Rewrite edns0 (#561)

* Add edns0 code rewrite

* check arg count

* change `new`; set EDNS0 if request doesn't have it set

* change set to replace_or_append

* change to append_or_replace

* return error in new

* update documents

* fixt UT

* return error

* go fmt

* Rework for more general EDNS0 use

Also changed how rules are created and validated. Implements
EDNS0 NSID in addition to local.

* go fmt

* README updates, NSID tests and fixes

* gofmt -s -w

* Fix tests for rewrite syntax change

* Add tests, fix error message

* Review nits

* Missed on nit

* More tests, integration test, fix edns0 parse issue

* Fix README, use RewriteIgnored

* go fmt

1 files changed, 94 insertions, 0 deletions

diff --git a/test/rewrite_test.go b/test/rewrite_test.go
new file mode 100644
index 000000000..9aedd7417
--- /dev/null
+++ b/test/rewrite_test.go
@@ -0,0 +1,94 @@
+package test
+
+import (
+	"bytes"
+	"io/ioutil"
+	"log"
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestRewrite(t *testing.T) {
+	t.Parallel()
+	corefile := `.:0 {
+       rewrite type MX a
+       rewrite edns0 local set 0xffee hello-world
+       erratic . {
+	drop 0
+	}
+}`
+
+	i, err := CoreDNSServer(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+
+	udp, _ := CoreDNSServerPorts(i, 0)
+	if udp == "" {
+		t.Fatalf("Could not get UDP listening port")
+	}
+	defer i.Stop()
+
+	log.SetOutput(ioutil.Discard)
+
+	testMX(t, udp)
+	testEdns0(t, udp)
+}
+
+func testMX(t *testing.T, server string) {
+	m := new(dns.Msg)
+	m.SetQuestion("example.com.", dns.TypeMX)
+
+	r, err := dns.Exchange(m, server)
+	if err != nil {
+		t.Fatalf("Expected to receive reply, but didn't: %s", err)
+	}
+
+	// expect answer section with A record in it
+	if len(r.Answer) == 0 {
+		t.Error("Expected to at least one RR in the answer section, got none")
+	}
+	if r.Answer[0].Header().Rrtype != dns.TypeA {
+		t.Errorf("Expected RR to A, got: %d", r.Answer[0].Header().Rrtype)
+	}
+	if r.Answer[0].(*dns.A).A.String() != "192.0.2.53" {
+		t.Errorf("Expected 192.0.2.53, got: %s", r.Answer[0].(*dns.A).A.String())
+	}
+}
+
+func testEdns0(t *testing.T, server string) {
+	m := new(dns.Msg)
+	m.SetQuestion("example.com.", dns.TypeA)
+
+	r, err := dns.Exchange(m, server)
+	if err != nil {
+		t.Fatalf("Expected to receive reply, but didn't: %s", err)
+	}
+
+	// expect answer section with A record in it
+	if len(r.Answer) == 0 {
+		t.Error("Expected to at least one RR in the answer section, got none")
+	}
+	if r.Answer[0].Header().Rrtype != dns.TypeA {
+		t.Errorf("Expected RR to A, got: %d", r.Answer[0].Header().Rrtype)
+	}
+	if r.Answer[0].(*dns.A).A.String() != "192.0.2.53" {
+		t.Errorf("Expected 192.0.2.53, got: %s", r.Answer[0].(*dns.A).A.String())
+	}
+	o := r.IsEdns0()
+	if o == nil || len(o.Option) == 0 {
+		t.Error("Expected EDNS0 options but got none")
+	} else {
+		if e, ok := o.Option[0].(*dns.EDNS0_LOCAL); ok {
+			if e.Code != 0xffee {
+				t.Errorf("Expected EDNS_LOCAL code 0xffee but got %x", e.Code)
+			}
+			if !bytes.Equal(e.Data, []byte("hello-world")) {
+				t.Errorf("Expected EDNS_LOCAL data 'hello-world' but got %q", e.Data)
+			}
+		} else {
+			t.Errorf("Expected EDNS0_LOCAL but got %v", o.Option[0])
+		}
+	}
+}