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|
package main
import (
"bytes"
"encoding/binary"
"strings"
)
type DNSHeader struct {
ID uint16
QR uint8
OPCODE uint8
AA uint8
TC uint8
RD uint8
RA uint8
Z uint8
RCODE uint8
QDCOUNT uint16
ANCOUNT uint16
NSCOUNT uint16
ARCOUNT uint16
}
type DNSMessage struct {
Header DNSHeader
Questions []DNSQuestion
Answers []DNSAnswer
}
type DNSAnswer struct {
Name []byte
Type uint16
Class uint16
TTL uint32
RDLength uint16
RData []byte
}
type DNSQuestion struct {
Name []byte
Type uint16
Class uint16
}
func (m *DNSHeader) Bytes() []byte {
bytes := make([]byte, 12)
binary.BigEndian.PutUint16(bytes[0:], m.ID)
binary.BigEndian.PutUint16(bytes[2:4], m.combineFlags())
binary.BigEndian.PutUint16(bytes[4:6], m.QDCOUNT)
binary.BigEndian.PutUint16(bytes[6:8], m.ANCOUNT)
binary.BigEndian.PutUint16(bytes[8:10], m.NSCOUNT)
binary.BigEndian.PutUint16(bytes[10:12], m.ARCOUNT)
return bytes
}
func (m *DNSHeader) combineFlags() uint16 {
return uint16(m.QR)<<15 | uint16(m.OPCODE)<<11 | uint16(m.AA)<<10 |
uint16(m.TC)<<9 | uint16(m.RD)<<8 | uint16(m.RA)<<7 | uint16(m.Z)<<4 |
uint16(m.RCODE)
}
func (m *DNSMessage) AddQuestion(q DNSQuestion) {
m.Questions = append(m.Questions, q)
m.Header.QDCOUNT = m.Header.QDCOUNT + 1
}
func (m *DNSMessage) Bytes() []byte {
headerBytes := m.Header.Bytes()
bytes := []byte{}
bytes = append(bytes, headerBytes...)
for _, question := range m.Questions {
questionBytes := question.Bytes()
bytes = append(bytes, questionBytes...)
}
for _, answer := range m.Answers {
answerBytes := answer.Bytes()
bytes = append(bytes, answerBytes...)
}
return bytes
}
func (a *DNSAnswer) Bytes() []byte {
bytes := []byte{}
bytes = append(bytes, a.Name...)
bytes = binary.BigEndian.AppendUint16(bytes, a.Type)
bytes = binary.BigEndian.AppendUint16(bytes, a.Class)
bytes = binary.BigEndian.AppendUint32(bytes, a.TTL)
bytes = binary.BigEndian.AppendUint16(bytes, a.RDLength)
bytes = binary.BigEndian.AppendUint32(bytes, binary.BigEndian.Uint32(a.RData))
return bytes
}
func (m *DNSMessage) AddAnswer(a DNSAnswer) {
m.Answers = append(m.Answers, a)
m.Header.ANCOUNT = m.Header.ANCOUNT + 1
}
func (q *DNSQuestion) Bytes() []byte {
bytes := []byte{}
bytes = append(bytes, q.Name...)
bytes = binary.BigEndian.AppendUint16(bytes, q.Type)
bytes = binary.BigEndian.AppendUint16(bytes, q.Class)
return bytes
}
func MakeMessage(header DNSHeader) DNSMessage {
return DNSMessage{Header: header, Questions: []DNSQuestion{}, Answers: []DNSAnswer{}}
}
func MakeAnswer(name []byte, rdata []byte) DNSAnswer {
return DNSAnswer{Name: name, Type: 1, Class: 1, TTL: 60, RDLength: uint16(len(rdata)), RData: rdata}
}
func ParseHeader(buf []byte) DNSHeader {
return DNSHeader{
ID: binary.BigEndian.Uint16(buf[0:2]),
QR: uint8(buf[2] >> 7),
OPCODE: uint8(buf[2] >> 3 & 0x0f),
AA: uint8(buf[2] >> 2 & 0x01),
TC: uint8(buf[2] >> 1 & 0x01),
RD: uint8(buf[2] & 0x01),
RA: uint8(buf[3] >> 7),
Z: uint8(buf[3] >> 4 & 0x07),
RCODE: uint8(buf[3] & 0x0f),
QDCOUNT: binary.BigEndian.Uint16(buf[4:6]),
ANCOUNT: binary.BigEndian.Uint16(buf[6:8]),
NSCOUNT: binary.BigEndian.Uint16(buf[8:10]),
ARCOUNT: binary.BigEndian.Uint16(buf[10:12]),
}
}
func MakeQuestion(name []byte) DNSQuestion {
return DNSQuestion{Name: name, Type: 1, Class: 1}
}
func ParseQuestions(buf []byte, questionCount uint16) []DNSQuestion {
questions := []DNSQuestion{}
offset := 12
for i := 0; i < int(questionCount); i++ {
len := bytes.Index(buf[offset:], []byte{0})
label := ParseDomain(buf[offset:offset+len+1], buf)
question := MakeQuestion(label)
questions = append(questions, question)
offset += len + 1 + 4
}
return questions
}
func ParseDomain(data []byte, source []byte) []byte {
domain := decodeDNSPacket(data, source)
segments := strings.Split(domain, ".")
var encodedDomain []byte
for _, segment := range segments {
encodedDomain = append(encodedDomain, byte(len(segment)))
encodedDomain = append(encodedDomain, []byte(segment)...)
}
encodedDomain = append(encodedDomain, 0x00)
return encodedDomain
}
func decodeDNSPacket(packet []byte, source []byte) string {
offset := 0
labels := []string{}
for {
if packet[offset] == 0 {
break
}
if (packet[offset]&0xc0)>>6 == 0b11 {
pointer := int(binary.BigEndian.Uint16(packet[offset:offset+2]) << 2 >> 2)
length := bytes.Index(source[pointer:], []byte{0})
labels = append(labels, decodeDNSPacket(source[pointer:pointer+length+1], source))
offset += 2
continue
}
length := int(packet[offset])
substring := packet[offset+1 : offset+1+length]
labels = append(labels, string(substring))
offset += length + 1
}
return strings.Join(labels, ".")
}
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