plugin/health: Poll localhost by default (#5934)

defaulting to localhost makes things explicit in CoreDNS code, and will give us valid URIs in
the logs

Signed-off-by: W. Trevor King <wking@tremily.us>

plugin/health/health.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"net"
 	"net/http"
+	"net/url"
 	"time"
 
 	clog "github.com/coredns/coredns/plugin/pkg/log"
@@ -16,8 +17,9 @@ var log = clog.NewWithPlugin("health")
 
 // Health implements healthchecks by exporting a HTTP endpoint.
 type health struct {
-	Addr     string
-	lameduck time.Duration
+	Addr      string
+	lameduck  time.Duration
+	healthURI *url.URL
 
 	ln      net.Listener
 	nlSetup bool
@@ -30,6 +32,19 @@ func (h *health) OnStartup() error {
 	if h.Addr == "" {
 		h.Addr = ":8080"
 	}
+
+	var err error
+	h.healthURI, err = url.Parse("http://" + h.Addr)
+	if err != nil {
+		return err
+	}
+
+	h.healthURI.Path = "/health"
+	if h.healthURI.Host == "" {
+		// while we can listen on multiple network interfaces, we need to pick one to poll
+		h.healthURI.Host = "localhost"
+	}
+
 	ln, err := reuseport.Listen("tcp", h.Addr)
 	if err != nil {
 		return err
@@ -39,7 +54,7 @@ func (h *health) OnStartup() error {
 	h.mux = http.NewServeMux()
 	h.nlSetup = true
 
-	h.mux.HandleFunc("/health", func(w http.ResponseWriter, r *http.Request) {
+	h.mux.HandleFunc(h.healthURI.Path, func(w http.ResponseWriter, r *http.Request) {
 		// We're always healthy.
 		w.WriteHeader(http.StatusOK)
 		io.WriteString(w, http.StatusText(http.StatusOK))

plugin/health/overloaded.go

@@ -32,8 +32,7 @@ func (h *health) overloaded(ctx context.Context) {
 		Transport: bypassProxy,
 	}
 
-	url := "http://" + h.Addr + "/health"
-	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, h.healthURI.String(), nil)
 	tick := time.NewTicker(1 * time.Second)
 	defer tick.Stop()
 
@@ -49,14 +48,14 @@ func (h *health) overloaded(ctx context.Context) {
 			if err != nil {
 				HealthDuration.Observe(time.Since(start).Seconds())
 				HealthFailures.Inc()
-				log.Warningf("Local health request to %q failed: %s", url, err)
+				log.Warningf("Local health request to %q failed: %s", req.URL.String(), err)
 				continue
 			}
 			resp.Body.Close()
 			elapsed := time.Since(start)
 			HealthDuration.Observe(elapsed.Seconds())
 			if elapsed > time.Second { // 1s is pretty random, but a *local* scrape taking that long isn't good
-				log.Warningf("Local health request to %q took more than 1s: %s", url, elapsed)
+				log.Warningf("Local health request to %q took more than 1s: %s", req.URL.String(), elapsed)
 			}
 
 		case <-ctx.Done():

plugin/health/overloaded_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
 	"testing"
 	"time"
 )
@@ -22,6 +23,13 @@ func Test_health_overloaded_cancellation(t *testing.T) {
 		stop: cancel,
 	}
 
+	var err error
+	h.healthURI, err = url.Parse(ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	h.healthURI.Path = "/health"
+
 	stopped := make(chan struct{})
 	go func() {
 		h.overloaded(ctx)