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Version: Pasley (2.0.0-rc.9)


Intercept allows developers to run a service locally and selectively intercept traffic from a remote environment to their local instance.

Use Case

Some services are not easy to debug by calling directly, and instead, it's far easier to rely on the interactions from higher-level services to call into our service. For example, a backend API is often easier to test by clicking through the frontend application.

Intercept allows developers to selectively intercept traffic for a remote service, so they do not need to run the entire stack locally, and they can debug against real-world traffic experienced within the remote environment.

Intercept Dataflow


Intercept works by creating an intermediate proxy for the service to inspect traffic and determine if the request should be directed to the original in-cluster service, or tunnelled to a developer's local machine.

A few scenarios are possible

  1. Intercept traffic with the X-C6O-INTERCEPT header to my local machine.
  2. Specify a custom intercept header instead, so that multiple developers can intercept the same services (as long as each developer uses a different header key/value pair).

How it Works

When an Intercept Session is initiated, we

  1. Open a local tunnel and give that tunnel a random DNS name. This local tunnel proxies requests through any NAT or firewall to a single locally running service.
  2. Deploy a proxy service in the cluster.
  3. Create a decoy Service that routes to the intercepted Service selectors.
  4. Hijack the existing service by setting the service selectors to route traffic to the proxy service Deployment.
  5. Create a Session record that stores all the changes made to the Kubernetes cluster.

The Proxy service selectively routes traffic based on the request headers.

Under the Hood

Intercept works by:

  1. Intercepting traffic for a remote service
  2. Inspecting for given traffic headers, and directing traffic to a. the original (in-cluster) service if no re-direct headers are found b. forwarding the request through a reverse tunnel to a developers local machine if a redirect header flag is given

Intercepting Traffic

In order to route local traffic to in cluster resources, teleport does several things:

  1. Creates a small light weight NGINX deployment to inspect headers and direct traffic.
  2. Modifies the existing Kubernetes Service resource to direct traffic to this proxy.

We try to minimize any modifications to your cluster, and revert all changes once finished. However, if any sessions close unexpectedly, run czctl session close to clean up any leftover residue or reissue the same command with a --close flag.

Propagating Headers

Unless you are intercepting all traffic for a particular service, traffic is directed to the intercepted service in-cluster vs. local depending on HTTP headers. When the intercepted service is upstream from the frontend service, these headers need to be propagated to the intercepted service to route the traffic properly.

For example, if we have the following route Frontend -> Core -> Leaf, in order to intercept the Leaf service, the Core service has to propagate intercept headers when calling the Leaf service.

Header propagation is commonly used for performance tracking, tracing, and other diagnostic functions.

Propagating headers is language-dependent and is not hard. It generally only requires a few lines of code.

In this pseudocode example that makes an outbound REST request, we only propagate headers that start with x-c6o but you are free to use your own convention:

  propagatedHeaders = []
for each header in request.headers
if (header.key.startsWith('x-c6o-'))

const result = await restRequest({ url, propagatedHeaders })

Reverse Tunnel

When a teleport session is opened, the developer's local machine creates a tunnel so that it can receive traffic requests from the remote cluster without needing to change any firewall or routing settings. We currently use ngrok to create a local tunnel however this will change in time.


A bonus of using ngrok under the hood, is it provides a simple web dashboard to view and inspect incoming traffic. When an intercept session is running, check out http://localhost:4040. You can have a maximum of 4 concurrent intercept tunnels running A tunnel expires after 24 hours. Restart intercept to continue after expiry.


To learn more about how ngrok works, see


This section describes what the intercept command creates within a cluster to accomplish its task and instructions on what to do if something breaks.

The intercept command creates three resources within the namespace of the deployment that is intercepted:

  1. A Session resource
  2. A reverse-proxy Deployment
  3. A decoy Service for routing un re-directed traffic to the original Deployment

The intercept command modifies the Service in front of the Deployment by backing up and then modifying the selectors to point to the intercept Deployment. This accomplishes the task of routing requests based on headers specified by the command.

Specifically, the following selector is used:

app: interceptor,
[Session hash, something like: 29ad008882b59faa516d733051a9d14b2d3b6836]

The ports will be pointed at port 80 by the interceptor if the targetPort is different:

- port: [Some port]
protocol: TCP
targetPort: 80

The hash is stored in a Session resource defined by a Session.CRD, which you can see by issuing the command kubectl get session intercept-[your namespace]-[your workload name] -o yaml.

You will see in the namespace these resources:

  • service/interceptor-[your workload name]-decoy (this will create endpoint: endpoints/interceptor-[your workload name]-decoy )
  • deployment/interceptor-[your workload name] (this run some pods, like: pod/interceptor-[your workload name]-6cd6c6b947-8gzcq)
  • configmap/interceptor-[your workload name]

These have the following responsibilities respectively:

  • The service is responsible for getting traffic to the original deployment
  • The interceptor for the workload type (like a deployment) is responsible for directing traffic based on the headers in the request
  • The config map configures the redirector's pods


Cleanup is accomplished by reissuing the command with a --clean parameter or using the czctl session close or czctl session close --all command. If this doesn't work, it is necessary to first correct the selector to point to the original deployment and then delete the three resource files the intercept has created:

First correct the selector and the ports updated by the interceptor. Get the original values from the Session:

> kubectl get session -n [your namespace] -o yaml
- op: replace
path: /spec/selector
[some key1]: [some value1]
[some key2]: [some value2]
- op: replace
path: /spec/ports
- name: unsecure
port: [some port]
protocol: TCP
targetPort: [some port]
apiVersion: v1
kind: Service
name: [workload name]
namespace: [your namespace
> kubectl edit service [your service name] -n [your namespace]

Find the section that has this:

app: interceptor
[some hash, like: 29ad008882b59faa516d733051a9d14b2d3b6836]

and set it to the original, correct the selector to:

[some key1]: [some value1]
[some key2]: [some value2]

and the ports' section will be directed to port 80:

- name: unsecure
port: [some port]
protocol: TCP
targetPort: 80

Correct this to the original port given in the Session resource.

- name: unsecure
port: [some port]
protocol: TCP
targetPort: [some port]

Then delete the intercept residue resources

kubectl delete service interceptor-[your workload name]-decoy -n [your namespace]
kubectl delete deployment interceptor-[your workload name] -n [your namespace]
kubectl delete configmap interceptor-[your workload name] -n [your namespace]
kubectl delete session interceptor-[your workload name] -n [your namespace]

The pods and endpoints will clean up upon deletion of the decoy service and the interceptor deployment.

Example Cleanup

First correct the selector:

kubectl edit service -n sample-project service/sample-project-server

The selector and ports looks like so when intercept is active:

- port: 3000
protocol: TCP
targetPort: 80
app: interceptor 29ad008882b59faa516d733051a9d14b2d3b6836

In this example, it should look like so when corrected:

- port: 3000
protocol: TCP
targetPort: 3000
app: sample-project
component: backend

Then delete the residue resources

kubectl delete service -n sample-project interceptor-sample-project-server-decoy
kubectl delete deployment -n sample-project interceptor-sample-project-server
kubectl delete configmap -n sample-project interceptor-sample-project-server
kubectl delete session -n sample-project intercept-sample-project-server

Ngrok may also be left running and can be stopped by force if necessary:

ps xau | grep 'ngrok' | grep -v 'grep' |  awk '{print $2 " -> " $11, $12}'
67103 -> /Users/username/.codezero/bin/ngrok/ngrok start
sudo kill -9 67103

Closing Intercept

Run czctl session close to end the Intercept session or reissue the same command with a --close flag. The session close command will clean up all the residue added to the Kubernetes cluster and restore the intercepted service to its original state.

Command Reference

See the Intercept command reference for more information.