Intercepts

When intercepting a service, Telepresence installs a traffic-agent sidecar in to the workload. That traffic-agent supports one or more intercept mechanisms that it uses to decide which traffic to intercept. Telepresence has a simple default traffic-agent, however you can configure a different traffic-agent with more sophisticated mechanisms either by setting the images.agentImage field in config.yml or by writing an extensions/${extension}.yml file that tells Telepresence about a traffic-agent that it can use, what mechanisms that traffic-agent supports, and command-line flags to expose to the user to configure that mechanism. You may tell Telepresence which known mechanism to use with the --mechanism=${mechanism} flag or by setting one of the --${mechansim}-XXX flags, which implicitly set the mechanism; for example, setting --http-match=auto implicitly sets --mechanism=http.

The default open-source traffic-agent only supports the tcp mechanism, which treats the raw layer 4 TCP streams as opaque and sends all of that traffic down to the developer's workstation. This means that it is a "global" intercept, affecting all users of the cluster.

In addition to the default open-source traffic-agent, Telepresence already knows about the Ambassador Cloud traffic-agent, which supports the http mechanism. The http mechanism operates at higher layer, working with layer 7 HTTP, and may intercept specific HTTP requests, allowing other HTTP requests through to the regular service. This allows for "personal" intercepts which only intercept traffic tagged as belonging to a given developer.

Intercept behavior when logged in to Ambassador Cloud

Logging in to Ambassador Cloud (with telepresence login) changes the Telepresence defaults in two ways.

First, being logged in to Ambassador Cloud causes Telepresence to default to --mechanism=http --http-match=auto (or just --http-match=auto, as --http-match implies --mechanism=http). If you hadn't been logged in it would have defaulted to --mechanism=tcp. This tells Telepresence to use the Ambassador Cloud traffic-agent to do smart "personal" intercepts and only intercept a subset of HTTP requests, rather than just intercepting the entirety of all TCP connections. This is important for working in a shared cluster with teammates, and is important for the preview URL functionality below. See telepresence intercept --help for information on using --http-match to customize which requests it intercepts.

Secondly, being logged in causes Telepresence to default to --preview-url=true. If you hadn't been logged in it would have defaulted to --preview-url=false. This tells Telepresence to take advantage of Ambassador Cloud to create a preview URL for this intercept, creating a shareable URL that automatically sets the appropriate headers to have requests coming from the preview URL be intercepted. In order to create the preview URL, it will prompt you for four settings about how your cluster's ingress is configured. For each, Telepresence tries to intelligently detect the correct value for your cluster; if it detects it correctly, may simply press "enter" and accept the default, otherwise you must tell Telepresence the correct value.

When you create an intercept with the http mechanism, Telepresence determines whether the application protocol uses HTTP/1.1 or HTTP/2. If the service's ports.appProtocol field is set, Telepresence uses that. If not, then Telepresence uses the configured application protocol strategy to determine the protocol. The default behavior (http2Probe strategy) sends a GET /telepresence-http2-check request to your service to determine if it supports HTTP/2. This is required for the intercepts to behave correctly.

TLS

If the intercepted service has been set up for --mechanism=http, Telepresence needs to terminate the TLS connection for the http mechanism to function in your intercepts. Additionally, you need to ensure the TLS annotations are properly entered in your workload’s Pod template to designate that requests leaving your service still speak TLS outside of the service as expected.

Use the --http-plaintext flag when doing an intercept when the service in the cluster is using TLS in case you want to use plaintext for the communication with the process on your local workstation.

Supported workloads

Kubernetes has various workloads. Currently, Telepresence supports intercepting (installing a traffic-agent on) Deployments, ReplicaSets, and StatefulSets.

Specifying a namespace for an intercept

The namespace of the intercepted workload is specified using the --namespace option. When this option is used, and --workload is not used, then the given name is interpreted as the name of the workload and the name of the intercept will be constructed from that name and the namespace.

This will intercept a workload named hello and name the intercept hello-myns. In order to remove the intercept, you will need to run telepresence leave hello-mydns instead of just telepresence leave hello.

The name of the intercept will be left unchanged if the workload is specified.

This will intercept a workload named hello and name the intercept myhello.

Importing environment variables

Telepresence can import the environment variables from the pod that is being intercepted, see this doc for more details.

Creating an intercept without a preview URL

If you are not logged in to Ambassador Cloud, the following command will intercept all traffic bound to the service and proxy it to your laptop. This includes traffic coming through your ingress controller, so use this option carefully as to not disrupt production environments.

If you are logged in to Ambassador Cloud, setting the --preview-url flag to false is necessary.

This will output an HTTP header that you can set on your request for that traffic to be intercepted:

Run telepresence status to see the list of active intercepts.

Finally, run telepresence leave <name of intercept> to stop the intercept.

Skipping the ingress dialogue

You can skip the ingress dialogue by setting the relevant parameters using flags. If any of the following flags are set, the dialogue will be skipped and the flag values will be used instead. If any of the required flags are missing, an error will be thrown.

Creating an intercept when a service has multiple ports

If you are trying to intercept a service that has multiple ports, you need to tell Telepresence which service port you are trying to intercept. To specify, you can either use the name of the service port or the port number itself. To see which options might be available to you and your service, use kubectl to describe your service or look in the object's YAML. For more information on multiple ports, see the Kubernetes documentation.

When intercepting a service that has multiple ports, the name of the service port that has been intercepted is also listed.

If you want to change which port has been intercepted, you can create a new intercept the same way you did above and it will change which service port is being intercepted.

Creating an intercept When multiple services match your workload

Oftentimes, there's a 1-to-1 relationship between a service and a workload, so telepresence is able to auto-detect which service it should intercept based on the workload you are trying to intercept. But if you use something like Argo, there may be two services (that use the same labels) to manage traffic between a canary and a stable service.

Fortunately, if you know which service you want to use when intercepting a workload, you can use the --service flag. So in the aforementioned example, if you wanted to use the echo-stable service when intercepting your workload, your command would look like this:

Port-forwarding an intercepted container's sidecars

Sidecars are containers that sit in the same pod as an application container; they usually provide auxiliary functionality to an application, and can usually be reached at localhost:${SIDECAR_PORT}. For example, a common use case for a sidecar is to proxy requests to a database, your application would connect to localhost:${SIDECAR_PORT}, and the sidecar would then connect to the database, perhaps augmenting the connection with TLS or authentication.

When intercepting a container that uses sidecars, you might want those sidecars' ports to be available to your local application at localhost:${SIDECAR_PORT}, exactly as they would be if running in-cluster. Telepresence's --to-pod ${PORT} flag implements this behavior, adding port-forwards for the port given.

If there are multiple ports that you need forwarded, simply repeat the flag (--to-pod=<sidecarPort0> --to-pod=<sidecarPort1>).

Intercepting headless services

Kubernetes supports creating services without a ClusterIP, which, when they have a pod selector, serve to provide a DNS record that will directly point to the service's backing pods. Telepresence supports intercepting these headless services as it would a regular service with a ClusterIP. So, for example, if you have the following service:

You can intercept it like any other: