some other system.

The service instance's port is set to the _first_ defined port of the
service unless set specifically with the `consul.hashicorp.com/service-port` annotation. Refer to [Service Ports](/consul/docs/register/service/k8s/service-sync#service-ports) for more information.

#### ClusterIP

ClusterIP services are synced by default as of `consul-k8s` version 0.3.0.
Each pod that is an endpoint for the service is synced as a Consul service
instance with its IP set to the pod IP and its port set to the `targetPort`.

The service instance's port can be overridden with the `consul.hashicorp.com/service-port` annotation. Refer to [Service Ports](/consul/docs/register/service/k8s/service-sync#service-ports) for more information.

-> In Kubernetes clusters where pod IPs are not accessible outside the cluster,
the services registered in Consul may not be routable. To
skip syncing ClusterIP services, set [`syncClusterIPServices`](/consul/docs/reference/k8s/helm#v-synccatalog-syncclusteripservices)
to `false` in the Helm chart values file.

### Enable and disable sync

By default, all valid service types are synced from every Kubernetes
namespace except for `kube-system` and `kube-public`.

To only sync specific services, first modify the annotation to set the default to `false`:

```yaml
syncCatalog:
  enabled: true
  default: false
```

Then, explicitly enable syncing specific services with the `consul.hashicorp.com/service-sync` annotation:

```yaml
kind: Service
apiVersion: v1
metadata:
  name: my-service
  annotations:
    'consul.hashicorp.com/service-sync': 'true'
```

-> **Note:** If the annotation is set to `false` when the default sync is `true`, the service does not sync.

You can allow or deny syncing from specific Kubernetes namespaces by setting the
`k8sAllowNamespaces` and `k8sDenyNamespaces` keys:

```yaml
syncCatalog:
  enabled: true
  default: true
  k8sAllowNamespaces: ['*']
  k8sDenyNamespaces: ['kube-system', 'kube-public']
```

In the default configuration, services from all namespaces except for
`kube-system` and `kube-public` are synced.

To only sync from specific namespaces, you can list only those
namespaces in the `k8sAllowNamespaces` key:

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

To sync from every namespace except specific namespaces, use `*` in the allow list and then specify the non-syncing namespaces in the deny list:

```yaml
syncCatalog:
  enabled: true
  default: true
  k8sAllowNamespaces: ['*']
  k8sDenyNamespaces: ['no-sync-ns-1', 'no-sync-ns-2']
```

-> **Note:** The deny list takes precedence over the allow list. If a namespace
is listed in both lists, it does not sync.

### Service name

When a Kubernetes service is synced to Consul, the name of the service in Consul
by default is the value of the `name` metadata on that Kubernetes service.
This makes it so that service sync works with zero configuration changes.
This setting can be overridden using an annotation to specify the Consul service name:

```yaml
kind: Service
apiVersion: v1
metadata:
  name: my-service
  annotations:
    'consul.hashicorp.com/service-name': my-consul-service
```

**If a conflicting service name exists in Consul,** the sync program
registers additional instances to that same service. Therefore, services inside
and outside of Kubernetes should have different names unless you want either
side to potentially connect. This default behavior also enables gracefully
transitioning a service between deployments inside and outside of Kubernetes.

### Service ports

When syncing the Kubernetes service to Consul, the Consul service port is the first defined port in the service. Additionally, all ports become registered in the service instance metadata with the key "port-X," where X is
the name of the port and the value is the externally accessible port.

The default service port can be overridden using an annotation:

```yaml
kind: Service
apiVersion: v1
metadata:
  name: my-service
  annotations:
    'consul.hashicorp.com/service-port': 'http'
```

The annotation value may be a name of a port (recommended) or an exact port value.

### Service tags

A service registered in Consul from Kubernetes always has the tag "k8s" added
to it. Additional tags can be specified with a comma-separated annotation value. These custom tags automatically include the "k8s" tag, which can't be disabled. When specifying values, use commas without whitespace.

```yaml
kind: Service
apiVersion: v1
metadata:
  name: my-service
  annotations:
    'consul.hashicorp.com/service-tags': 'primary,foo'
```

### Service meta

A service registered in Consul from Kubernetes sets the `external-source` key to
`kubernetes`. This can be used from the API, CLI and UI to filter
service instances that are set in k8s. The Consul UI displays a Kubernetes icon next to all externally
registered services from Kubernetes.

Additional metadata can be specified using annotations. The "KEY" below can be
set to any key. This allows setting multiple meta values.

```yaml
kind: Service
apiVersion: v1
metadata:
  name: my-service
  annotations:
    'consul.hashicorp.com/service-meta-KEY': 'value'
```

### Consul Enterprise Namespaces

Consul Enterprise supports Consul namespaces. These can be used when syncing
from Kubernetes to Consul. However, namespaces are not supported when syncing from Consul to Kubernetes.

There are three options available:

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

    ```yaml
    global:
      enableConsulNamespaces: true

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

1.  **Mirror Namespaces** - Each Kubernetes service is synced to a Consul namespace with the same
    name as its Kubernetes namespace.

    For example, service `foo` in Kubernetes namespace `ns-1` is synced to the Consul namespace `ns-1`.
    If a mirrored namespace does not exist in Consul, it is created automatically.

    ```yaml
    global:
      enableConsulNamespaces: true

    syncCatalog:
      enabled: true
      consulNamespaces:
        mirroringK8S: true

    addK8SNamespaceSuffix: false
    ```

1.  **Mirror Namespaces With Prefix** - Each Kubernetes service is synced to a Consul namespace
    with the same name as its Kubernetes namespace with a prefix. For example, given a prefix
    `k8s-`, service `foo` in Kubernetes namespace `ns-1` syncs to the Consul namespace
    `k8s-ns-1`.

    ```yaml
    global:
      enableConsulNamespaces: true

    syncCatalog:
      enabled: true
      consulNamespaces:
        mirroringK8S: true
        mirroringK8SPrefix: 'k8s-'

    addK8SNamespaceSuffix: false
    ```

-> Note that in both mirroring examples, `addK8SNamespaceSuffix` is set to `false`. If set to its default value, `true`, the Kubernetes namespace is added as a suffix to each
Consul service name. For example Kubernetes service `foo` in namespace `k8s-ns`
would be registered into Consul with the name `foo-k8s-ns`.
This is useful when syncing from multiple Kubernetes namespaces to
a single Consul namespace. However, you may want to disable this behavior
when mirroring namespaces so that services do not overlap with services from
other namespaces.

## Consul to Kubernetes

This syncs Consul services into first-class Kubernetes services.
The sync creates an [`ExternalName`](https://kubernetes.io/docs/concepts/services-networking/service/#externalname) for each Consul service. The "external name" is the Consul DNS name.

For example, given a Consul service `foo`:

```yaml
apiVersion: v1
kind: Service
metadata:
  name: foo
  ...
spec:
  externalName: foo.service.consul
  type: ExternalName
```

With Consul To Kubernetes syncing enabled, DNS requests of the form `<consul-service-name>`
are serviced by Consul DNS. From a different Kubernetes namespace than where Consul
is deployed, the DNS request would need to be `<consul-service-name>.<consul-namespace>`.

-> **Note:** Consul to Kubernetes syncing is not required if you enabled [Consul DNS on Kubernetes](/consul/docs/k8s/dns).
All you need to do is address services in the form `<consul-service-name>.service.consul`, so you do not need Kubernetes `Service` objects created.

~> **Requires Consul DNS via CoreDNS in Kubernetes:** This feature requires that
[Consul DNS](/consul/docs/k8s/dns) is configured within Kubernetes.
Additionally, [CoreDNS](https://kubernetes.io/docs/tasks/administer-cluster/dns-custom-nameservers/#config-coredns)
is required instead of kube-dns to resolve an
issue with resolving `externalName` services pointing to custom domains.

### Enable and disable sync

All Consul services visible to the sync process based on its given ACL token
are synced to Kubernetes.

There is no way to change this behavior per service. For the opposite sync
direction (Kubernetes to Consul), you can use Kubernetes annotations to disable
a sync per service. This is not currently possible for Consul to Kubernetes
sync and the ACL token must be used to limit what services are synced.

In the future, we hope to support per-service configuration.

### Service Name

When a Consul service is synced to Kubernetes, the name of the Kubernetes
service matches the name of the Consul service exactly.

To change this default exact match behavior, it is possible to specify a
prefix to be added to service names within Kubernetes by using the
`-k8s-service-prefix` flag. This can also be specified in the Helm
configuration.

**If a conflicting service is found**: the service is not synced. This
does not match the Kubernetes to Consul behavior, but given the current
implementation we must do this because Kubernetes cannot mix both CNAME and
Endpoint-based services.

### Kubernetes service labels and annotations

Any Consul services synced to Kubernetes are labeled and annotated.
An annotation `consul.hashicorp.com/synced` is set to `true` to note
that this is a synced service from Consul.

Additionally, a label `consul=true` is specified so that label selectors
can be used with `kubectl` and other tooling to easily filter all Consul-synced
services.
