feat: Add RCode DNS client

dns/rcode.go

@@ -0,0 +1,54 @@
+package dns
+
+import (
+	"context"
+	"fmt"
+
+	D "github.com/miekg/dns"
+)
+
+func newRCodeClient(addr string) rcodeClient {
+	var rcode int
+	switch addr {
+	case "success":
+		rcode = D.RcodeSuccess
+	case "format_error":
+		rcode = D.RcodeFormatError
+	case "server_failure":
+		rcode = D.RcodeServerFailure
+	case "name_error":
+		rcode = D.RcodeNameError
+	case "not_implemented":
+		rcode = D.RcodeNotImplemented
+	case "refused":
+		rcode = D.RcodeRefused
+	default:
+		panic(fmt.Errorf("unsupported RCode type: %s", addr))
+	}
+
+	return rcodeClient{
+		rcode: rcode,
+		addr:  "rcode://" + addr,
+	}
+}
+
+type rcodeClient struct {
+	rcode int
+	addr  string
+}
+
+var _ dnsClient = rcodeClient{}
+
+func (r rcodeClient) Exchange(m *D.Msg) (*D.Msg, error) {
+	m.Response = true
+	m.Rcode = r.rcode
+	return m, nil
+}
+
+func (r rcodeClient) ExchangeContext(ctx context.Context, m *D.Msg) (*D.Msg, error) {
+	return r.Exchange(m)
+}
+
+func (r rcodeClient) Address() string {
+	return r.addr
+}