Join External Services to Consul on Kubernetes

Services running on non-Kubernetes nodes can join a Consul cluster running within Kubernetes.

Auto-join

The recommended way to join a cluster running within Kubernetes is to use the "k8s" cloud auto-join provider.

The auto-join provider dynamically discovers IP addresses to join using the Kubernetes API. It authenticates with Kubernetes using a standard kubeconfig file. Auto-join works with all major hosted Kubernetes offerings as well as self-hosted installations. The token in the kubeconfig file needs to have permissions to list pods in the namespace where Consul servers are deployed.

The auto-join string below joins a Consul server agent to a cluster using the official Helm chart:

$ consul agent -retry-join 'provider=k8s label_selector="app=consul,component=server"'

Auto-join on the Pod network

In the default Consul Helm chart installation, Consul servers are routable through their pod IPs for server RPCs. As a result, any external agents joining the Consul cluster running on Kubernetes need to be able to connect to those pod IPs.

In many hosted Kubernetes environments, you need to explicitly configure your hosting provider to ensure that pod IPs are routable from external VMs. For more information, refer to Azure AKS CNI, AWS EKS CNI and GKE VPC-native clusters.

To join external agents with Consul on Kubernetes deployments installed with default values through the official Helm chart:

1. Make sure the pod IPs of the servers in Kubernetes are routable from the VM and that the VM can access port 8301 (for gossip) and port 8300 (for server RPC) on those pod IPs.

2. Make sure that the server pods running in Kubernetes can route to the VM's advertise IP on its gossip port (default 8301).

3. Make sure you have the kubeconfig file for the Kubernetes cluster in $HOME/.kube/config on the external VM.

4. On the external VM, run:

   consul agent \
     -advertise="$ADVERTISE_IP" \
     -retry-join='provider=k8s label_selector="app=consul,component=server"' \
     -bind=0.0.0.0 \
     -hcl='leave_on_terminate = true' \
     -hcl='ports { grpc = 8502 }' \
     -config-dir=$CONFIG_DIR \
     -datacenter=$DATACENTER \
     -data-dir=$DATA_DIR \
   

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5. Run consul members to check if the join was successful.

   / $ consul members
   Node                                           Address           Status  Type    Build  Protocol  DC   Segment
   consul-consul-server-0                         10.138.0.43:9301  alive    server  1.9.1  2         dc1  <all>
   external-agent                                 10.138.0.38:8301  alive    client  1.9.0  2         dc1  <default>
   gke-external-agent-default-pool-32d15192-grs4  10.138.0.43:8301  alive    client  1.9.1  2         dc1  <default>
   gke-external-agent-default-pool-32d15192-otge  10.138.0.44:8301  alive    client  1.9.1  2         dc1  <default>
   gke-external-agent-default-pool-32d15192-vo7k  10.138.0.42:8301  alive    client  1.9.1  2         dc1  <default>
   

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Auto-join through host ports

If your external VMs cannot connect to Kubernetes pod IPs but they can connect to the internal host IPs of the nodes in the Kubernetes cluster, you can join the two by exposing ports on the host IP instead.

1. Install the official Helm chart with the following values:

   client:
     exposeGossipPorts: true # exposes client gossip ports as hostPorts
   server:
     exposeGossipAndRPCPorts: true # exposes the server gossip and RPC ports as hostPorts
     ports:
       # Configures the server gossip port
       serflan:
         # Note that this needs to be different than 8301, to avoid conflicting with the client gossip hostPort
         port: 9301
   

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   This installation exposes the client gossip ports, the server gossip ports and the server RPC port at hostIP:hostPort. Note that hostIP is the internal IP of the VM that the client/server pods are deployed on.

2. Make sure the IPs of the Kubernetes nodes are routable from the VM and that the VM can access ports 8301 and 9301 (for gossip) and port 8300 (for server RPC) on those node IPs.

3. Make sure the server pods running in Kubernetes can route to the VM's advertise IP on its gossip port (default 8301).

4. Make sure you have the kubeconfig file for the Kubernetes cluster in $HOME/.kube/config on the external VM.

5. On the external VM, run:

   consul agent \
     -advertise="$ADVERTISE_IP" \
     -retry-join='provider=k8s host_network=true label_selector="app=consul,component=server"'
     -bind=0.0.0.0 \
     -hcl='leave_on_terminate = true' \
     -hcl='ports { grpc = 8502 }' \
     -config-dir=$CONFIG_DIR \
     -datacenter=$DATACENTER \
     -data-dir=$DATA_DIR \
   

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   Note the addition of host_network=true in the retry-join argument.

6. Run consul members to check if the join was successful.

   / $ consul members
   Node                                           Address           Status  Type    Build  Protocol  DC   Segment
   consul-consul-server-0                         10.138.0.43:9301  alive    server  1.9.1  2         dc1  <all>
   external-agent                                 10.138.0.38:8301  alive    client  1.9.0  2         dc1  <default>
   gke-external-agent-default-pool-32d15192-grs4  10.138.0.43:8301  alive    client  1.9.1  2         dc1  <default>
   gke-external-agent-default-pool-32d15192-otge  10.138.0.44:8301  alive    client  1.9.1  2         dc1  <default>
   gke-external-agent-default-pool-32d15192-vo7k  10.138.0.42:8301  alive    client  1.9.1  2         dc1  <default>
   

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Manual join

If you are unable to use auto-join, try following the instructions in either of the auto-join sections, but instead of using a provider key in the -retry-join flag, pass the address of at least one Consul server. Example: -retry-join=$CONSUL_SERVER_IP:$SERVER_SERFLAN_PORT.

A kubeconfig file is not required when using manual join.

Instead of hardcoding an IP address, we recommend you set up a DNS entry that resolves to the pod IPs or host IPs that the Consul server pods are running on.