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>

6 files changed, 286 insertions, 1 deletions

diff --git a/plugin/minimal/README.md b/plugin/minimal/README.md
new file mode 100644
index 000000000..e667fee25
--- /dev/null
+++ b/plugin/minimal/README.md
@@ -0,0 +1,36 @@
+# minimal
+
+## Name
+
+*minimal* - minimizes size of the DNS response message whenever possible.
+
+## Description
+
+The *minimal* plugin tries to minimize the size of the response. Depending on the response type it 
+removes resource records from the AUTHORITY and ADDITIONAL sections.
+
+Specifically this plugin looks at successful responses (this excludes negative responses, i.e.
+nodata or name error). If the successful response isn't a delegation only the RRs in the answer
+section are written to the client.
+
+## Syntax
+
+~~~ txt
+minimal
+~~~
+
+## Examples
+
+Enable minimal responses:
+
+~~~ corefile
+example.org {
+    whoami
+    forward . 8.8.8.8
+    minimal
+}
+~~~
+
+## See Also
+
+[BIND 9 Configuration Reference](https://bind9.readthedocs.io/en/latest/reference.html#boolean-options)
diff --git a/plugin/minimal/minimal.go b/plugin/minimal/minimal.go
new file mode 100644
index 000000000..6b5a7a731
--- /dev/null
+++ b/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
+}
diff --git a/plugin/minimal/minimal_test.go b/plugin/minimal/minimal_test.go
new file mode 100644
index 000000000..b579a9970
--- /dev/null
+++ b/plugin/minimal/minimal_test.go
@@ -0,0 +1,152 @@
+package minimal
+
+import (
+	"context"
+	"testing"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	"github.com/coredns/coredns/plugin/test"
+	"github.com/miekg/dns"
+)
+
+// testHandler implements plugin.Handler and will be used to create a stub handler for the test
+type testHandler struct {
+	Response *test.Case
+	Next     plugin.Handler
+}
+
+func (t *testHandler) Name() string { return "test-handler" }
+
+func (t *testHandler) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	d := new(dns.Msg)
+	d.SetReply(r)
+	if t.Response != nil {
+		d.Answer = t.Response.Answer
+		d.Ns = t.Response.Ns
+		d.Extra = t.Response.Extra
+		d.Rcode = t.Response.Rcode
+	}
+	w.WriteMsg(d)
+	return 0, nil
+}
+
+func TestMinimizeResponse(t *testing.T) {
+	baseAnswer := []dns.RR{
+		test.A("example.com.  293 IN A 142.250.76.46"),
+	}
+	baseNs := []dns.RR{
+		test.NS("example.com.  157127 IN NS ns2.example.com."),
+		test.NS("example.com.  157127 IN NS ns1.example.com."),
+		test.NS("example.com.  157127 IN NS ns3.example.com."),
+		test.NS("example.com.  157127 IN NS ns4.example.com."),
+	}
+
+	baseExtra := []dns.RR{
+		test.A("ns2.example.com. 316273 IN A 216.239.34.10"),
+		test.AAAA("ns2.example.com. 157127 IN AAAA 2001:4860:4802:34::a"),
+		test.A("ns3.example.com. 316274 IN A 216.239.36.10"),
+		test.AAAA("ns3.example.com. 157127 IN AAAA 2001:4860:4802:36::a"),
+		test.A("ns1.example.com. 165555 IN A 216.239.32.10"),
+		test.AAAA("ns1.example.com. 165555 IN AAAA 2001:4860:4802:32::a"),
+		test.A("ns4.example.com. 190188 IN A 216.239.38.10"),
+		test.AAAA("ns4.example.com. 157127 IN AAAA 2001:4860:4802:38::a"),
+	}
+
+	tests := []struct {
+		active   bool
+		original test.Case
+		minimal  test.Case
+	}{
+		{ // minimization possible NoError case
+			original: test.Case{
+				Answer: baseAnswer,
+				Ns:     nil,
+				Extra:  baseExtra,
+				Rcode:  0,
+			},
+			minimal: test.Case{
+				Answer: baseAnswer,
+				Ns:     nil,
+				Extra:  nil,
+				Rcode:  0,
+			},
+		},
+		{ // delegate response case
+			original: test.Case{
+				Answer: nil,
+				Ns:     baseNs,
+				Extra:  baseExtra,
+				Rcode:  0,
+			},
+			minimal: test.Case{
+				Answer: nil,
+				Ns:     baseNs,
+				Extra:  baseExtra,
+				Rcode:  0,
+			},
+		}, { // negative response case
+			original: test.Case{
+				Answer: baseAnswer,
+				Ns:     baseNs,
+				Extra:  baseExtra,
+				Rcode:  2,
+			},
+			minimal: test.Case{
+				Answer: baseAnswer,
+				Ns:     baseNs,
+				Extra:  baseExtra,
+				Rcode:  2,
+			},
+		},
+	}
+
+	for i, tc := range tests {
+		req := new(dns.Msg)
+		req.SetQuestion("example.com", dns.TypeA)
+
+		tHandler := &testHandler{
+			Response: &tc.original,
+			Next:     nil,
+		}
+		o := &minimalHandler{Next: tHandler}
+		rec := dnstest.NewRecorder(&test.ResponseWriter{})
+		_, err := o.ServeDNS(context.TODO(), rec, req)
+
+		if err != nil {
+			t.Errorf("Expected no error, but got %q", err)
+		}
+
+		if len(tc.minimal.Answer) != len(rec.Msg.Answer) {
+			t.Errorf("Test %d: Expected %d Answer, but got %d", i, len(tc.minimal.Answer), len(req.Answer))
+			continue
+		}
+		if len(tc.minimal.Ns) != len(rec.Msg.Ns) {
+			t.Errorf("Test %d: Expected %d Ns, but got %d", i, len(tc.minimal.Ns), len(req.Ns))
+			continue
+		}
+
+		if len(tc.minimal.Extra) != len(rec.Msg.Extra) {
+			t.Errorf("Test %d: Expected %d Extras, but got %d", i, len(tc.minimal.Extra), len(req.Extra))
+			continue
+		}
+
+		for j, a := range rec.Msg.Answer {
+			if tc.minimal.Answer[j].String() != a.String() {
+				t.Errorf("Test %d: Expected Answer %d to be %v, but got %v", i, j, tc.minimal.Answer[j], a)
+			}
+		}
+
+		for j, a := range rec.Msg.Ns {
+			if tc.minimal.Ns[j].String() != a.String() {
+				t.Errorf("Test %d: Expected NS %d to be %v, but got %v", i, j, tc.minimal.Ns[j], a)
+			}
+		}
+
+		for j, a := range rec.Msg.Extra {
+			if tc.minimal.Extra[j].String() != a.String() {
+				t.Errorf("Test %d: Expected Extra %d to be %v, but got %v", i, j, tc.minimal.Extra[j], a)
+			}
+		}
+	}
+}
diff --git a/plugin/minimal/setup.go b/plugin/minimal/setup.go
new file mode 100644
index 000000000..1bf37a652
--- /dev/null
+++ b/plugin/minimal/setup.go
@@ -0,0 +1,24 @@
+package minimal
+
+import (
+	"github.com/coredns/caddy"
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+)
+
+func init() {
+	plugin.Register("minimal", setup)
+}
+
+func setup(c *caddy.Controller) error {
+	c.Next()
+	if c.NextArg() {
+		return plugin.Error("minimal", c.ArgErr())
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		return &minimalHandler{Next: next}
+	})
+
+	return nil
+}
diff --git a/plugin/minimal/setup_test.go b/plugin/minimal/setup_test.go
new file mode 100644
index 000000000..49341c441
--- /dev/null
+++ b/plugin/minimal/setup_test.go
@@ -0,0 +1,19 @@
+package minimal
+
+import (
+	"testing"
+
+	"github.com/coredns/caddy"
+)
+
+func TestSetup(t *testing.T) {
+	c := caddy.NewTestController("dns", `minimal-response`)
+	if err := setup(c); err != nil {
+		t.Fatalf("Expected no errors, but got: %v", err)
+	}
+
+	c = caddy.NewTestController("dns", `minimal-response example.org`)
+	if err := setup(c); err == nil {
+		t.Fatalf("Expected errors, but got: %v", err)
+	}
+}
diff --git a/plugin/plugin.go b/plugin/plugin.go
index ee1bf8429..9bac48885 100644
--- a/plugin/plugin.go
+++ b/plugin/plugin.go
@@ -69,7 +69,7 @@ func (f HandlerFunc) Name() string { return "handlerfunc" }
 // Error returns err with 'plugin/name: ' prefixed to it.
 func Error(name string, err error) error { return fmt.Errorf("%s/%s: %s", "plugin", name, err) }
 
-// NextOrFailure calls next.ServeDNS when next is not nil, otherwise it will return, a ServerFailure and a nil error.
+// NextOrFailure calls next.ServeDNS when next is not nil, otherwise it will return, a ServerFailure and a `no next plugin found` error.
 func NextOrFailure(name string, next Handler, ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) { // nolint: golint
 	if next != nil {
 		if span := ot.SpanFromContext(ctx); span != nil {