middleware/proxy: Allow non-HTTP upstreams to be health checked (#589)

Allow HTTP health check to be performed against a regular DNS upstream server.

TODO: Add tests.

test/proxy_http_health_test.go

@@ -0,0 +1,95 @@
+package test
+
+import (
+	"io"
+	"io/ioutil"
+	"log"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+
+	"github.com/coredns/coredns/middleware/proxy"
+	"github.com/coredns/coredns/middleware/test"
+	"github.com/coredns/coredns/request"
+	"github.com/miekg/dns"
+)
+
+func TestProxyWithHTTPCheckOK(t *testing.T) {
+	log.SetOutput(ioutil.Discard)
+
+	healthCheckServer := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusOK)
+			io.WriteString(w, "OK\n")
+		}))
+	defer healthCheckServer.Close()
+
+	healthCheckURL, err := url.Parse(healthCheckServer.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	healthCheckPort := healthCheckURL.Port()
+
+	name, rm, err := test.TempFile(".", exampleOrg)
+	if err != nil {
+		t.Fatalf("failed to create zone: %s", err)
+	}
+	defer rm()
+
+	// We have to bind to 127.0.0.1 because the server started by
+	// httptest.NewServer does, and the IP addresses of the backend
+	// DNS and HTTP servers must match.
+	authoritativeCorefile := `example.org:0 {
+	   bind 127.0.0.1
+       file ` + name + `
+}
+`
+
+	authoritativeInstance, err := CoreDNSServer(authoritativeCorefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS authoritative instance: %s", err)
+	}
+
+	authoritativeAddr, _ := CoreDNSServerPorts(authoritativeInstance, 0)
+	if authoritativeAddr == "" {
+		t.Fatalf("Could not get CoreDNS authoritative instance UDP listening port")
+	}
+	defer authoritativeInstance.Stop()
+
+	proxyCorefile := `example.org:0 {
+    proxy . ` + authoritativeAddr + ` {
+		health_check /health:` + healthCheckPort + ` 1s
+
+	}
+}
+`
+
+	proxyInstance, err := CoreDNSServer(proxyCorefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS proxy instance: %s", err)
+	}
+
+	proxyAddr, _ := CoreDNSServerPorts(proxyInstance, 0)
+	if proxyAddr == "" {
+		t.Fatalf("Could not get CoreDNS proxy instance UDP listening port")
+	}
+	defer proxyInstance.Stop()
+
+	p := proxy.NewLookup([]string{proxyAddr})
+	state := request.Request{W: &test.ResponseWriter{}, Req: new(dns.Msg)}
+	resp, err := p.Lookup(state, "example.org.", dns.TypeA)
+	if err != nil {
+		t.Fatal("Expected to receive reply, but didn't")
+	}
+	// expect answer section with A record in it
+	if len(resp.Answer) == 0 {
+		t.Fatalf("Expected to at least one RR in the answer section, got none: %s", resp)
+	}
+	if resp.Answer[0].Header().Rrtype != dns.TypeA {
+		t.Errorf("Expected RR to A, got: %d", resp.Answer[0].Header().Rrtype)
+	}
+	if resp.Answer[0].(*dns.A).A.String() != "127.0.0.1" {
+		t.Errorf("Expected 127.0.0.1, got: %s", resp.Answer[0].(*dns.A).A.String())
+	}
+}