Add _grpc_config replies

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/traffic/lb.go

+package traffic
+
+import "github.com/miekg/dns"
+
+// See https://github.com/grpc/grpc/blob/master/doc/service_config.md for the fields in this proto.
+// We encode it as json and return it in a TXT field.
+var lbTXT = `grpc_config=[{"serviceConfig":{"loadBalancingConfig":[{"xds_experimental":{"lrs_load_reporting_server_name":""}}]}}]`
+
+func txt(z string) []dns.RR {
+	return []dns.RR{&dns.TXT{
+		Hdr: dns.RR_Header{Name: z, Rrtype: dns.TypeTXT, Class: dns.ClassINET, Ttl: 5},
+		Txt: []string{lbTXT},
+	}}
+}

plugin/traffic/setup.go

@@ -2,6 +2,7 @@ package traffic
 
 import (
 	"crypto/tls"
+	"encoding/json"
 	"fmt"
 	"math/rand"
 	"strings"
@@ -31,6 +32,9 @@ func setup(c *caddy.Controller) error {
 	if err != nil {
 		return plugin.Error("traffic", err)
 	}
+	if _, err := json.Marshal(lbTXT); err != nil {
+		return fmt.Errorf("failed to marshal grpc serverConfig: %s", err)
+	}
 
 	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
 		t.Next = next

plugin/traffic/setup_test.go

 package traffic
 
 import (
+	"encoding/json"
 	"testing"
 
 	"github.com/caddyserver/caddy"
@@ -13,6 +14,16 @@ func TestSetup(t *testing.T) {
 	}
 }
 
+func TestLBTxt(t *testing.T) {
+	_, err := json.Marshal(lbTXT)
+	if err != nil {
+		t.Fatalf("Failed to marshal grpc serverConfig: %s", err)
+	}
+	if len(lbTXT) > 255 {
+		t.Fatalf("Too long grpc serverConfig (>255): %d", len(lbTXT))
+	}
+}
+
 func TestParseTraffic(t *testing.T) {
 	tests := []struct {
 		input     string

plugin/traffic/traffic.go

@@ -69,10 +69,9 @@ func (t *Traffic) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 				return 0, nil
 			}
 			if strings.HasPrefix(strings.ToLower(labels[0]), "_grpc_config") {
-				// this is the grpc config blob encoded in a TXT record, we just return a NXDOMAIN
-				// make this a separate so we can insert some logic later.
-				m.Ns = soa(state.Zone)
-				m.Rcode = dns.RcodeNameError
+				// this is the grpc config blob encoded in a TXT record, see documentation for lbTXT.
+				m.Answer = txt(state.Zone)
+				m.Rcode = dns.RcodeSuccess
 				w.WriteMsg(m)
 				return 0, nil
 

plugin/traffic/traffic_test.go

@@ -125,7 +125,7 @@ func TestTraffic(t *testing.T) {
 					{"127.0.0.2", 18008, corepb.HealthStatus_HEALTHY},
 				}),
 			},
-			cluster: "_grpc_config.web", qtype: dns.TypeTXT, rcode: dns.RcodeNameError, ns: true,
+			cluster: "_grpc_config.web", qtype: dns.TypeTXT, rcode: dns.RcodeSuccess,
 		},
 	}