plugin/grpc: New gRPC plugin (#2667)

* plugin/grpc: New gRPC plugin

* some changes after the first review:

- remove healthcheck. gRPC already has this implicitly implemented
- some naming and stetic changes
- fix some comments
- other minor fixes

* plugin/grpc: New gRPC plugin

* some changes after the first review:

- remove healthcheck. gRPC already has this implicitly implemented
- some naming and stetic changes
- fix some comments
- other minor fixes

* add OWNERS file and change plugin order

* remove Rcode checker

plugin/grpc/grpc.go

@@ -0,0 +1,130 @@
+package grpc
+
+import (
+	"context"
+	"crypto/tls"
+	"time"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/debug"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+	ot "github.com/opentracing/opentracing-go"
+)
+
+// GRPC represents a plugin instance that can proxy requests to another (DNS) server via gRPC protocol.
+// It has a list of proxies each representing one upstream proxy.
+type GRPC struct {
+	proxies []*Proxy
+	p       Policy
+
+	from    string
+	ignored []string
+
+	tlsConfig     *tls.Config
+	tlsServerName string
+
+	Next plugin.Handler
+}
+
+// ServeDNS implements the plugin.Handler interface.
+func (g *GRPC) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	state := request.Request{W: w, Req: r}
+	if !g.match(state) {
+		return plugin.NextOrFailure(g.Name(), g.Next, ctx, w, r)
+	}
+
+	var (
+		span, child ot.Span
+		ret         *dns.Msg
+		err         error
+		i           int
+	)
+	span = ot.SpanFromContext(ctx)
+	list := g.list()
+	deadline := time.Now().Add(defaultTimeout)
+
+	for time.Now().Before(deadline) {
+		if i >= len(list) {
+			// reached the end of list without any answer
+			if ret != nil {
+				// write empty response and finish
+				w.WriteMsg(ret)
+			}
+			break
+		}
+
+		proxy := list[i]
+		i++
+
+		if span != nil {
+			child = span.Tracer().StartSpan("query", ot.ChildOf(span.Context()))
+			ctx = ot.ContextWithSpan(ctx, child)
+		}
+
+		ret, err = proxy.query(ctx, r)
+		if err != nil {
+			// Continue with the next proxy
+			continue
+		}
+
+		if child != nil {
+			child.Finish()
+		}
+
+		// Check if the reply is correct; if not return FormErr.
+		if !state.Match(ret) {
+			debug.Hexdumpf(ret, "Wrong reply for id: %d, %s %d", ret.Id, state.QName(), state.QType())
+
+			formerr := state.ErrorMessage(dns.RcodeFormatError)
+			w.WriteMsg(formerr)
+			return 0, nil
+		}
+
+		w.WriteMsg(ret)
+		return 0, nil
+	}
+
+	return 0, nil
+}
+
+// NewGRPC returns a new GRPC.
+func newGRPC() *GRPC {
+	g := &GRPC{
+		p: new(random),
+	}
+	return g
+}
+
+// Name implements the Handler interface.
+func (g *GRPC) Name() string { return "grpc" }
+
+// Len returns the number of configured proxies.
+func (g *GRPC) len() int { return len(g.proxies) }
+
+func (g *GRPC) match(state request.Request) bool {
+	if !plugin.Name(g.from).Matches(state.Name()) || !g.isAllowedDomain(state.Name()) {
+		return false
+	}
+
+	return true
+}
+
+func (g *GRPC) isAllowedDomain(name string) bool {
+	if dns.Name(name) == dns.Name(g.from) {
+		return true
+	}
+
+	for _, ignore := range g.ignored {
+		if plugin.Name(ignore).Matches(name) {
+			return false
+		}
+	}
+	return true
+}
+
+// List returns a set of proxies to be used for this client depending on the policy in p.
+func (g *GRPC) list() []*Proxy { return g.p.List(g.proxies) }
+
+const defaultTimeout = 5 * time.Second