lib/dns: refactoring the section header

This changes rename the SectionHeader into MessageHeader.

The pack() method is optimized with the following results,

benchmark                         old ns/op     new ns/op     delta
BenchmarkMessageHeader_pack-8     66.2          21.7          -67.31%

benchmark                         old allocs     new allocs     delta
BenchmarkMessageHeader_pack-8     3              1              -66.67%

benchmark                         old bytes     new bytes     delta
BenchmarkMessageHeader_pack-8     32            16            -50.00%

The unpack() method is simplified by minimizing the if-condition.

This changes also fix the pack and unpack OpCode for value other then 0,
due to wrong shift value, 2 instead of 3.

17 files changed, 463 insertions, 140 deletions

diff --git a/.gitignore b/.gitignore
index 69b1ab51..d3afe736 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
+**/testdata/bench
+**/testdata/pprof
 *.bak
 *.bench
 *.cprof
diff --git a/lib/dns/answer_test.go b/lib/dns/answer_test.go
index 6f95695c..1c083233 100644
--- a/lib/dns/answer_test.go
+++ b/lib/dns/answer_test.go
@@ -16,7 +16,7 @@ func TestNewAnswer(t *testing.T) {
 	at := time.Now().Unix()
 
 	msg1 := &Message{
-		Header: SectionHeader{
+		Header: MessageHeader{
 			ID: 1,
 		},
 		Question: SectionQuestion{
@@ -120,7 +120,7 @@ func TestAnswerClear(t *testing.T) {
 func TestAnswerGet(t *testing.T) {
 	// kilabit.info A
 	res := &Message{
-		Header: SectionHeader{
+		Header: MessageHeader{
 			ID:      1,
 			QDCount: 1,
 			ANCount: 1,
@@ -183,7 +183,7 @@ func TestAnswerGet(t *testing.T) {
 		test.Assert(t, "ReceivedAt", an.ReceivedAt >= at-5, true)
 		test.Assert(t, "AccessedAt", an.AccessedAt >= at, true)
 		got := &Message{
-			Header:   SectionHeader{},
+			Header:   MessageHeader{},
 			Question: SectionQuestion{},
 			packet:   gotPacket,
 		}
@@ -200,12 +200,12 @@ func TestAnswerGet(t *testing.T) {
 func TestAnswerUpdate(t *testing.T) {
 	at := time.Now().Unix() - 5
 	msg1 := &Message{
-		Header: SectionHeader{
+		Header: MessageHeader{
 			ID: 1,
 		},
 	}
 	msg2 := &Message{
-		Header: SectionHeader{
+		Header: MessageHeader{
 			ID: 1,
 		},
 	}
diff --git a/lib/dns/answers_test.go b/lib/dns/answers_test.go
index b17a305f..f3f758c3 100644
--- a/lib/dns/answers_test.go
+++ b/lib/dns/answers_test.go
@@ -159,7 +159,7 @@ func TestAnswersUpdate(t *testing.T) {
 		RType:  1,
 		RClass: 1,
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID: 1,
 			},
 		},
@@ -178,7 +178,7 @@ func TestAnswersUpdate(t *testing.T) {
 		RType:  1,
 		RClass: 1,
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID: 2,
 			},
 		},
diff --git a/lib/dns/caches_test.go b/lib/dns/caches_test.go
index 17781d93..6d00a059 100644
--- a/lib/dns/caches_test.go
+++ b/lib/dns/caches_test.go
@@ -48,7 +48,7 @@ func TestCachesGet(t *testing.T) {
 		RType:      1,
 		RClass:     1,
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID: 1,
 			},
 		},
@@ -59,7 +59,7 @@ func TestCachesGet(t *testing.T) {
 		RType:      2,
 		RClass:     1,
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID: 2,
 			},
 		},
@@ -70,7 +70,7 @@ func TestCachesGet(t *testing.T) {
 		RType:      3,
 		RClass:     1,
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID: 3,
 			},
 		},
@@ -126,7 +126,7 @@ func TestCachesPrune(t *testing.T) {
 		RType:      1,
 		RClass:     1,
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID: 1,
 			},
 		},
@@ -138,7 +138,7 @@ func TestCachesPrune(t *testing.T) {
 		RType:      2,
 		RClass:     1,
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID: 2,
 			},
 		},
@@ -150,7 +150,7 @@ func TestCachesPrune(t *testing.T) {
 		RType:      3,
 		RClass:     1,
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID: 3,
 			},
 		},
@@ -195,7 +195,7 @@ func TestCachesUpsert(t *testing.T) {
 		RType:      1,
 		RClass:     1,
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID: 1,
 			},
 		},
@@ -207,7 +207,7 @@ func TestCachesUpsert(t *testing.T) {
 		RType:      1,
 		RClass:     1,
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID: 3,
 			},
 		},
@@ -219,7 +219,7 @@ func TestCachesUpsert(t *testing.T) {
 		RType:      2,
 		RClass:     1,
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID: 2,
 			},
 		},
@@ -231,7 +231,7 @@ func TestCachesUpsert(t *testing.T) {
 		RType:      2,
 		RClass:     1,
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID: 4,
 			},
 		},
diff --git a/lib/dns/dns.go b/lib/dns/dns.go
index 8342933d..28a62d32 100644
--- a/lib/dns/dns.go
+++ b/lib/dns/dns.go
@@ -93,9 +93,9 @@ type OpCode byte
 
 // List of valid operation code.
 const (
-	OpCodeQuery  OpCode = iota // a standard query (QUERY)
-	OpCodeIQuery               // an inverse query (IQUERY), obsolete by RFC3425
-	OpCodeStatus               // a server status request (STATUS)
+	OpCodeQuery  OpCode = iota // A standard query (QUERY)
+	OpCodeIQuery               // An inverse query (IQUERY), obsolete by RFC3425
+	OpCodeStatus               // A server status request (STATUS)
 )
 
 // ResponseCode define response code in message header.
diff --git a/lib/dns/doh_client_test.go b/lib/dns/doh_client_test.go
index b8a3e2ab..3309a1ff 100644
--- a/lib/dns/doh_client_test.go
+++ b/lib/dns/doh_client_test.go
@@ -31,7 +31,7 @@ func TestDoHClient_Lookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0,
 				IsAA:    true,
 				QDCount: 1,
@@ -59,7 +59,7 @@ func TestDoHClient_Lookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0,
 				IsAA:    true,
 				QDCount: 1,
@@ -94,7 +94,7 @@ func TestDoHClient_Lookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0,
 				IsAA:    true,
 				QDCount: 1,
@@ -121,7 +121,7 @@ func TestDoHClient_Lookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0,
 				IsAA:    false,
 				RCode:   RCodeErrServer,
@@ -176,7 +176,7 @@ func TestDoHClient_Post(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0,
 				IsAA:    true,
 				QDCount: 1,
@@ -204,7 +204,7 @@ func TestDoHClient_Post(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0,
 				IsAA:    true,
 				QDCount: 1,
@@ -239,7 +239,7 @@ func TestDoHClient_Post(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0,
 				IsAA:    true,
 				QDCount: 1,
@@ -266,7 +266,7 @@ func TestDoHClient_Post(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0,
 				IsAA:    false,
 				RCode:   RCodeErrServer,
diff --git a/lib/dns/dot_client_test.go b/lib/dns/dot_client_test.go
index 25701993..ee7c55fb 100644
--- a/lib/dns/dot_client_test.go
+++ b/lib/dns/dot_client_test.go
@@ -29,7 +29,7 @@ func TestDoTClient_Lookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      5,
 				IsAA:    true,
 				QDCount: 1,
@@ -57,7 +57,7 @@ func TestDoTClient_Lookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      6,
 				IsAA:    true,
 				QDCount: 1,
@@ -92,7 +92,7 @@ func TestDoTClient_Lookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      7,
 				IsAA:    true,
 				QDCount: 1,
diff --git a/lib/dns/message.go b/lib/dns/message.go
index 8164f36b..0fa4532a 100644
--- a/lib/dns/message.go
+++ b/lib/dns/message.go
@@ -51,7 +51,7 @@ import (
 // [1] RFC 1035 - 4.1. Format
 //
 type Message struct {
-	Header     SectionHeader
+	Header     MessageHeader
 	Question   SectionQuestion
 	Answer     []ResourceRecord
 	Authority  []ResourceRecord
@@ -74,7 +74,7 @@ type Message struct {
 //
 func NewMessage() *Message {
 	return &Message{
-		Header: SectionHeader{
+		Header: MessageHeader{
 			IsQuery: true,
 			IsRD:    true,
 			QDCount: 1,
@@ -120,7 +120,7 @@ func NewMessageAddress(hname []byte, addresses [][]byte) (msg *Message) {
 	}
 
 	msg = &Message{
-		Header: SectionHeader{
+		Header: MessageHeader{
 			IsAA:    true,
 			QDCount: 1,
 			ANCount: 1,
@@ -164,7 +164,7 @@ func NewMessageAddress(hname []byte, addresses [][]byte) (msg *Message) {
 //
 func NewMessageFromRR(rr *ResourceRecord) (msg *Message, err error) {
 	msg = &Message{
-		Header: SectionHeader{
+		Header: MessageHeader{
 			IsAA:    true,
 			QDCount: 1,
 			ANCount: 1,
diff --git a/lib/dns/section_header.go b/lib/dns/message_header.go
index 2844fcfb..a0d7d7bb 100644
--- a/lib/dns/section_header.go
+++ b/lib/dns/message_header.go
@@ -10,22 +10,25 @@ import (
 
 const (
 	headerIsQuery    byte = 0x00
-	headerIsResponse byte = 0x80
-	headerIsAA       byte = 0x04
-	headerIsTC       byte = 0x02
-	headerIsRD       byte = 0x01
-	headerIsRA       byte = 0x80
+	headerIsResponse byte = 0x80 // 1000.0000
+	headerMaskOpCode byte = 0x78 // 0111.1000
+	headerIsAA       byte = 0x04 // 0000.0100
+	headerIsTC       byte = 0x02 // 0000.0010
+	headerIsRD       byte = 0x01 // 0000.0001
+	headerIsRA       byte = 0x80 //          1000.0000
+	headerMaskRCode  byte = 0x0F //          0000.1111
 )
 
 //
-// SectionHeader The header section is always present.  The header includes
-// fields that specify which of the remaining sections are present, and also
-// specify whether the message is a query or a response, a standard query or
-// some other opcode, etc. [1]
+// MessageHeader the header includes fields that specify which of the
+// remaining sections are present, and also specify whether the message is a
+// query or a response, a standard query or some other opcode, etc. [1]
+//
+// The header section is always present.
 //
 // [1] RFC 1035 P-25 - 4.1. Format
 //
-type SectionHeader struct {
+type MessageHeader struct {
 	//
 	// A 16 bit identifier assigned by the program that generates
 	// any kind of query.  This identifier is copied the corresponding
@@ -82,27 +85,25 @@ type SectionHeader struct {
 	//
 	RCode ResponseCode
 
-	// An unsigned 16 bit integer specifying the number of entries in the
-	// question section.
+	// The number of entries in the question section.
 	QDCount uint16
 
-	// An unsigned 16 bit integer specifying the number of resource
-	// records in the answer section.
+	// The number of resource records in the answer section.
 	ANCount uint16
 
-	// An unsigned 16 bit integer specifying the number of name server
-	// resource records in the authority records section.
+	// The number of name server resource records in the authority records
+	// section.
 	NSCount uint16
 
-	// An unsigned 16 bit integer specifying the number of resource
-	// records in the additional records section.
+	// The number of resource records in the additional records section.
 	ARCount uint16
 }
 
 //
-// Reset the header to default (query) values.
+// Reset the header to default (query) values, which mean the IsQuery is true,
+// the Op code is 0, with recursion enabled, and query count set tot 1.
 //
-func (hdr *SectionHeader) Reset() {
+func (hdr *MessageHeader) Reset() {
 	hdr.ID = 0
 	hdr.IsQuery = true
 	hdr.Op = OpCodeQuery
@@ -111,7 +112,7 @@ func (hdr *SectionHeader) Reset() {
 	hdr.IsRD = true
 	hdr.IsRA = false
 	hdr.RCode = RCodeOK
-	hdr.QDCount = 0
+	hdr.QDCount = 1
 	hdr.ANCount = 0
 	hdr.NSCount = 0
 	hdr.ARCount = 0
@@ -120,10 +121,11 @@ func (hdr *SectionHeader) Reset() {
 //
 // pack the section header into slice of bytes.
 //
-func (hdr *SectionHeader) pack() []byte {
-	var b0, b1 byte
-
-	packet := make([]byte, 4)
+func (hdr *MessageHeader) pack() []byte {
+	var (
+		b0, b1 byte
+		packet [12]byte
+	)
 
 	packet[0] = byte(hdr.ID >> 8)
 	packet[1] = byte(hdr.ID)
@@ -134,7 +136,7 @@ func (hdr *SectionHeader) pack() []byte {
 		b0 = headerIsResponse
 	}
 
-	b0 |= (0x78 & byte(hdr.Op<<2))
+	b0 |= (headerMaskOpCode & byte(hdr.Op<<3))
 
 	if hdr.IsRD {
 		b0 |= headerIsRD
@@ -150,48 +152,33 @@ func (hdr *SectionHeader) pack() []byte {
 		if hdr.IsRA {
 			b1 |= headerIsRA
 		}
-		b1 |= (0x0F & byte(hdr.RCode))
+		b1 |= (headerMaskRCode & byte(hdr.RCode))
 	}
 
 	packet[2] = b0
 	packet[3] = b1
 
-	packet = libbytes.AppendUint16(packet, hdr.QDCount)
-	packet = libbytes.AppendUint16(packet, hdr.ANCount)
-	packet = libbytes.AppendUint16(packet, hdr.NSCount)
-	packet = libbytes.AppendUint16(packet, hdr.ARCount)
+	libbytes.WriteUint16(packet[:], 4, hdr.QDCount)
+	libbytes.WriteUint16(packet[:], 6, hdr.ANCount)
+	libbytes.WriteUint16(packet[:], 8, hdr.NSCount)
+	libbytes.WriteUint16(packet[:], 10, hdr.ARCount)
 
-	return packet
+	return packet[:]
 }
 
 //
 // unpack the DNS header section.
 //
-func (hdr *SectionHeader) unpack(packet []byte) {
+func (hdr *MessageHeader) unpack(packet []byte) {
 	hdr.ID = libbytes.ReadUint16(packet, 0)
 
-	if packet[2]&headerIsResponse == headerIsResponse {
-		hdr.IsQuery = false
-	}
-	hdr.Op = OpCode((packet[2] & 0x78) >> 2)
-
-	if packet[2]&headerIsAA == headerIsAA {
-		hdr.IsAA = true
-	}
-	if packet[2]&headerIsTC == headerIsTC {
-		hdr.IsTC = true
-	}
-	if packet[2]&headerIsRD == headerIsRD {
-		hdr.IsRD = true
-	} else {
-		hdr.IsRD = false
-	}
-
-	if packet[3]&headerIsRA == headerIsRA {
-		hdr.IsRA = true
-	}
-
-	hdr.RCode = ResponseCode(0x0F & packet[3])
+	hdr.IsQuery = packet[2]&headerIsResponse != headerIsResponse
+	hdr.Op = OpCode((packet[2] & headerMaskOpCode) >> 3)
+	hdr.IsAA = packet[2]&headerIsAA == headerIsAA
+	hdr.IsTC = packet[2]&headerIsTC == headerIsTC
+	hdr.IsRD = packet[2]&headerIsRD == headerIsRD
+	hdr.IsRA = packet[3]&headerIsRA == headerIsRA
+	hdr.RCode = ResponseCode(headerMaskRCode & packet[3])
 
 	hdr.QDCount = libbytes.ReadUint16(packet, 4)
 	hdr.ANCount = libbytes.ReadUint16(packet, 6)
diff --git a/lib/dns/message_header_bench_test.go b/lib/dns/message_header_bench_test.go
new file mode 100644
index 00000000..16ce802e
--- /dev/null
+++ b/lib/dns/message_header_bench_test.go
@@ -0,0 +1,76 @@
+// Copyright 2021, Shulhan <ms@kilabit.info>. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dns
+
+import (
+	"testing"
+)
+
+/**
+
+Before 2021-11-14
+
+$ go test -benchmem -bench=MessageHeader_pack -memprofile testdata/pprof/MessageHeader_unpack.mem.old > testdata/bench/MessageHeader_pack.old
+
+goos: linux
+goarch: amd64
+pkg: github.com/shuLhan/share/lib/dns
+cpu: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
+BenchmarkMessageHeader_pack-8           19629273                66.25 ns/op           32 B/op          3 allocs/op
+
+After
+
+$ go test -benchmem -bench=MessageHeader_pack -memprofile testdata/pprof/MessageHeader_unpack.mem.new > testdata/bench/MessageHeader_pack.new
+
+goos: linux
+goarch: amd64
+pkg: github.com/shuLhan/share/lib/dns
+cpu: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
+BenchmarkMessageHeader_pack-8           54183505                21.66 ns/op           16 B/op          1 allocs/op
+
+**/
+func BenchmarkMessageHeader_pack(b *testing.B) {
+	hdr := &MessageHeader{
+		ID:      0xABCD,
+		Op:      OpCodeQuery,
+		IsAA:    true,
+		IsRD:    true,
+		QDCount: 1,
+		ANCount: 4,
+		NSCount: 1,
+		ARCount: 1,
+	}
+
+	for x := 0; x < b.N; x++ {
+		_ = hdr.pack()
+	}
+}
+
+/**
+
+$ go test -benchmem -bench=MessageHeader_unpack -memprofile testdata/pprof/MessageHeader_unpack.mem.new > testdata/bench/MessageHeader_unpack.new
+
+goos: linux
+goarch: amd64
+pkg: github.com/shuLhan/share/lib/dns
+cpu: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
+BenchmarkMessageHeader_unpack-8         310460067                3.848 ns/op           0 B/op          0 allocs/op
+
+**/
+func BenchmarkMessageHeader_unpack(b *testing.B) {
+	hdr := &MessageHeader{}
+	packet := []byte{
+		0xab, 0xcd,
+		0x85, 0x00,
+		0x00, 0x01,
+		0x00, 0x04,
+		0x00, 0x01,
+		0x00, 0x01,
+	}
+
+	for x := 0; x < b.N; x++ {
+		hdr.unpack(packet)
+	}
+}
diff --git a/lib/dns/message_header_test.go b/lib/dns/message_header_test.go
new file mode 100644
index 00000000..c7c47315
--- /dev/null
+++ b/lib/dns/message_header_test.go
@@ -0,0 +1,258 @@
+// Copyright 2021, Shulhan <ms@kilabit.info>. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dns
+
+import (
+	"testing"
+
+	"github.com/shuLhan/share/lib/test"
+)
+
+type testMessageHeader struct {
+	desc   string
+	hdr    MessageHeader
+	packet []byte
+}
+
+func testMessageHeaderCases() []testMessageHeader {
+	return []testMessageHeader{{
+		desc: "As query",
+		hdr: MessageHeader{
+			ID:      0xABCD,
+			IsQuery: true,
+			Op:      OpCodeQuery,
+			QDCount: 1,
+		},
+		packet: []byte{
+			0xab, 0xcd,
+			0x00, 0x00,
+			0x00, 0x01,
+			0x00, 0x00,
+			0x00, 0x00,
+			0x00, 0x00,
+		},
+	}, {
+		desc: "As inverse query",
+		hdr: MessageHeader{
+			ID:      0xABCD,
+			IsQuery: true,
+			Op:      OpCodeIQuery,
+			QDCount: 1,
+		},
+		packet: []byte{
+			0xab, 0xcd,
+			0x08, 0x00,
+			0x00, 0x01,
+			0x00, 0x00,
+			0x00, 0x00,
+			0x00, 0x00,
+		},
+	}, {
+		desc: "As query server status",
+		hdr: MessageHeader{
+			ID:      0xABCD,
+			IsQuery: true,
+			Op:      OpCodeStatus,
+		},
+		packet: []byte{
+			0xab, 0xcd,
+			0x10, 0x00,
+			0x00, 0x00,
+			0x00, 0x00,
+			0x00, 0x00,
+			0x00, 0x00,
+		},
+	}, {
+		desc: "As query, RD=1",
+		hdr: MessageHeader{
+			ID:      0xABCD,
+			IsQuery: true,
+			Op:      OpCodeQuery,
+			IsRD:    true,
+			QDCount: 1,
+		},
+		packet: []byte{
+			0xab, 0xcd,
+			0x01, 0x00,
+			0x00, 0x01,
+			0x00, 0x00,
+			0x00, 0x00,
+			0x00, 0x00,
+		},
+	}, {
+		desc: "As answer",
+		hdr: MessageHeader{
+			ID:      0xABCD,
+			Op:      OpCodeQuery,
+			QDCount: 1,
+			ANCount: 0x04,
+		},
+		packet: []byte{
+			0xab, 0xcd,
+			0x80, 0x00,
+			0x00, 0x01,
+			0x00, 0x04,
+			0x00, 0x00,
+			0x00, 0x00,
+		},
+	}, {
+		desc: "As answer, RCode=5",
+		hdr: MessageHeader{
+			ID:      0xABCD,
+			Op:      OpCodeQuery,
+			RCode:   RCodeRefused,
+			QDCount: 1,
+		},
+		packet: []byte{
+			0xab, 0xcd,
+			0x80, 0x05,
+			0x00, 0x01,
+			0x00, 0x00,
+			0x00, 0x00,
+			0x00, 0x00,
+		},
+	}, {
+		desc: "As answer, IsAA=1",
+		hdr: MessageHeader{
+			ID:      0xABCD,
+			Op:      OpCodeQuery,
+			IsAA:    true,
+			QDCount: 1,
+		},
+		packet: []byte{
+			0xab, 0xcd,
+			0x84, 0x00,
+			0x00, 0x01,
+			0x00, 0x00,
+			0x00, 0x00,
+			0x00, 0x00,
+		},
+	}, {
+		desc: "As answer, IsTC=1",
+		hdr: MessageHeader{
+			ID:      0xABCD,
+			Op:      OpCodeQuery,
+			IsTC:    true,
+			QDCount: 1,
+		},
+		packet: []byte{
+			0xab, 0xcd,
+			0x82, 0x00,
+			0x00, 0x01,
+			0x00, 0x00,
+			0x00, 0x00,
+			0x00, 0x00,
+		},
+	}, {
+		desc: "As answer, IsRD=1",
+		hdr: MessageHeader{
+			ID:      0xABCD,
+			Op:      OpCodeQuery,
+			IsRD:    true,
+			QDCount: 1,
+		},
+		packet: []byte{
+			0xab, 0xcd,
+			0x81, 0x00,
+			0x00, 0x01,
+			0x00, 0x00,
+			0x00, 0x00,
+			0x00, 0x00,
+		},
+	}, {
+		desc: "As answer, IsTC=1 IsRD=1",
+		hdr: MessageHeader{
+			ID:      0xABCD,
+			Op:      OpCodeQuery,
+			IsTC:    true,
+			IsRD:    true,
+			QDCount: 1,
+		},
+		packet: []byte{
+			0xab, 0xcd,
+			0x83, 0x00,
+			0x00, 0x01,
+			0x00, 0x00,
+			0x00, 0x00,
+			0x00, 0x00,
+		},
+	}, {
+		desc: "As answer, IsAA=1 IsRD=1",
+		hdr: MessageHeader{
+			ID:      0xABCD,
+			Op:      OpCodeQuery,
+			IsAA:    true,
+			IsRD:    true,
+			QDCount: 1,
+			ANCount: 3,
+		},
+		packet: []byte{
+			0xab, 0xcd,
+			0x85, 0x00,
+			0x00, 0x01,
+			0x00, 0x03,
+			0x00, 0x00,
+			0x00, 0x00,
+		},
+	}, {
+		desc: "As answer, IsAA=1 IsTC=1 IsRD=1",
+		hdr: MessageHeader{
+			ID:      0xABCD,
+			Op:      OpCodeQuery,
+			IsAA:    true,
+			IsTC:    true,
+			IsRD:    true,
+			QDCount: 1,
+			ANCount: 3,
+		},
+		packet: []byte{
+			0xab, 0xcd,
+			0x87, 0x00,
+			0x00, 0x01,
+			0x00, 0x03,
+			0x00, 0x00,
+			0x00, 0x00,
+		},
+	}, {
+		desc: "As answer, IsAA=1 IsRD=1 all counts",
+		hdr: MessageHeader{
+			ID:      0xABCD,
+			Op:      OpCodeQuery,
+			IsAA:    true,
+			IsRD:    true,
+			QDCount: 1,
+			ANCount: 4,
+			NSCount: 1,
+			ARCount: 1,
+		},
+		packet: []byte{
+			0xab, 0xcd,
+			0x85, 0x00,
+			0x00, 0x01,
+			0x00, 0x04,
+			0x00, 0x01,
+			0x00, 0x01,
+		},
+	}}
+}
+
+func TestMessageHeader_pack(t *testing.T) {
+	cases := testMessageHeaderCases()
+
+	for _, c := range cases {
+		got := c.hdr.pack()
+		test.Assert(t, c.desc, c.packet, got)
+	}
+}
+
+func TestMessageHeader_unpack(t *testing.T) {
+	cases := testMessageHeaderCases()
+
+	for _, c := range cases {
+		got := MessageHeader{}
+		got.unpack(c.packet)
+		test.Assert(t, c.desc, c.hdr, got)
+	}
+}
diff --git a/lib/dns/message_test.go b/lib/dns/message_test.go
index 43647309..3b57d435 100644
--- a/lib/dns/message_test.go
+++ b/lib/dns/message_test.go
@@ -159,7 +159,7 @@ func TestMessagePack(t *testing.T) {
 	}{{
 		desc: "Simple query",
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      1,
 				IsQuery: true,
 				QDCount: 1,
@@ -187,7 +187,7 @@ func TestMessagePack(t *testing.T) {
 	}, {
 		desc: "Response with A RDATA",
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0x8cdb,
 				IsQuery: false,
 				Op:      OpCodeQuery,
@@ -275,7 +275,7 @@ func TestMessagePack(t *testing.T) {
 			0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		},
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0xd4a6,
 				IsQuery: false,
 				Op:      OpCodeQuery,
@@ -342,7 +342,7 @@ func TestMessagePack(t *testing.T) {
 			0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		},
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0x8a38,
 				IsQuery: false,
 				Op:      OpCodeQuery,
@@ -389,7 +389,7 @@ func TestMessagePack(t *testing.T) {
 			0x51, 0x80,
 		},
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      2,
 				IsQuery: false,
 				Op:      OpCodeQuery,
@@ -473,7 +473,7 @@ func TestMessagePack(t *testing.T) {
 			0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		},
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0x9eef,
 				IsQuery: false,
 				Op:      OpCodeQuery,
@@ -573,7 +573,7 @@ func TestMessagePack(t *testing.T) {
 			0x00, 0x00, 0x29, 0x05, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		},
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      16253,
 				IsQuery: false,
 				Op:      OpCodeQuery,
@@ -634,7 +634,7 @@ func TestMessagePack(t *testing.T) {
 			0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		},
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0x2c90,
 				IsRD:    true,
 				IsRA:    true,
@@ -732,7 +732,7 @@ func TestMessagePack(t *testing.T) {
 			0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		},
 		msg: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0x2cb4,
 				IsRD:    true,
 				IsRA:    true,
@@ -837,7 +837,7 @@ func TestMessagePack(t *testing.T) {
 
 func TestMessageSetAuthoritativeAnswer(t *testing.T) {
 	msgQuery := &Message{
-		Header: SectionHeader{
+		Header: MessageHeader{
 			ID:      1,
 			IsQuery: true,
 			IsAA:    true,
@@ -854,7 +854,7 @@ func TestMessageSetAuthoritativeAnswer(t *testing.T) {
 	}
 
 	msgResponse := &Message{
-		Header: SectionHeader{
+		Header: MessageHeader{
 			ID:   1,
 			IsAA: true,
 			IsRD: true,
@@ -906,7 +906,7 @@ func TestMessageSetAuthoritativeAnswer(t *testing.T) {
 
 func TestMessageSetQuery(t *testing.T) {
 	msgQuery := &Message{
-		Header: SectionHeader{
+		Header: MessageHeader{
 			ID:      1,
 			IsQuery: true,
 			IsAA:    true,
@@ -949,7 +949,7 @@ func TestMessageSetQuery(t *testing.T) {
 
 func TestMessageSetRecursionDesired(t *testing.T) {
 	msgQuery := &Message{
-		Header: SectionHeader{
+		Header: MessageHeader{
 			ID:      1,
 			IsQuery: true,
 			IsAA:    true,
@@ -992,7 +992,7 @@ func TestMessageSetRecursionDesired(t *testing.T) {
 
 func TestMessageSetResponseCode(t *testing.T) {
 	msgQuery := &Message{
-		Header: SectionHeader{
+		Header: MessageHeader{
 			ID:      1,
 			IsQuery: true,
 			IsAA:    true,
@@ -1059,7 +1059,7 @@ func TestMessageUnpack(t *testing.T) {
 			0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		},
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0x8cdb,
 				IsQuery: false,
 				Op:      OpCodeQuery,
@@ -1131,7 +1131,7 @@ func TestMessageUnpack(t *testing.T) {
 			0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		},
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0xd4a6,
 				IsQuery: false,
 				Op:      OpCodeQuery,
@@ -1210,7 +1210,7 @@ func TestMessageUnpack(t *testing.T) {
 			0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		},
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0x8a38,
 				IsQuery: false,
 				Op:      OpCodeQuery,
@@ -1269,7 +1269,7 @@ func TestMessageUnpack(t *testing.T) {
 			0x00, 0x01, 0x51, 0x80,
 		},
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      2,
 				IsQuery: false,
 				Op:      OpCodeQuery,
@@ -1365,7 +1365,7 @@ func TestMessageUnpack(t *testing.T) {
 			0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		},
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0x9eef,
 				IsQuery: false,
 				Op:      OpCodeQuery,
@@ -1512,7 +1512,7 @@ func TestMessageUnpack(t *testing.T) {
 			0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		},
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      16253,
 				IsQuery: false,
 				Op:      OpCodeQuery,
@@ -1604,7 +1604,7 @@ func TestMessageUnpack(t *testing.T) {
 			0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		},
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0x2c90,
 				IsRD:    true,
 				IsRA:    true,
@@ -1708,7 +1708,7 @@ func TestMessageUnpack(t *testing.T) {
 			0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		},
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      0x2cb4,
 				IsRD:    true,
 				IsRA:    true,
diff --git a/lib/dns/tcp_client_test.go b/lib/dns/tcp_client_test.go
index 6ab3d5f3..7726bfc3 100644
--- a/lib/dns/tcp_client_test.go
+++ b/lib/dns/tcp_client_test.go
@@ -29,7 +29,7 @@ func TestTCPClientLookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      5,
 				IsAA:    true,
 				QDCount: 1,
@@ -57,7 +57,7 @@ func TestTCPClientLookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      6,
 				IsAA:    true,
 				QDCount: 1,
@@ -92,7 +92,7 @@ func TestTCPClientLookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      7,
 				IsAA:    true,
 				QDCount: 1,
diff --git a/lib/dns/udp_client_example_test.go b/lib/dns/udp_client_example_test.go
index 8fc2354b..0ccd9951 100644
--- a/lib/dns/udp_client_example_test.go
+++ b/lib/dns/udp_client_example_test.go
@@ -19,7 +19,7 @@ func ExampleUDPClient() {
 	}
 
 	req := &dns.Message{
-		Header: dns.SectionHeader{},
+		Header: dns.MessageHeader{},
 		Question: dns.SectionQuestion{
 			Name:  "kilabit.info",
 			Type:  dns.RecordTypeA,
diff --git a/lib/dns/udp_client_test.go b/lib/dns/udp_client_test.go
index 4f81c1de..44489500 100644
--- a/lib/dns/udp_client_test.go
+++ b/lib/dns/udp_client_test.go
@@ -29,7 +29,7 @@ func TestUDPClientLookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      8,
 				IsAA:    true,
 				QDCount: 1,
@@ -57,7 +57,7 @@ func TestUDPClientLookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      9,
 				IsAA:    true,
 				QDCount: 1,
@@ -92,7 +92,7 @@ func TestUDPClientLookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      10,
 				IsAA:    true,
 				QDCount: 1,
@@ -119,7 +119,7 @@ func TestUDPClientLookup(t *testing.T) {
 		rclass: RecordClassIN,
 		qname:  "kilabit.info",
 		exp: &Message{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				ID:      11,
 				IsAA:    false,
 				RCode:   RCodeErrServer,
diff --git a/lib/dns/zone_file.go b/lib/dns/zone_file.go
index 55adf329..896f9df9 100644
--- a/lib/dns/zone_file.go
+++ b/lib/dns/zone_file.go
@@ -161,7 +161,7 @@ func (zone *ZoneFile) Add(rr *ResourceRecord) (err error) {
 	}
 
 	msg := &Message{
-		Header: SectionHeader{
+		Header: MessageHeader{
 			IsAA:    true,
 			QDCount: 1,
 			ANCount: 1,
diff --git a/lib/dns/zone_file_test.go b/lib/dns/zone_file_test.go
index 66381e50..c69b1fa2 100644
--- a/lib/dns/zone_file_test.go
+++ b/lib/dns/zone_file_test.go
@@ -164,7 +164,7 @@ VAXA    A       10.2.0.27
 
 `,
 		exp: []*Message{{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,
@@ -190,7 +190,7 @@ VAXA    A       10.2.0.27
 				},
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 3,
@@ -220,7 +220,7 @@ VAXA    A       10.2.0.27
 				Value: "vaxa.isi.edu",
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 2,
@@ -250,7 +250,7 @@ VAXA    A       10.2.0.27
 				},
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,
@@ -268,7 +268,7 @@ VAXA    A       10.2.0.27
 				Value: "26.3.0.103",
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 2,
@@ -292,7 +292,7 @@ VAXA    A       10.2.0.27
 				Value: "128.9.0.32",
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 2,
@@ -395,7 +395,7 @@ mail IN CNAME @
 relay IN CNAME relay.pair.com.
 `,
 		exp: []*Message{{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,
@@ -421,7 +421,7 @@ relay IN CNAME relay.pair.com.
 				},
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 2,
@@ -445,7 +445,7 @@ relay IN CNAME relay.pair.com.
 				Value: "ns0.ns0.com",
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,
@@ -463,7 +463,7 @@ relay IN CNAME relay.pair.com.
 				Value: "127.0.0.1",
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,
@@ -481,7 +481,7 @@ relay IN CNAME relay.pair.com.
 				Value: "209.68.14.80",
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,
@@ -502,7 +502,7 @@ relay IN CNAME relay.pair.com.
 				},
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,
@@ -520,7 +520,7 @@ relay IN CNAME relay.pair.com.
 				Value: "pcguide.com",
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,
@@ -538,7 +538,7 @@ relay IN CNAME relay.pair.com.
 				Value: "pcguide.com",
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,
@@ -556,7 +556,7 @@ relay IN CNAME relay.pair.com.
 				Value: "pcguide.com",
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,
@@ -641,7 +641,7 @@ dev.kilabit.com.   A  127.0.0.1
 angularjs.doc       A  127.0.0.1
 `,
 		exp: []*Message{{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,
@@ -659,7 +659,7 @@ angularjs.doc       A  127.0.0.1
 				Value: "127.0.0.1",
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,
@@ -677,7 +677,7 @@ angularjs.doc       A  127.0.0.1
 				Value: "127.0.0.1",
 			}},
 		}, {
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,
@@ -750,7 +750,7 @@ func TestZoneParseTXT(t *testing.T) {
 	}{{
 		in: `@ IN TXT "This is a test"`,
 		exp: []*Message{{
-			Header: SectionHeader{
+			Header: MessageHeader{
 				IsAA:    true,
 				QDCount: 1,
 				ANCount: 1,