talk to an in-cluster http service by proxying via the apiserver

talk-to-http-service.go

package main

import (
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strings"

	"k8s.io/client-go/rest"
	"sigs.k8s.io/controller-runtime/pkg/client/config"
)

func must(err error) {
	if err != nil {
		panic(err)
	}
}

func main() {
	restConfig := config.GetConfigOrDie()

	transport, err := rest.TransportFor(restConfig)
	must(err)

	// Construct an http.Client that that authenticates with the apiserver.
	client := &http.Client{
		Transport: transport,
	}

	// Construct request url:
	host := restConfig.Host
	if !strings.HasSuffix(host, "/") {
		host = host + "/"
	}
	requestUrl, err := url.Parse(host)
	must(err)

	targetNamespace := "package-operator-system"
	targetService := "package-operator-metrics"
	targetPort := 8080
	targetPath := "/metrics"

	requestUrl.Path = fmt.Sprintf(
		"/api/v1/namespaces/%s/services/%s:%d/proxy%s",
		targetNamespace,
		targetService,
		targetPort,
		targetPath,
	)

	resp, err := client.Get(requestUrl.String())
	must(err)
	defer resp.Body.Close()

	body, err := io.ReadAll(resp.Body)
	must(err)

	fmt.Println(string(body))
}

Full blown port-forward alternative: https://microcumul.us/blog/k8s-port-forwarding/