dns.go

// Copyright 2014 Google, Inc. All rights reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the LICENSE file in the root of the source
// tree.

package layers

import (
	"encoding/binary"
	"errors"
	"fmt"
	"net"

	"github.com/MarconiProtocol/gopacket"
)

// DNSClass defines the class associated with a request/response.  Different DNS
// classes can be thought of as an array of parallel namespace trees.
type DNSClass uint16

// DNSClass known values.
const (
	DNSClassIN  DNSClass = 1   // Internet
	DNSClassCS  DNSClass = 2   // the CSNET class (Obsolete)
	DNSClassCH  DNSClass = 3   // the CHAOS class
	DNSClassHS  DNSClass = 4   // Hesiod [Dyer 87]
	DNSClassAny DNSClass = 255 // AnyClass
)

func (dc DNSClass) String() string {
	switch dc {
	default:
		return "Unknown"
	case DNSClassIN:
		return "IN"
	case DNSClassCS:
		return "CS"
	case DNSClassCH:
		return "CH"
	case DNSClassHS:
		return "HS"
	case DNSClassAny:
		return "Any"
	}
}

// DNSType defines the type of data being requested/returned in a
// question/answer.
type DNSType uint16

// DNSType known values.
const (
	DNSTypeA     DNSType = 1  // a host address
	DNSTypeNS    DNSType = 2  // an authoritative name server
	DNSTypeMD    DNSType = 3  // a mail destination (Obsolete - use MX)
	DNSTypeMF    DNSType = 4  // a mail forwarder (Obsolete - use MX)
	DNSTypeCNAME DNSType = 5  // the canonical name for an alias
	DNSTypeSOA   DNSType = 6  // marks the start of a zone of authority
	DNSTypeMB    DNSType = 7  // a mailbox domain name (EXPERIMENTAL)
	DNSTypeMG    DNSType = 8  // a mail group member (EXPERIMENTAL)
	DNSTypeMR    DNSType = 9  // a mail rename domain name (EXPERIMENTAL)
	DNSTypeNULL  DNSType = 10 // a null RR (EXPERIMENTAL)
	DNSTypeWKS   DNSType = 11 // a well known service description
	DNSTypePTR   DNSType = 12 // a domain name pointer
	DNSTypeHINFO DNSType = 13 // host information
	DNSTypeMINFO DNSType = 14 // mailbox or mail list information
	DNSTypeMX    DNSType = 15 // mail exchange
	DNSTypeTXT   DNSType = 16 // text strings
	DNSTypeAAAA  DNSType = 28 // a IPv6 host address [RFC3596]
	DNSTypeSRV   DNSType = 33 // server discovery [RFC2782] [RFC6195]
)

func (dt DNSType) String() string {
	switch dt {
	default:
		return "Unknown"
	case DNSTypeA:
		return "A"
	case DNSTypeNS:
		return "NS"
	case DNSTypeMD:
		return "MD"
	case DNSTypeMF:
		return "MF"
	case DNSTypeCNAME:
		return "CNAME"
	case DNSTypeSOA:
		return "SOA"
	case DNSTypeMB:
		return "MB"
	case DNSTypeMG:
		return "MG"
	case DNSTypeMR:
		return "MR"
	case DNSTypeNULL:
		return "NULL"
	case DNSTypeWKS:
		return "WKS"
	case DNSTypePTR:
		return "PTR"
	case DNSTypeHINFO:
		return "HINFO"
	case DNSTypeMINFO:
		return "MINFO"
	case DNSTypeMX:
		return "MX"
	case DNSTypeTXT:
		return "TXT"
	case DNSTypeAAAA:
		return "AAAA"
	case DNSTypeSRV:
		return "SRV"
	}
}

// DNSResponseCode provides response codes for question answers.
type DNSResponseCode uint8

// DNSResponseCode known values.
const (
	DNSResponseCodeNoErr    DNSResponseCode = 0  // No error
	DNSResponseCodeFormErr  DNSResponseCode = 1  // Format Error                       [RFC1035]
	DNSResponseCodeServFail DNSResponseCode = 2  // Server Failure                     [RFC1035]
	DNSResponseCodeNXDomain DNSResponseCode = 3  // Non-Existent Domain                [RFC1035]
	DNSResponseCodeNotImp   DNSResponseCode = 4  // Not Implemented                    [RFC1035]
	DNSResponseCodeRefused  DNSResponseCode = 5  // Query Refused                      [RFC1035]
	DNSResponseCodeYXDomain DNSResponseCode = 6  // Name Exists when it should not     [RFC2136]
	DNSResponseCodeYXRRSet  DNSResponseCode = 7  // RR Set Exists when it should not   [RFC2136]
	DNSResponseCodeNXRRSet  DNSResponseCode = 8  // RR Set that should exist does not  [RFC2136]
	DNSResponseCodeNotAuth  DNSResponseCode = 9  // Server Not Authoritative for zone  [RFC2136]
	DNSResponseCodeNotZone  DNSResponseCode = 10 // Name not contained in zone         [RFC2136]
	DNSResponseCodeBadVers  DNSResponseCode = 16 // Bad OPT Version                    [RFC2671]
	DNSResponseCodeBadSig   DNSResponseCode = 16 // TSIG Signature Failure             [RFC2845]
	DNSResponseCodeBadKey   DNSResponseCode = 17 // Key not recognized                 [RFC2845]
	DNSResponseCodeBadTime  DNSResponseCode = 18 // Signature out of time window       [RFC2845]
	DNSResponseCodeBadMode  DNSResponseCode = 19 // Bad TKEY Mode                      [RFC2930]
	DNSResponseCodeBadName  DNSResponseCode = 20 // Duplicate key name                 [RFC2930]
	DNSResponseCodeBadAlg   DNSResponseCode = 21 // Algorithm not supported            [RFC2930]
	DNSResponseCodeBadTruc  DNSResponseCode = 22 // Bad Truncation                     [RFC4635]
)

func (drc DNSResponseCode) String() string {
	switch drc {
	default:
		return "Unknown"
	case DNSResponseCodeNoErr:
		return "No Error"
	case DNSResponseCodeFormErr:
		return "Format Error"
	case DNSResponseCodeServFail:
		return "Server Failure "
	case DNSResponseCodeNXDomain:
		return "Non-Existent Domain"
	case DNSResponseCodeNotImp:
		return "Not Implemented"
	case DNSResponseCodeRefused:
		return "Query Refused"
	case DNSResponseCodeYXDomain:
		return "Name Exists when it should not"
	case DNSResponseCodeYXRRSet:
		return "RR Set Exists when it should not"
	case DNSResponseCodeNXRRSet:
		return "RR Set that should exist does not"
	case DNSResponseCodeNotAuth:
		return "Server Not Authoritative for zone"
	case DNSResponseCodeNotZone:
		return "Name not contained in zone"
	case DNSResponseCodeBadVers:
		return "Bad OPT Version"
	case DNSResponseCodeBadKey:
		return "Key not recognized"
	case DNSResponseCodeBadTime:
		return "Signature out of time window"
	case DNSResponseCodeBadMode:
		return "Bad TKEY Mode"
	case DNSResponseCodeBadName:
		return "Duplicate key name"
	case DNSResponseCodeBadAlg:
		return "Algorithm not supported"
	case DNSResponseCodeBadTruc:
		return "Bad Truncation"
	}
}

// DNSOpCode defines a set of different operation types.
type DNSOpCode uint8

// DNSOpCode known values.
const (
	DNSOpCodeQuery  DNSOpCode = 0 // Query                  [RFC1035]
	DNSOpCodeIQuery DNSOpCode = 1 // Inverse Query Obsolete [RFC3425]
	DNSOpCodeStatus DNSOpCode = 2 // Status                 [RFC1035]
	DNSOpCodeNotify DNSOpCode = 4 // Notify                 [RFC1996]
	DNSOpCodeUpdate DNSOpCode = 5 // Update                 [RFC2136]
)

func (doc DNSOpCode) String() string {
	switch doc {
	default:
		return "Unknown"
	case DNSOpCodeQuery:
		return "Query"
	case DNSOpCodeIQuery:
		return "Inverse Query"
	case DNSOpCodeStatus:
		return "Status"
	case DNSOpCodeNotify:
		return "Notify"
	case DNSOpCodeUpdate:
		return "Update"
	}
}

// DNS is specified in RFC 1034 / RFC 1035
// +---------------------+
// |        Header       |
// +---------------------+
// |       Question      | the question for the name server
// +---------------------+
// |        Answer       | RRs answering the question
// +---------------------+
// |      Authority      | RRs pointing toward an authority
// +---------------------+
// |      Additional     | RRs holding additional information
// +---------------------+
//
//  DNS Header
//  0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
//  |                      ID                       |
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
//  |QR|   Opcode  |AA|TC|RD|RA|   Z    |   RCODE   |
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
//  |                    QDCOUNT                    |
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
//  |                    ANCOUNT                    |
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
//  |                    NSCOUNT                    |
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
//  |                    ARCOUNT                    |
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

// DNS contains data from a single Domain Name Service packet.
type DNS struct {
	BaseLayer

	// Header fields
	ID     uint16
	QR     bool
	OpCode DNSOpCode

	AA bool  // Authoritative answer
	TC bool  // Truncated
	RD bool  // Recursion desired
	RA bool  // Recursion available
	Z  uint8 // Reserved for future use

	ResponseCode DNSResponseCode
	QDCount      uint16 // Number of questions to expect
	ANCount      uint16 // Number of answers to expect
	NSCount      uint16 // Number of authorities to expect
	ARCount      uint16 // Number of additional records to expect

	// Entries
	Questions   []DNSQuestion
	Answers     []DNSResourceRecord
	Authorities []DNSResourceRecord
	Additionals []DNSResourceRecord

	// buffer for doing name decoding.  We use a single reusable buffer to avoid
	// name decoding on a single object via multiple DecodeFromBytes calls
	// requiring constant allocation of small byte slices.
	buffer []byte
}

// LayerType returns gopacket.LayerTypeDNS.
func (d *DNS) LayerType() gopacket.LayerType { return LayerTypeDNS }

// decodeDNS decodes the byte slice into a DNS type. It also
// setups the application Layer in PacketBuilder.
func decodeDNS(data []byte, p gopacket.PacketBuilder) error {
	d := &DNS{}
	err := d.DecodeFromBytes(data, p)
	if err != nil {
		return err
	}
	p.AddLayer(d)
	p.SetApplicationLayer(d)
	return nil
}

// DecodeFromBytes decodes the slice into the DNS struct.
func (d *DNS) DecodeFromBytes(data []byte, df gopacket.DecodeFeedback) error {
	d.buffer = d.buffer[:0]

	if len(data) < 12 {
		df.SetTruncated()
		return errors.New("DNS packet too short")
	}

	// since there are no further layers, the baselayer's content is
	// pointing to this layer
	d.BaseLayer = BaseLayer{Contents: data[:len(data)]}
	d.ID = binary.BigEndian.Uint16(data[:2])
	d.QR = data[2]&0x80 != 0
	d.OpCode = DNSOpCode(data[2]>>3) & 0x0F
	d.AA = data[2]&0x04 != 0
	d.TC = data[2]&0x02 != 0
	d.RD = data[2]&0x01 != 0
	d.RA = data[3]&0x80 != 0
	d.Z = uint8(data[3]>>4) & 0x7
	d.ResponseCode = DNSResponseCode(data[3] & 0xF)
	d.QDCount = binary.BigEndian.Uint16(data[4:6])
	d.ANCount = binary.BigEndian.Uint16(data[6:8])
	d.NSCount = binary.BigEndian.Uint16(data[8:10])
	d.ARCount = binary.BigEndian.Uint16(data[10:12])

	d.Questions = d.Questions[:0]
	d.Answers = d.Answers[:0]
	d.Authorities = d.Authorities[:0]
	d.Additionals = d.Additionals[:0]

	offset := 12
	var err error
	for i := 0; i < int(d.QDCount); i++ {
		var q DNSQuestion
		if offset, err = q.decode(data, offset, df, &d.buffer); err != nil {
			return err
		}
		d.Questions = append(d.Questions, q)
	}

	// For some horrible reason, if we do the obvious thing in this loop:
	//   var r DNSResourceRecord
	//   if blah := r.decode(blah); err != nil {
	//     return err
	//   }
	//   d.Foo = append(d.Foo, r)
	// the Go compiler thinks that 'r' escapes to the heap, causing a malloc for
	// every Answer, Authority, and Additional.  To get around this, we do
	// something really silly:  we append an empty resource record to our slice,
	// then use the last value in the slice to call decode.  Since the value is
	// already in the slice, there's no WAY it can escape... on the other hand our
	// code is MUCH uglier :(
	for i := 0; i < int(d.ANCount); i++ {
		d.Answers = append(d.Answers, DNSResourceRecord{})
		if offset, err = d.Answers[i].decode(data, offset, df, &d.buffer); err != nil {
			d.Answers = d.Answers[:i] // strip off erroneous value
			return err
		}
	}
	for i := 0; i < int(d.NSCount); i++ {
		d.Authorities = append(d.Authorities, DNSResourceRecord{})
		if offset, err = d.Authorities[i].decode(data, offset, df, &d.buffer); err != nil {
			d.Authorities = d.Authorities[:i] // strip off erroneous value
			return err
		}
	}
	for i := 0; i < int(d.ARCount); i++ {
		d.Additionals = append(d.Additionals, DNSResourceRecord{})
		if offset, err = d.Additionals[i].decode(data, offset, df, &d.buffer); err != nil {
			d.Additionals = d.Additionals[:i] // strip off erroneous value
			return err
		}
	}

	if uint16(len(d.Questions)) != d.QDCount {
		return errors.New("Invalid query decoding, not the right number of questions")
	} else if uint16(len(d.Answers)) != d.ANCount {
		return errors.New("Invalid query decoding, not the right number of answers")
	} else if uint16(len(d.Authorities)) != d.NSCount {
		return errors.New("Invalid query decoding, not the right number of authorities")
	} else if uint16(len(d.Additionals)) != d.ARCount {
		return errors.New("Invalid query decoding, not the right number of additionals info")
	}
	return nil
}

// CanDecode implements gopacket.DecodingLayer.
func (d *DNS) CanDecode() gopacket.LayerClass {
	return LayerTypeDNS
}

// NextLayerType implements gopacket.DecodingLayer.
func (d *DNS) NextLayerType() gopacket.LayerType {
	return gopacket.LayerTypePayload
}

// Payload returns nil.
func (d *DNS) Payload() []byte {
	return nil
}

func b2i(b bool) int {
	if b {
		return 1
	}
	return 0
}

func recSize(rr *DNSResourceRecord) int {
	switch rr.Type {
	case DNSTypeA:
		return 4
	case DNSTypeAAAA:
		return 16
	case DNSTypeNS:
		return len(rr.NS) + 2
	case DNSTypeCNAME:
		return len(rr.CNAME) + 2
	case DNSTypePTR:
		return len(rr.PTR) + 2
	case DNSTypeSOA:
		return len(rr.SOA.MName) + 2 + len(rr.SOA.RName) + 2 + 20
	case DNSTypeMX:
		return 2 + len(rr.MX.Name) + 2
	case DNSTypeTXT:
		l := len(rr.TXTs)
		for _, txt := range rr.TXTs {
			l += len(txt)
		}
		return l
	case DNSTypeSRV:
		return 6 + len(rr.SRV.Name) + 2
	}

	return 0
}

func computeSize(recs []DNSResourceRecord) int {
	sz := 0
	for _, rr := range recs {
		sz += len(rr.Name) + 12
		sz += recSize(&rr)
	}
	return sz
}

// SerializeTo writes the serialized form of this layer into the
// SerializationBuffer, implementing gopacket.SerializableLayer.
func (d *DNS) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.SerializeOptions) error {
	dsz := 0
	for _, q := range d.Questions {
		dsz += len(q.Name) + 6
	}
	dsz += computeSize(d.Answers)
	dsz += computeSize(d.Authorities)
	dsz += computeSize(d.Additionals)

	bytes, err := b.PrependBytes(12 + dsz)
	if err != nil {
		return err
	}
	binary.BigEndian.PutUint16(bytes, d.ID)
	bytes[2] = byte((b2i(d.QR) << 7) | (int(d.OpCode) << 3) | (b2i(d.AA) << 2) | (b2i(d.TC) << 1) | b2i(d.RD))
	bytes[3] = byte((b2i(d.RA) << 7) | (int(d.Z) << 4) | int(d.ResponseCode))

	if opts.FixLengths {
		d.QDCount = uint16(len(d.Questions))
		d.ANCount = uint16(len(d.Answers))
		d.NSCount = uint16(len(d.Authorities))
		d.ARCount = uint16(len(d.Additionals))
	}
	binary.BigEndian.PutUint16(bytes[4:], d.QDCount)
	binary.BigEndian.PutUint16(bytes[6:], d.ANCount)
	binary.BigEndian.PutUint16(bytes[8:], d.NSCount)
	binary.BigEndian.PutUint16(bytes[10:], d.ARCount)

	off := 12
	for _, qd := range d.Questions {
		n := qd.encode(bytes, off)
		off += n
	}

	for i := range d.Answers {
		// done this way so we can modify DNSResourceRecord to fix
		// lengths if requested
		qa := &d.Answers[i]
		n, err := qa.encode(bytes, off, opts)
		if err != nil {
			return err
		}
		off += n
	}

	for i := range d.Authorities {
		qa := &d.Authorities[i]
		n, err := qa.encode(bytes, off, opts)
		if err != nil {
			return err
		}
		off += n
	}
	for i := range d.Additionals {
		qa := &d.Additionals[i]
		n, err := qa.encode(bytes, off, opts)
		if err != nil {
			return err
		}
		off += n
	}

	return nil
}

var errMaxRecursion = errors.New("max DNS recursion level hit")

const maxRecursionLevel = 255

func decodeName(data []byte, offset int, buffer *[]byte, level int) ([]byte, int, error) {
	if level > maxRecursionLevel {
		return nil, 0, errMaxRecursion
	} else if offset >= len(data) {
		return nil, 0, errors.New("dns name offset too high")
	} else if offset < 0 {
		return nil, 0, errors.New("dns name offset is negative")
	}
	start := len(*buffer)
	index := offset
	if data[index] == 0x00 {
		return nil, index + 1, nil
	}
loop:
	for data[index] != 0x00 {
		switch data[index] & 0xc0 {
		default:
			/* RFC 1035
			   A domain name represented as a sequence of labels, where
			   each label consists of a length octet followed by that
			   number of octets.  The domain name terminates with the
			   zero length octet for the null label of the root.  Note
			   that this field may be an odd number of octets; no
			   padding is used.
			*/
			index2 := index + int(data[index]) + 1
			if index2-offset > 255 {
				return nil, 0, errors.New("dns name is too long")
			} else if index2 < index+1 || index2 > len(data) {
				return nil, 0, errors.New("dns name uncomputable: invalid index")
			}
			*buffer = append(*buffer, '.')
			*buffer = append(*buffer, data[index+1:index2]...)
			index = index2

		case 0xc0:
			/* RFC 1035
			   The pointer takes the form of a two octet sequence.

			   The first two bits are ones.  This allows a pointer to
			   be distinguished from a label, since the label must
			   begin with two zero bits because labels are restricted
			   to 63 octets or less.  (The 10 and 01 combinations are
			   reserved for future use.)  The OFFSET field specifies
			   an offset from the start of the message (i.e., the
			   first octet of the ID field in the domain header).  A
			   zero offset specifies the first byte of the ID field,
			   etc.

			   The compression scheme allows a domain name in a message to be
			   represented as either:
			      - a sequence of labels ending in a zero octet
			      - a pointer
			      - a sequence of labels ending with a pointer
			*/
			if index+2 > len(data) {
				return nil, 0, errors.New("dns offset pointer too high")
			}
			offsetp := int(binary.BigEndian.Uint16(data[index:index+2]) & 0x3fff)
			if offsetp > len(data) {
				return nil, 0, errors.New("dns offset pointer too high")
			}
			// This looks a little tricky, but actually isn't.  Because of how
			// decodeName is written, calling it appends the decoded name to the
			// current buffer.  We already have the start of the buffer, then, so
			// once this call is done buffer[start:] will contain our full name.
			_, _, err := decodeName(data, offsetp, buffer, level+1)
			if err != nil {
				return nil, 0, err
			}
			index++ // pointer is two bytes, so add an extra byte here.
			break loop
		/* EDNS, or other DNS option ? */
		case 0x40: // RFC 2673
			return nil, 0, fmt.Errorf("qname '0x40' - RFC 2673 unsupported yet (data=%x index=%d)",
				data[index], index)

		case 0x80:
			return nil, 0, fmt.Errorf("qname '0x80' unsupported yet (data=%x index=%d)",
				data[index], index)
		}
		if index >= len(data) {
			return nil, 0, errors.New("dns index walked out of range")
		}
	}
	if len(*buffer) <= start {
		return nil, 0, errors.New("no dns data found for name")
	}
	return (*buffer)[start+1:], index + 1, nil
}

// DNSQuestion wraps a single request (question) within a DNS query.
type DNSQuestion struct {
	Name  []byte
	Type  DNSType
	Class DNSClass
}

func (q *DNSQuestion) decode(data []byte, offset int, df gopacket.DecodeFeedback, buffer *[]byte) (int, error) {
	name, endq, err := decodeName(data, offset, buffer, 1)
	if err != nil {
		return 0, err
	}

	q.Name = name
	q.Type = DNSType(binary.BigEndian.Uint16(data[endq : endq+2]))
	q.Class = DNSClass(binary.BigEndian.Uint16(data[endq+2 : endq+4]))

	return endq + 4, nil
}

func (q *DNSQuestion) encode(data []byte, offset int) int {
	noff := encodeName(q.Name, data, offset)
	binary.BigEndian.PutUint16(data[noff:], uint16(q.Type))
	binary.BigEndian.PutUint16(data[noff+2:], uint16(q.Class))
	return len(q.Name) + 6
}

//  DNSResourceRecord
//  0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
//  |                                               |
//  /                                               /
//  /                      NAME                     /
//  |                                               |
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
//  |                      TYPE                     |
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
//  |                     CLASS                     |
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
//  |                      TTL                      |
//  |                                               |
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
//  |                   RDLENGTH                    |
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
//  /                     RDATA                     /
//  /                                               /
//  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

// DNSResourceRecord wraps the data from a single DNS resource within a
// response.
type DNSResourceRecord struct {
	// Header
	Name  []byte
	Type  DNSType
	Class DNSClass
	TTL   uint32

	// RDATA Raw Values
	DataLength uint16
	Data       []byte

	// RDATA Decoded Values
	IP             net.IP
	NS, CNAME, PTR []byte
	TXTs           [][]byte
	SOA            DNSSOA
	SRV            DNSSRV
	MX             DNSMX

	// Undecoded TXT for backward compatibility
	TXT []byte
}

// decode decodes the resource record, returning the total length of the record.
func (rr *DNSResourceRecord) decode(data []byte, offset int, df gopacket.DecodeFeedback, buffer *[]byte) (int, error) {
	name, endq, err := decodeName(data, offset, buffer, 1)
	if err != nil {
		return 0, err
	}

	rr.Name = name
	rr.Type = DNSType(binary.BigEndian.Uint16(data[endq : endq+2]))
	rr.Class = DNSClass(binary.BigEndian.Uint16(data[endq+2 : endq+4]))
	rr.TTL = binary.BigEndian.Uint32(data[endq+4 : endq+8])
	rr.DataLength = binary.BigEndian.Uint16(data[endq+8 : endq+10])
	end := endq + 10 + int(rr.DataLength)
	if end > len(data) {
		return 0, fmt.Errorf("resource record length exceeds data")
	}
	rr.Data = data[endq+10 : end]

	if err = rr.decodeRData(data, endq+10, buffer); err != nil {
		return 0, err
	}

	return endq + 10 + int(rr.DataLength), nil
}

func encodeName(name []byte, data []byte, offset int) int {
	l := 0
	for i := range name {
		if name[i] == '.' {
			data[offset+i-l] = byte(l)
			l = 0
		} else {
			// skip one to write the length
			data[offset+i+1] = name[i]
			l++
		}
	}
	// length for final portion
	data[offset+len(name)-l] = byte(l)
	data[offset+len(name)+1] = 0x00 // terminal
	return offset + len(name) + 2
}

func (rr *DNSResourceRecord) encode(data []byte, offset int, opts gopacket.SerializeOptions) (int, error) {

	noff := encodeName(rr.Name, data, offset)

	binary.BigEndian.PutUint16(data[noff:], uint16(rr.Type))
	binary.BigEndian.PutUint16(data[noff+2:], uint16(rr.Class))
	binary.BigEndian.PutUint32(data[noff+4:], uint32(rr.TTL))

	switch rr.Type {
	case DNSTypeA:
		copy(data[noff+10:], rr.IP.To4())
	case DNSTypeAAAA:
		copy(data[noff+10:], rr.IP)
	case DNSTypeNS:
		encodeName(rr.NS, data, noff+10)
	case DNSTypeCNAME:
		encodeName(rr.CNAME, data, noff+10)
	case DNSTypePTR:
		encodeName(rr.PTR, data, noff+10)
	case DNSTypeSOA:
		noff2 := encodeName(rr.SOA.MName, data, noff+10)
		noff2 = encodeName(rr.SOA.RName, data, noff2)
		binary.BigEndian.PutUint32(data[noff2:], rr.SOA.Serial)
		binary.BigEndian.PutUint32(data[noff2+4:], rr.SOA.Refresh)
		binary.BigEndian.PutUint32(data[noff2+8:], rr.SOA.Retry)
		binary.BigEndian.PutUint32(data[noff2+12:], rr.SOA.Expire)
		binary.BigEndian.PutUint32(data[noff2+16:], rr.SOA.Minimum)
	case DNSTypeMX:
		binary.BigEndian.PutUint16(data[noff+10:], rr.MX.Preference)
		encodeName(rr.MX.Name, data, noff+12)
	case DNSTypeTXT:
		noff2 := noff + 10
		for _, txt := range rr.TXTs {
			data[noff2] = byte(len(txt))
			copy(data[noff2+1:], txt)
			noff2 += 1 + len(txt)
		}
	case DNSTypeSRV:
		binary.BigEndian.PutUint16(data[noff+10:], rr.SRV.Priority)
		binary.BigEndian.PutUint16(data[noff+12:], rr.SRV.Weight)
		binary.BigEndian.PutUint16(data[noff+14:], rr.SRV.Port)
		encodeName(rr.SRV.Name, data, noff+16)
	default:
		return 0, fmt.Errorf("serializing resource record of type %v not supported", rr.Type)
	}

	// DataLength
	dSz := recSize(rr)
	binary.BigEndian.PutUint16(data[noff+8:], uint16(dSz))

	if opts.FixLengths {
		rr.DataLength = uint16(dSz)
	}

	return len(rr.Name) + 1 + 11 + dSz, nil
}

func (rr *DNSResourceRecord) String() string {

	if rr.Class == DNSClassIN {
		switch rr.Type {
		case DNSTypeA, DNSTypeAAAA:
			return rr.IP.String()
		case DNSTypeNS:
			return "NS " + string(rr.NS)
		case DNSTypeCNAME:
			return "CNAME " + string(rr.CNAME)
		case DNSTypePTR:
			return "PTR " + string(rr.PTR)
		case DNSTypeTXT:
			return "TXT " + string(rr.TXT)
		}
	}

	return fmt.Sprintf("<%v, %v>", rr.Class, rr.Type)
}

func decodeCharacterStrings(data []byte) ([][]byte, error) {
	strings := make([][]byte, 0, 1)
	end := len(data)
	for index, index2 := 0, 0; index != end; index = index2 {
		index2 = index + 1 + int(data[index]) // index increases by 1..256 and does not overflow
		if index2 > end {
			return nil, errors.New("Insufficient data for a <character-string>")
		}
		strings = append(strings, data[index+1:index2])
	}
	return strings, nil
}

func (rr *DNSResourceRecord) decodeRData(data []byte, offset int, buffer *[]byte) error {
	switch rr.Type {
	case DNSTypeA:
		rr.IP = rr.Data
	case DNSTypeAAAA:
		rr.IP = rr.Data
	case DNSTypeTXT, DNSTypeHINFO:
		rr.TXT = rr.Data
		txts, err := decodeCharacterStrings(rr.Data)
		if err != nil {
			return err
		}
		rr.TXTs = txts
	case DNSTypeNS:
		name, _, err := decodeName(data, offset, buffer, 1)
		if err != nil {
			return err
		}
		rr.NS = name
	case DNSTypeCNAME:
		name, _, err := decodeName(data, offset, buffer, 1)
		if err != nil {
			return err
		}
		rr.CNAME = name
	case DNSTypePTR:
		name, _, err := decodeName(data, offset, buffer, 1)
		if err != nil {
			return err
		}
		rr.PTR = name
	case DNSTypeSOA:
		name, endq, err := decodeName(data, offset, buffer, 1)
		if err != nil {
			return err
		}
		rr.SOA.MName = name
		name, endq, err = decodeName(data, endq, buffer, 1)
		if err != nil {
			return err
		}
		rr.SOA.RName = name
		rr.SOA.Serial = binary.BigEndian.Uint32(data[endq : endq+4])
		rr.SOA.Refresh = binary.BigEndian.Uint32(data[endq+4 : endq+8])
		rr.SOA.Retry = binary.BigEndian.Uint32(data[endq+8 : endq+12])
		rr.SOA.Expire = binary.BigEndian.Uint32(data[endq+12 : endq+16])
		rr.SOA.Minimum = binary.BigEndian.Uint32(data[endq+16 : endq+20])
	case DNSTypeMX:
		rr.MX.Preference = binary.BigEndian.Uint16(data[offset : offset+2])
		name, _, err := decodeName(data, offset+2, buffer, 1)
		if err != nil {
			return err
		}
		rr.MX.Name = name
	case DNSTypeSRV:
		rr.SRV.Priority = binary.BigEndian.Uint16(data[offset : offset+2])
		rr.SRV.Weight = binary.BigEndian.Uint16(data[offset+2 : offset+4])
		rr.SRV.Port = binary.BigEndian.Uint16(data[offset+4 : offset+6])
		name, _, err := decodeName(data, offset+6, buffer, 1)
		if err != nil {
			return err
		}
		rr.SRV.Name = name
	}
	return nil
}

// DNSSOA is a Start of Authority record.  Each domain requires a SOA record at
// the cutover where a domain is delegated from its parent.
type DNSSOA struct {
	MName, RName                            []byte
	Serial, Refresh, Retry, Expire, Minimum uint32
}

// DNSSRV is a Service record, defining a location (hostname/port) of a
// server/service.
type DNSSRV struct {
	Priority, Weight, Port uint16
	Name                   []byte
}

// DNSMX is a mail exchange record, defining a mail server for a recipient's
// domain.
type DNSMX struct {
	Preference uint16
	Name       []byte
}