Added minimal-responses plugin (#4417)

* Added minimal-responses plugin

Signed-off-by: Soumya Ghosh Dastidar <gdsoumya@gmail.com>

* Removed unnecessary comments

* Updated tests

Signed-off-by: Soumya Ghosh Dastidar <gdsoumya@gmail.com>

* Reformated imports

Signed-off-by: Soumya Ghosh Dastidar <gdsoumya@gmail.com>

* Updated package name

Signed-off-by: Soumya Ghosh Dastidar <gdsoumya@gmail.com>

* Removed  unnecessary comments

Co-authored-by: Miek Gieben <miek@miek.nl>

* Added changes

Signed-off-by: Soumya Ghosh Dastidar <gdsoumya@gmail.com>

* updated

Signed-off-by: Soumya Ghosh Dastidar <gdsoumya@gmail.com>

* Updated comment for NextOrFailure

Signed-off-by: Soumya Ghosh Dastidar <gdsoumya@gmail.com>

* Updated to test.Case for testing

Signed-off-by: Soumya Ghosh Dastidar <gdsoumya@gmail.com>

* Formated imports using goimports

Signed-off-by: Soumya Ghosh Dastidar <gdsoumya@gmail.com>

Co-authored-by: Miek Gieben <miek@miek.nl>

plugin/minimal/minimal.go

@@ -0,0 +1,54 @@
+package minimal
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/nonwriter"
+	"github.com/coredns/coredns/plugin/pkg/response"
+	"github.com/miekg/dns"
+)
+
+// minimalHandler implements the plugin.Handler interface.
+type minimalHandler struct {
+	Next plugin.Handler
+}
+
+func (m *minimalHandler) Name() string { return "minimal" }
+
+func (m *minimalHandler) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	nw := nonwriter.New(w)
+
+	rcode, err := plugin.NextOrFailure(m.Name(), m.Next, ctx, nw, r)
+	if err != nil {
+		return rcode, err
+	}
+
+	ty, _ := response.Typify(nw.Msg, time.Now().UTC())
+	cl := response.Classify(ty)
+
+	// if response is Denial or Error pass through also if the type is Delegation pass through
+	if cl == response.Denial || cl == response.Error || ty == response.Delegation {
+		w.WriteMsg(nw.Msg)
+		return 0, nil
+	}
+	if ty != response.NoError {
+		w.WriteMsg(nw.Msg)
+		return 0, plugin.Error("minimal", fmt.Errorf("unhandled response type %q for %q", ty, nw.Msg.Question[0].Name))
+	}
+
+	// copy over the original Msg params, deep copy not required as RRs are not modified
+	d := &dns.Msg{
+		MsgHdr:   nw.Msg.MsgHdr,
+		Compress: nw.Msg.Compress,
+		Question: nw.Msg.Question,
+		Answer:   nw.Msg.Answer,
+		Ns:       nil,
+		Extra:    nil,
+	}
+
+	w.WriteMsg(d)
+	return 0, nil
+}