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))