Add tracing option (#487)

Adds a middleware to enable tracing with OpenTracing/OpenZipkin.
Enabling tracing will have a large impact on performance so it is
not advisable in production.

middleware/trace/setup.go

@@ -0,0 +1,87 @@
+package trace
+
+import (
+	"fmt"
+	"strings"
+	"sync"
+
+	"github.com/miekg/coredns/core/dnsserver"
+	"github.com/miekg/coredns/middleware"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("trace", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	t, err := traceParse(c)
+	if err != nil {
+		return middleware.Error("trace", err)
+	}
+
+	dnsserver.GetConfig(c).AddMiddleware(func(next middleware.Handler) middleware.Handler {
+		t.Next = next
+		return t
+	})
+
+	traceOnce.Do(func() {
+		c.OnStartup(t.OnStartup)
+	})
+
+	return nil
+}
+
+func traceParse(c *caddy.Controller) (*Trace, error) {
+	var (
+		tr = &Trace{Endpoint: defEP, EndpointType: defEpType}
+		err error
+	)
+
+	cfg := dnsserver.GetConfig(c)
+	tr.ServiceEndpoint = cfg.ListenHost + ":" + cfg.Port
+	for c.Next() {
+		if c.Val() == "trace" {
+			var err error
+			args := c.RemainingArgs()
+			switch len(args) {
+			case 0:
+				tr.Endpoint, err = normalizeEndpoint(tr.EndpointType, defEP)
+			case 1:
+				tr.Endpoint, err = normalizeEndpoint(defEpType, args[0])
+			case 2:
+				tr.EndpointType = strings.ToLower(args[0])
+				tr.Endpoint, err = normalizeEndpoint(tr.EndpointType, args[1])
+			default:
+				err = c.ArgErr()
+			}
+			if err != nil {
+				return tr, err
+			}
+		}
+	}
+	return tr, err
+}
+
+func normalizeEndpoint(epType, ep string) (string, error) {
+	switch epType {
+	case "zipkin":
+		if strings.Index(ep, "http") == -1 {
+			ep = "http://" + ep + "/api/v1/spans"
+		}
+		return ep, nil
+	default:
+		return "", fmt.Errorf("Tracing endpoint type '%s' is not supported.", epType)
+	}
+}
+
+var traceOnce sync.Once
+
+const (
+	defEP = "localhost:9411"
+	defEpType = "zipkin"
+)