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/proxy.go

@@ -0,0 +1,81 @@
+package grpc
+
+import (
+	"context"
+	"crypto/tls"
+	"strconv"
+	"time"
+
+	"github.com/coredns/coredns/pb"
+
+	"github.com/miekg/dns"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/status"
+)
+
+// Proxy defines an upstream host.
+type Proxy struct {
+	addr string
+
+	// connection
+	client   pb.DnsServiceClient
+	dialOpts []grpc.DialOption
+}
+
+// newProxy returns a new proxy.
+func newProxy(addr string, tlsConfig *tls.Config) (*Proxy, error) {
+	p := &Proxy{
+		addr: addr,
+	}
+
+	if tlsConfig != nil {
+		p.dialOpts = append(p.dialOpts, grpc.WithTransportCredentials(credentials.NewTLS(tlsConfig)))
+	} else {
+		p.dialOpts = append(p.dialOpts, grpc.WithInsecure())
+	}
+
+	conn, err := grpc.Dial(p.addr, p.dialOpts...)
+	if err != nil {
+		return nil, err
+	}
+	p.client = pb.NewDnsServiceClient(conn)
+
+	return p, nil
+}
+
+// query sends the request and waits for a response.
+func (p *Proxy) query(ctx context.Context, req *dns.Msg) (*dns.Msg, error) {
+	start := time.Now()
+
+	msg, err := req.Pack()
+	if err != nil {
+		return nil, err
+	}
+
+	reply, err := p.client.Query(ctx, &pb.DnsPacket{Msg: msg})
+	if err != nil {
+		// if not found message, return empty message with NXDomain code
+		if status.Code(err) == codes.NotFound {
+			m := new(dns.Msg).SetRcode(req, dns.RcodeNameError)
+			return m, nil
+		}
+		return nil, err
+	}
+	ret := new(dns.Msg)
+	if err := ret.Unpack(reply.Msg); err != nil {
+		return nil, err
+	}
+
+	rc, ok := dns.RcodeToString[ret.Rcode]
+	if !ok {
+		rc = strconv.Itoa(ret.Rcode)
+	}
+
+	RequestCount.WithLabelValues(p.addr).Add(1)
+	RcodeCount.WithLabelValues(rc, p.addr).Add(1)
+	RequestDuration.WithLabelValues(p.addr).Observe(time.Since(start).Seconds())
+
+	return ret, nil
+}