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Source file src/vendor/golang.org/x/net/dns/dnsmessage/message.go

Documentation: vendor/golang.org/x/net/dns/dnsmessage

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package dnsmessage provides a mostly RFC 1035 compliant implementation of
     6  // DNS message packing and unpacking.
     7  //
     8  // The package also supports messages with Extension Mechanisms for DNS
     9  // (EDNS(0)) as defined in RFC 6891.
    10  //
    11  // This implementation is designed to minimize heap allocations and avoid
    12  // unnecessary packing and unpacking as much as possible.
    13  package dnsmessage
    14  
    15  import (
    16  	"errors"
    17  )
    18  
    19  // Message formats
    20  
    21  // A Type is a type of DNS request and response.
    22  type Type uint16
    23  
    24  const (
    25  	// ResourceHeader.Type and Question.Type
    26  	TypeA     Type = 1
    27  	TypeNS    Type = 2
    28  	TypeCNAME Type = 5
    29  	TypeSOA   Type = 6
    30  	TypePTR   Type = 12
    31  	TypeMX    Type = 15
    32  	TypeTXT   Type = 16
    33  	TypeAAAA  Type = 28
    34  	TypeSRV   Type = 33
    35  	TypeOPT   Type = 41
    36  
    37  	// Question.Type
    38  	TypeWKS   Type = 11
    39  	TypeHINFO Type = 13
    40  	TypeMINFO Type = 14
    41  	TypeAXFR  Type = 252
    42  	TypeALL   Type = 255
    43  )
    44  
    45  var typeNames = map[Type]string{
    46  	TypeA:     "TypeA",
    47  	TypeNS:    "TypeNS",
    48  	TypeCNAME: "TypeCNAME",
    49  	TypeSOA:   "TypeSOA",
    50  	TypePTR:   "TypePTR",
    51  	TypeMX:    "TypeMX",
    52  	TypeTXT:   "TypeTXT",
    53  	TypeAAAA:  "TypeAAAA",
    54  	TypeSRV:   "TypeSRV",
    55  	TypeOPT:   "TypeOPT",
    56  	TypeWKS:   "TypeWKS",
    57  	TypeHINFO: "TypeHINFO",
    58  	TypeMINFO: "TypeMINFO",
    59  	TypeAXFR:  "TypeAXFR",
    60  	TypeALL:   "TypeALL",
    61  }
    62  
    63  // String implements fmt.Stringer.String.
    64  func (t Type) String() string {
    65  	if n, ok := typeNames[t]; ok {
    66  		return n
    67  	}
    68  	return printUint16(uint16(t))
    69  }
    70  
    71  // GoString implements fmt.GoStringer.GoString.
    72  func (t Type) GoString() string {
    73  	if n, ok := typeNames[t]; ok {
    74  		return "dnsmessage." + n
    75  	}
    76  	return printUint16(uint16(t))
    77  }
    78  
    79  // A Class is a type of network.
    80  type Class uint16
    81  
    82  const (
    83  	// ResourceHeader.Class and Question.Class
    84  	ClassINET   Class = 1
    85  	ClassCSNET  Class = 2
    86  	ClassCHAOS  Class = 3
    87  	ClassHESIOD Class = 4
    88  
    89  	// Question.Class
    90  	ClassANY Class = 255
    91  )
    92  
    93  var classNames = map[Class]string{
    94  	ClassINET:   "ClassINET",
    95  	ClassCSNET:  "ClassCSNET",
    96  	ClassCHAOS:  "ClassCHAOS",
    97  	ClassHESIOD: "ClassHESIOD",
    98  	ClassANY:    "ClassANY",
    99  }
   100  
   101  // String implements fmt.Stringer.String.
   102  func (c Class) String() string {
   103  	if n, ok := classNames[c]; ok {
   104  		return n
   105  	}
   106  	return printUint16(uint16(c))
   107  }
   108  
   109  // GoString implements fmt.GoStringer.GoString.
   110  func (c Class) GoString() string {
   111  	if n, ok := classNames[c]; ok {
   112  		return "dnsmessage." + n
   113  	}
   114  	return printUint16(uint16(c))
   115  }
   116  
   117  // An OpCode is a DNS operation code.
   118  type OpCode uint16
   119  
   120  // GoString implements fmt.GoStringer.GoString.
   121  func (o OpCode) GoString() string {
   122  	return printUint16(uint16(o))
   123  }
   124  
   125  // An RCode is a DNS response status code.
   126  type RCode uint16
   127  
   128  const (
   129  	// Message.Rcode
   130  	RCodeSuccess        RCode = 0
   131  	RCodeFormatError    RCode = 1
   132  	RCodeServerFailure  RCode = 2
   133  	RCodeNameError      RCode = 3
   134  	RCodeNotImplemented RCode = 4
   135  	RCodeRefused        RCode = 5
   136  )
   137  
   138  var rCodeNames = map[RCode]string{
   139  	RCodeSuccess:        "RCodeSuccess",
   140  	RCodeFormatError:    "RCodeFormatError",
   141  	RCodeServerFailure:  "RCodeServerFailure",
   142  	RCodeNameError:      "RCodeNameError",
   143  	RCodeNotImplemented: "RCodeNotImplemented",
   144  	RCodeRefused:        "RCodeRefused",
   145  }
   146  
   147  // String implements fmt.Stringer.String.
   148  func (r RCode) String() string {
   149  	if n, ok := rCodeNames[r]; ok {
   150  		return n
   151  	}
   152  	return printUint16(uint16(r))
   153  }
   154  
   155  // GoString implements fmt.GoStringer.GoString.
   156  func (r RCode) GoString() string {
   157  	if n, ok := rCodeNames[r]; ok {
   158  		return "dnsmessage." + n
   159  	}
   160  	return printUint16(uint16(r))
   161  }
   162  
   163  func printPaddedUint8(i uint8) string {
   164  	b := byte(i)
   165  	return string([]byte{
   166  		b/100 + '0',
   167  		b/10%10 + '0',
   168  		b%10 + '0',
   169  	})
   170  }
   171  
   172  func printUint8Bytes(buf []byte, i uint8) []byte {
   173  	b := byte(i)
   174  	if i >= 100 {
   175  		buf = append(buf, b/100+'0')
   176  	}
   177  	if i >= 10 {
   178  		buf = append(buf, b/10%10+'0')
   179  	}
   180  	return append(buf, b%10+'0')
   181  }
   182  
   183  func printByteSlice(b []byte) string {
   184  	if len(b) == 0 {
   185  		return ""
   186  	}
   187  	buf := make([]byte, 0, 5*len(b))
   188  	buf = printUint8Bytes(buf, uint8(b[0]))
   189  	for _, n := range b[1:] {
   190  		buf = append(buf, ',', ' ')
   191  		buf = printUint8Bytes(buf, uint8(n))
   192  	}
   193  	return string(buf)
   194  }
   195  
   196  const hexDigits = "0123456789abcdef"
   197  
   198  func printString(str []byte) string {
   199  	buf := make([]byte, 0, len(str))
   200  	for i := 0; i < len(str); i++ {
   201  		c := str[i]
   202  		if c == '.' || c == '-' || c == ' ' ||
   203  			'A' <= c && c <= 'Z' ||
   204  			'a' <= c && c <= 'z' ||
   205  			'0' <= c && c <= '9' {
   206  			buf = append(buf, c)
   207  			continue
   208  		}
   209  
   210  		upper := c >> 4
   211  		lower := (c << 4) >> 4
   212  		buf = append(
   213  			buf,
   214  			'\\',
   215  			'x',
   216  			hexDigits[upper],
   217  			hexDigits[lower],
   218  		)
   219  	}
   220  	return string(buf)
   221  }
   222  
   223  func printUint16(i uint16) string {
   224  	return printUint32(uint32(i))
   225  }
   226  
   227  func printUint32(i uint32) string {
   228  	// Max value is 4294967295.
   229  	buf := make([]byte, 10)
   230  	for b, d := buf, uint32(1000000000); d > 0; d /= 10 {
   231  		b[0] = byte(i/d%10 + '0')
   232  		if b[0] == '0' && len(b) == len(buf) && len(buf) > 1 {
   233  			buf = buf[1:]
   234  		}
   235  		b = b[1:]
   236  		i %= d
   237  	}
   238  	return string(buf)
   239  }
   240  
   241  func printBool(b bool) string {
   242  	if b {
   243  		return "true"
   244  	}
   245  	return "false"
   246  }
   247  
   248  var (
   249  	// ErrNotStarted indicates that the prerequisite information isn't
   250  	// available yet because the previous records haven't been appropriately
   251  	// parsed, skipped or finished.
   252  	ErrNotStarted = errors.New("parsing/packing of this type isn't available yet")
   253  
   254  	// ErrSectionDone indicated that all records in the section have been
   255  	// parsed or finished.
   256  	ErrSectionDone = errors.New("parsing/packing of this section has completed")
   257  
   258  	errBaseLen            = errors.New("insufficient data for base length type")
   259  	errCalcLen            = errors.New("insufficient data for calculated length type")
   260  	errReserved           = errors.New("segment prefix is reserved")
   261  	errTooManyPtr         = errors.New("too many pointers (>10)")
   262  	errInvalidPtr         = errors.New("invalid pointer")
   263  	errNilResouceBody     = errors.New("nil resource body")
   264  	errResourceLen        = errors.New("insufficient data for resource body length")
   265  	errSegTooLong         = errors.New("segment length too long")
   266  	errZeroSegLen         = errors.New("zero length segment")
   267  	errResTooLong         = errors.New("resource length too long")
   268  	errTooManyQuestions   = errors.New("too many Questions to pack (>65535)")
   269  	errTooManyAnswers     = errors.New("too many Answers to pack (>65535)")
   270  	errTooManyAuthorities = errors.New("too many Authorities to pack (>65535)")
   271  	errTooManyAdditionals = errors.New("too many Additionals to pack (>65535)")
   272  	errNonCanonicalName   = errors.New("name is not in canonical format (it must end with a .)")
   273  	errStringTooLong      = errors.New("character string exceeds maximum length (255)")
   274  	errCompressedSRV      = errors.New("compressed name in SRV resource data")
   275  )
   276  
   277  // Internal constants.
   278  const (
   279  	// packStartingCap is the default initial buffer size allocated during
   280  	// packing.
   281  	//
   282  	// The starting capacity doesn't matter too much, but most DNS responses
   283  	// Will be <= 512 bytes as it is the limit for DNS over UDP.
   284  	packStartingCap = 512
   285  
   286  	// uint16Len is the length (in bytes) of a uint16.
   287  	uint16Len = 2
   288  
   289  	// uint32Len is the length (in bytes) of a uint32.
   290  	uint32Len = 4
   291  
   292  	// headerLen is the length (in bytes) of a DNS header.
   293  	//
   294  	// A header is comprised of 6 uint16s and no padding.
   295  	headerLen = 6 * uint16Len
   296  )
   297  
   298  type nestedError struct {
   299  	// s is the current level's error message.
   300  	s string
   301  
   302  	// err is the nested error.
   303  	err error
   304  }
   305  
   306  // nestedError implements error.Error.
   307  func (e *nestedError) Error() string {
   308  	return e.s + ": " + e.err.Error()
   309  }
   310  
   311  // Header is a representation of a DNS message header.
   312  type Header struct {
   313  	ID                 uint16
   314  	Response           bool
   315  	OpCode             OpCode
   316  	Authoritative      bool
   317  	Truncated          bool
   318  	RecursionDesired   bool
   319  	RecursionAvailable bool
   320  	RCode              RCode
   321  }
   322  
   323  func (m *Header) pack() (id uint16, bits uint16) {
   324  	id = m.ID
   325  	bits = uint16(m.OpCode)<<11 | uint16(m.RCode)
   326  	if m.RecursionAvailable {
   327  		bits |= headerBitRA
   328  	}
   329  	if m.RecursionDesired {
   330  		bits |= headerBitRD
   331  	}
   332  	if m.Truncated {
   333  		bits |= headerBitTC
   334  	}
   335  	if m.Authoritative {
   336  		bits |= headerBitAA
   337  	}
   338  	if m.Response {
   339  		bits |= headerBitQR
   340  	}
   341  	return
   342  }
   343  
   344  // GoString implements fmt.GoStringer.GoString.
   345  func (m *Header) GoString() string {
   346  	return "dnsmessage.Header{" +
   347  		"ID: " + printUint16(m.ID) + ", " +
   348  		"Response: " + printBool(m.Response) + ", " +
   349  		"OpCode: " + m.OpCode.GoString() + ", " +
   350  		"Authoritative: " + printBool(m.Authoritative) + ", " +
   351  		"Truncated: " + printBool(m.Truncated) + ", " +
   352  		"RecursionDesired: " + printBool(m.RecursionDesired) + ", " +
   353  		"RecursionAvailable: " + printBool(m.RecursionAvailable) + ", " +
   354  		"RCode: " + m.RCode.GoString() + "}"
   355  }
   356  
   357  // Message is a representation of a DNS message.
   358  type Message struct {
   359  	Header
   360  	Questions   []Question
   361  	Answers     []Resource
   362  	Authorities []Resource
   363  	Additionals []Resource
   364  }
   365  
   366  type section uint8
   367  
   368  const (
   369  	sectionNotStarted section = iota
   370  	sectionHeader
   371  	sectionQuestions
   372  	sectionAnswers
   373  	sectionAuthorities
   374  	sectionAdditionals
   375  	sectionDone
   376  
   377  	headerBitQR = 1 << 15 // query/response (response=1)
   378  	headerBitAA = 1 << 10 // authoritative
   379  	headerBitTC = 1 << 9  // truncated
   380  	headerBitRD = 1 << 8  // recursion desired
   381  	headerBitRA = 1 << 7  // recursion available
   382  )
   383  
   384  var sectionNames = map[section]string{
   385  	sectionHeader:      "header",
   386  	sectionQuestions:   "Question",
   387  	sectionAnswers:     "Answer",
   388  	sectionAuthorities: "Authority",
   389  	sectionAdditionals: "Additional",
   390  }
   391  
   392  // header is the wire format for a DNS message header.
   393  type header struct {
   394  	id          uint16
   395  	bits        uint16
   396  	questions   uint16
   397  	answers     uint16
   398  	authorities uint16
   399  	additionals uint16
   400  }
   401  
   402  func (h *header) count(sec section) uint16 {
   403  	switch sec {
   404  	case sectionQuestions:
   405  		return h.questions
   406  	case sectionAnswers:
   407  		return h.answers
   408  	case sectionAuthorities:
   409  		return h.authorities
   410  	case sectionAdditionals:
   411  		return h.additionals
   412  	}
   413  	return 0
   414  }
   415  
   416  // pack appends the wire format of the header to msg.
   417  func (h *header) pack(msg []byte) []byte {
   418  	msg = packUint16(msg, h.id)
   419  	msg = packUint16(msg, h.bits)
   420  	msg = packUint16(msg, h.questions)
   421  	msg = packUint16(msg, h.answers)
   422  	msg = packUint16(msg, h.authorities)
   423  	return packUint16(msg, h.additionals)
   424  }
   425  
   426  func (h *header) unpack(msg []byte, off int) (int, error) {
   427  	newOff := off
   428  	var err error
   429  	if h.id, newOff, err = unpackUint16(msg, newOff); err != nil {
   430  		return off, &nestedError{"id", err}
   431  	}
   432  	if h.bits, newOff, err = unpackUint16(msg, newOff); err != nil {
   433  		return off, &nestedError{"bits", err}
   434  	}
   435  	if h.questions, newOff, err = unpackUint16(msg, newOff); err != nil {
   436  		return off, &nestedError{"questions", err}
   437  	}
   438  	if h.answers, newOff, err = unpackUint16(msg, newOff); err != nil {
   439  		return off, &nestedError{"answers", err}
   440  	}
   441  	if h.authorities, newOff, err = unpackUint16(msg, newOff); err != nil {
   442  		return off, &nestedError{"authorities", err}
   443  	}
   444  	if h.additionals, newOff, err = unpackUint16(msg, newOff); err != nil {
   445  		return off, &nestedError{"additionals", err}
   446  	}
   447  	return newOff, nil
   448  }
   449  
   450  func (h *header) header() Header {
   451  	return Header{
   452  		ID:                 h.id,
   453  		Response:           (h.bits & headerBitQR) != 0,
   454  		OpCode:             OpCode(h.bits>>11) & 0xF,
   455  		Authoritative:      (h.bits & headerBitAA) != 0,
   456  		Truncated:          (h.bits & headerBitTC) != 0,
   457  		RecursionDesired:   (h.bits & headerBitRD) != 0,
   458  		RecursionAvailable: (h.bits & headerBitRA) != 0,
   459  		RCode:              RCode(h.bits & 0xF),
   460  	}
   461  }
   462  
   463  // A Resource is a DNS resource record.
   464  type Resource struct {
   465  	Header ResourceHeader
   466  	Body   ResourceBody
   467  }
   468  
   469  func (r *Resource) GoString() string {
   470  	return "dnsmessage.Resource{" +
   471  		"Header: " + r.Header.GoString() +
   472  		", Body: &" + r.Body.GoString() +
   473  		"}"
   474  }
   475  
   476  // A ResourceBody is a DNS resource record minus the header.
   477  type ResourceBody interface {
   478  	// pack packs a Resource except for its header.
   479  	pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error)
   480  
   481  	// realType returns the actual type of the Resource. This is used to
   482  	// fill in the header Type field.
   483  	realType() Type
   484  
   485  	// GoString implements fmt.GoStringer.GoString.
   486  	GoString() string
   487  }
   488  
   489  // pack appends the wire format of the Resource to msg.
   490  func (r *Resource) pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error) {
   491  	if r.Body == nil {
   492  		return msg, errNilResouceBody
   493  	}
   494  	oldMsg := msg
   495  	r.Header.Type = r.Body.realType()
   496  	msg, lenOff, err := r.Header.pack(msg, compression, compressionOff)
   497  	if err != nil {
   498  		return msg, &nestedError{"ResourceHeader", err}
   499  	}
   500  	preLen := len(msg)
   501  	msg, err = r.Body.pack(msg, compression, compressionOff)
   502  	if err != nil {
   503  		return msg, &nestedError{"content", err}
   504  	}
   505  	if err := r.Header.fixLen(msg, lenOff, preLen); err != nil {
   506  		return oldMsg, err
   507  	}
   508  	return msg, nil
   509  }
   510  
   511  // A Parser allows incrementally parsing a DNS message.
   512  //
   513  // When parsing is started, the Header is parsed. Next, each Question can be
   514  // either parsed or skipped. Alternatively, all Questions can be skipped at
   515  // once. When all Questions have been parsed, attempting to parse Questions
   516  // will return (nil, nil) and attempting to skip Questions will return
   517  // (true, nil). After all Questions have been either parsed or skipped, all
   518  // Answers, Authorities and Additionals can be either parsed or skipped in the
   519  // same way, and each type of Resource must be fully parsed or skipped before
   520  // proceeding to the next type of Resource.
   521  //
   522  // Note that there is no requirement to fully skip or parse the message.
   523  type Parser struct {
   524  	msg    []byte
   525  	header header
   526  
   527  	section        section
   528  	off            int
   529  	index          int
   530  	resHeaderValid bool
   531  	resHeader      ResourceHeader
   532  }
   533  
   534  // Start parses the header and enables the parsing of Questions.
   535  func (p *Parser) Start(msg []byte) (Header, error) {
   536  	if p.msg != nil {
   537  		*p = Parser{}
   538  	}
   539  	p.msg = msg
   540  	var err error
   541  	if p.off, err = p.header.unpack(msg, 0); err != nil {
   542  		return Header{}, &nestedError{"unpacking header", err}
   543  	}
   544  	p.section = sectionQuestions
   545  	return p.header.header(), nil
   546  }
   547  
   548  func (p *Parser) checkAdvance(sec section) error {
   549  	if p.section < sec {
   550  		return ErrNotStarted
   551  	}
   552  	if p.section > sec {
   553  		return ErrSectionDone
   554  	}
   555  	p.resHeaderValid = false
   556  	if p.index == int(p.header.count(sec)) {
   557  		p.index = 0
   558  		p.section++
   559  		return ErrSectionDone
   560  	}
   561  	return nil
   562  }
   563  
   564  func (p *Parser) resource(sec section) (Resource, error) {
   565  	var r Resource
   566  	var err error
   567  	r.Header, err = p.resourceHeader(sec)
   568  	if err != nil {
   569  		return r, err
   570  	}
   571  	p.resHeaderValid = false
   572  	r.Body, p.off, err = unpackResourceBody(p.msg, p.off, r.Header)
   573  	if err != nil {
   574  		return Resource{}, &nestedError{"unpacking " + sectionNames[sec], err}
   575  	}
   576  	p.index++
   577  	return r, nil
   578  }
   579  
   580  func (p *Parser) resourceHeader(sec section) (ResourceHeader, error) {
   581  	if p.resHeaderValid {
   582  		return p.resHeader, nil
   583  	}
   584  	if err := p.checkAdvance(sec); err != nil {
   585  		return ResourceHeader{}, err
   586  	}
   587  	var hdr ResourceHeader
   588  	off, err := hdr.unpack(p.msg, p.off)
   589  	if err != nil {
   590  		return ResourceHeader{}, err
   591  	}
   592  	p.resHeaderValid = true
   593  	p.resHeader = hdr
   594  	p.off = off
   595  	return hdr, nil
   596  }
   597  
   598  func (p *Parser) skipResource(sec section) error {
   599  	if p.resHeaderValid {
   600  		newOff := p.off + int(p.resHeader.Length)
   601  		if newOff > len(p.msg) {
   602  			return errResourceLen
   603  		}
   604  		p.off = newOff
   605  		p.resHeaderValid = false
   606  		p.index++
   607  		return nil
   608  	}
   609  	if err := p.checkAdvance(sec); err != nil {
   610  		return err
   611  	}
   612  	var err error
   613  	p.off, err = skipResource(p.msg, p.off)
   614  	if err != nil {
   615  		return &nestedError{"skipping: " + sectionNames[sec], err}
   616  	}
   617  	p.index++
   618  	return nil
   619  }
   620  
   621  // Question parses a single Question.
   622  func (p *Parser) Question() (Question, error) {
   623  	if err := p.checkAdvance(sectionQuestions); err != nil {
   624  		return Question{}, err
   625  	}
   626  	var name Name
   627  	off, err := name.unpack(p.msg, p.off)
   628  	if err != nil {
   629  		return Question{}, &nestedError{"unpacking Question.Name", err}
   630  	}
   631  	typ, off, err := unpackType(p.msg, off)
   632  	if err != nil {
   633  		return Question{}, &nestedError{"unpacking Question.Type", err}
   634  	}
   635  	class, off, err := unpackClass(p.msg, off)
   636  	if err != nil {
   637  		return Question{}, &nestedError{"unpacking Question.Class", err}
   638  	}
   639  	p.off = off
   640  	p.index++
   641  	return Question{name, typ, class}, nil
   642  }
   643  
   644  // AllQuestions parses all Questions.
   645  func (p *Parser) AllQuestions() ([]Question, error) {
   646  	// Multiple questions are valid according to the spec,
   647  	// but servers don't actually support them. There will
   648  	// be at most one question here.
   649  	//
   650  	// Do not pre-allocate based on info in p.header, since
   651  	// the data is untrusted.
   652  	qs := []Question{}
   653  	for {
   654  		q, err := p.Question()
   655  		if err == ErrSectionDone {
   656  			return qs, nil
   657  		}
   658  		if err != nil {
   659  			return nil, err
   660  		}
   661  		qs = append(qs, q)
   662  	}
   663  }
   664  
   665  // SkipQuestion skips a single Question.
   666  func (p *Parser) SkipQuestion() error {
   667  	if err := p.checkAdvance(sectionQuestions); err != nil {
   668  		return err
   669  	}
   670  	off, err := skipName(p.msg, p.off)
   671  	if err != nil {
   672  		return &nestedError{"skipping Question Name", err}
   673  	}
   674  	if off, err = skipType(p.msg, off); err != nil {
   675  		return &nestedError{"skipping Question Type", err}
   676  	}
   677  	if off, err = skipClass(p.msg, off); err != nil {
   678  		return &nestedError{"skipping Question Class", err}
   679  	}
   680  	p.off = off
   681  	p.index++
   682  	return nil
   683  }
   684  
   685  // SkipAllQuestions skips all Questions.
   686  func (p *Parser) SkipAllQuestions() error {
   687  	for {
   688  		if err := p.SkipQuestion(); err == ErrSectionDone {
   689  			return nil
   690  		} else if err != nil {
   691  			return err
   692  		}
   693  	}
   694  }
   695  
   696  // AnswerHeader parses a single Answer ResourceHeader.
   697  func (p *Parser) AnswerHeader() (ResourceHeader, error) {
   698  	return p.resourceHeader(sectionAnswers)
   699  }
   700  
   701  // Answer parses a single Answer Resource.
   702  func (p *Parser) Answer() (Resource, error) {
   703  	return p.resource(sectionAnswers)
   704  }
   705  
   706  // AllAnswers parses all Answer Resources.
   707  func (p *Parser) AllAnswers() ([]Resource, error) {
   708  	// The most common query is for A/AAAA, which usually returns
   709  	// a handful of IPs.
   710  	//
   711  	// Pre-allocate up to a certain limit, since p.header is
   712  	// untrusted data.
   713  	n := int(p.header.answers)
   714  	if n > 20 {
   715  		n = 20
   716  	}
   717  	as := make([]Resource, 0, n)
   718  	for {
   719  		a, err := p.Answer()
   720  		if err == ErrSectionDone {
   721  			return as, nil
   722  		}
   723  		if err != nil {
   724  			return nil, err
   725  		}
   726  		as = append(as, a)
   727  	}
   728  }
   729  
   730  // SkipAnswer skips a single Answer Resource.
   731  func (p *Parser) SkipAnswer() error {
   732  	return p.skipResource(sectionAnswers)
   733  }
   734  
   735  // SkipAllAnswers skips all Answer Resources.
   736  func (p *Parser) SkipAllAnswers() error {
   737  	for {
   738  		if err := p.SkipAnswer(); err == ErrSectionDone {
   739  			return nil
   740  		} else if err != nil {
   741  			return err
   742  		}
   743  	}
   744  }
   745  
   746  // AuthorityHeader parses a single Authority ResourceHeader.
   747  func (p *Parser) AuthorityHeader() (ResourceHeader, error) {
   748  	return p.resourceHeader(sectionAuthorities)
   749  }
   750  
   751  // Authority parses a single Authority Resource.
   752  func (p *Parser) Authority() (Resource, error) {
   753  	return p.resource(sectionAuthorities)
   754  }
   755  
   756  // AllAuthorities parses all Authority Resources.
   757  func (p *Parser) AllAuthorities() ([]Resource, error) {
   758  	// Authorities contains SOA in case of NXDOMAIN and friends,
   759  	// otherwise it is empty.
   760  	//
   761  	// Pre-allocate up to a certain limit, since p.header is
   762  	// untrusted data.
   763  	n := int(p.header.authorities)
   764  	if n > 10 {
   765  		n = 10
   766  	}
   767  	as := make([]Resource, 0, n)
   768  	for {
   769  		a, err := p.Authority()
   770  		if err == ErrSectionDone {
   771  			return as, nil
   772  		}
   773  		if err != nil {
   774  			return nil, err
   775  		}
   776  		as = append(as, a)
   777  	}
   778  }
   779  
   780  // SkipAuthority skips a single Authority Resource.
   781  func (p *Parser) SkipAuthority() error {
   782  	return p.skipResource(sectionAuthorities)
   783  }
   784  
   785  // SkipAllAuthorities skips all Authority Resources.
   786  func (p *Parser) SkipAllAuthorities() error {
   787  	for {
   788  		if err := p.SkipAuthority(); err == ErrSectionDone {
   789  			return nil
   790  		} else if err != nil {
   791  			return err
   792  		}
   793  	}
   794  }
   795  
   796  // AdditionalHeader parses a single Additional ResourceHeader.
   797  func (p *Parser) AdditionalHeader() (ResourceHeader, error) {
   798  	return p.resourceHeader(sectionAdditionals)
   799  }
   800  
   801  // Additional parses a single Additional Resource.
   802  func (p *Parser) Additional() (Resource, error) {
   803  	return p.resource(sectionAdditionals)
   804  }
   805  
   806  // AllAdditionals parses all Additional Resources.
   807  func (p *Parser) AllAdditionals() ([]Resource, error) {
   808  	// Additionals usually contain OPT, and sometimes A/AAAA
   809  	// glue records.
   810  	//
   811  	// Pre-allocate up to a certain limit, since p.header is
   812  	// untrusted data.
   813  	n := int(p.header.additionals)
   814  	if n > 10 {
   815  		n = 10
   816  	}
   817  	as := make([]Resource, 0, n)
   818  	for {
   819  		a, err := p.Additional()
   820  		if err == ErrSectionDone {
   821  			return as, nil
   822  		}
   823  		if err != nil {
   824  			return nil, err
   825  		}
   826  		as = append(as, a)
   827  	}
   828  }
   829  
   830  // SkipAdditional skips a single Additional Resource.
   831  func (p *Parser) SkipAdditional() error {
   832  	return p.skipResource(sectionAdditionals)
   833  }
   834  
   835  // SkipAllAdditionals skips all Additional Resources.
   836  func (p *Parser) SkipAllAdditionals() error {
   837  	for {
   838  		if err := p.SkipAdditional(); err == ErrSectionDone {
   839  			return nil
   840  		} else if err != nil {
   841  			return err
   842  		}
   843  	}
   844  }
   845  
   846  // CNAMEResource parses a single CNAMEResource.
   847  //
   848  // One of the XXXHeader methods must have been called before calling this
   849  // method.
   850  func (p *Parser) CNAMEResource() (CNAMEResource, error) {
   851  	if !p.resHeaderValid || p.resHeader.Type != TypeCNAME {
   852  		return CNAMEResource{}, ErrNotStarted
   853  	}
   854  	r, err := unpackCNAMEResource(p.msg, p.off)
   855  	if err != nil {
   856  		return CNAMEResource{}, err
   857  	}
   858  	p.off += int(p.resHeader.Length)
   859  	p.resHeaderValid = false
   860  	p.index++
   861  	return r, nil
   862  }
   863  
   864  // MXResource parses a single MXResource.
   865  //
   866  // One of the XXXHeader methods must have been called before calling this
   867  // method.
   868  func (p *Parser) MXResource() (MXResource, error) {
   869  	if !p.resHeaderValid || p.resHeader.Type != TypeMX {
   870  		return MXResource{}, ErrNotStarted
   871  	}
   872  	r, err := unpackMXResource(p.msg, p.off)
   873  	if err != nil {
   874  		return MXResource{}, err
   875  	}
   876  	p.off += int(p.resHeader.Length)
   877  	p.resHeaderValid = false
   878  	p.index++
   879  	return r, nil
   880  }
   881  
   882  // NSResource parses a single NSResource.
   883  //
   884  // One of the XXXHeader methods must have been called before calling this
   885  // method.
   886  func (p *Parser) NSResource() (NSResource, error) {
   887  	if !p.resHeaderValid || p.resHeader.Type != TypeNS {
   888  		return NSResource{}, ErrNotStarted
   889  	}
   890  	r, err := unpackNSResource(p.msg, p.off)
   891  	if err != nil {
   892  		return NSResource{}, err
   893  	}
   894  	p.off += int(p.resHeader.Length)
   895  	p.resHeaderValid = false
   896  	p.index++
   897  	return r, nil
   898  }
   899  
   900  // PTRResource parses a single PTRResource.
   901  //
   902  // One of the XXXHeader methods must have been called before calling this
   903  // method.
   904  func (p *Parser) PTRResource() (PTRResource, error) {
   905  	if !p.resHeaderValid || p.resHeader.Type != TypePTR {
   906  		return PTRResource{}, ErrNotStarted
   907  	}
   908  	r, err := unpackPTRResource(p.msg, p.off)
   909  	if err != nil {
   910  		return PTRResource{}, err
   911  	}
   912  	p.off += int(p.resHeader.Length)
   913  	p.resHeaderValid = false
   914  	p.index++
   915  	return r, nil
   916  }
   917  
   918  // SOAResource parses a single SOAResource.
   919  //
   920  // One of the XXXHeader methods must have been called before calling this
   921  // method.
   922  func (p *Parser) SOAResource() (SOAResource, error) {
   923  	if !p.resHeaderValid || p.resHeader.Type != TypeSOA {
   924  		return SOAResource{}, ErrNotStarted
   925  	}
   926  	r, err := unpackSOAResource(p.msg, p.off)
   927  	if err != nil {
   928  		return SOAResource{}, err
   929  	}
   930  	p.off += int(p.resHeader.Length)
   931  	p.resHeaderValid = false
   932  	p.index++
   933  	return r, nil
   934  }
   935  
   936  // TXTResource parses a single TXTResource.
   937  //
   938  // One of the XXXHeader methods must have been called before calling this
   939  // method.
   940  func (p *Parser) TXTResource() (TXTResource, error) {
   941  	if !p.resHeaderValid || p.resHeader.Type != TypeTXT {
   942  		return TXTResource{}, ErrNotStarted
   943  	}
   944  	r, err := unpackTXTResource(p.msg, p.off, p.resHeader.Length)
   945  	if err != nil {
   946  		return TXTResource{}, err
   947  	}
   948  	p.off += int(p.resHeader.Length)
   949  	p.resHeaderValid = false
   950  	p.index++
   951  	return r, nil
   952  }
   953  
   954  // SRVResource parses a single SRVResource.
   955  //
   956  // One of the XXXHeader methods must have been called before calling this
   957  // method.
   958  func (p *Parser) SRVResource() (SRVResource, error) {
   959  	if !p.resHeaderValid || p.resHeader.Type != TypeSRV {
   960  		return SRVResource{}, ErrNotStarted
   961  	}
   962  	r, err := unpackSRVResource(p.msg, p.off)
   963  	if err != nil {
   964  		return SRVResource{}, err
   965  	}
   966  	p.off += int(p.resHeader.Length)
   967  	p.resHeaderValid = false
   968  	p.index++
   969  	return r, nil
   970  }
   971  
   972  // AResource parses a single AResource.
   973  //
   974  // One of the XXXHeader methods must have been called before calling this
   975  // method.
   976  func (p *Parser) AResource() (AResource, error) {
   977  	if !p.resHeaderValid || p.resHeader.Type != TypeA {
   978  		return AResource{}, ErrNotStarted
   979  	}
   980  	r, err := unpackAResource(p.msg, p.off)
   981  	if err != nil {
   982  		return AResource{}, err
   983  	}
   984  	p.off += int(p.resHeader.Length)
   985  	p.resHeaderValid = false
   986  	p.index++
   987  	return r, nil
   988  }
   989  
   990  // AAAAResource parses a single AAAAResource.
   991  //
   992  // One of the XXXHeader methods must have been called before calling this
   993  // method.
   994  func (p *Parser) AAAAResource() (AAAAResource, error) {
   995  	if !p.resHeaderValid || p.resHeader.Type != TypeAAAA {
   996  		return AAAAResource{}, ErrNotStarted
   997  	}
   998  	r, err := unpackAAAAResource(p.msg, p.off)
   999  	if err != nil {
  1000  		return AAAAResource{}, err
  1001  	}
  1002  	p.off += int(p.resHeader.Length)
  1003  	p.resHeaderValid = false
  1004  	p.index++
  1005  	return r, nil
  1006  }
  1007  
  1008  // OPTResource parses a single OPTResource.
  1009  //
  1010  // One of the XXXHeader methods must have been called before calling this
  1011  // method.
  1012  func (p *Parser) OPTResource() (OPTResource, error) {
  1013  	if !p.resHeaderValid || p.resHeader.Type != TypeOPT {
  1014  		return OPTResource{}, ErrNotStarted
  1015  	}
  1016  	r, err := unpackOPTResource(p.msg, p.off, p.resHeader.Length)
  1017  	if err != nil {
  1018  		return OPTResource{}, err
  1019  	}
  1020  	p.off += int(p.resHeader.Length)
  1021  	p.resHeaderValid = false
  1022  	p.index++
  1023  	return r, nil
  1024  }
  1025  
  1026  // Unpack parses a full Message.
  1027  func (m *Message) Unpack(msg []byte) error {
  1028  	var p Parser
  1029  	var err error
  1030  	if m.Header, err = p.Start(msg); err != nil {
  1031  		return err
  1032  	}
  1033  	if m.Questions, err = p.AllQuestions(); err != nil {
  1034  		return err
  1035  	}
  1036  	if m.Answers, err = p.AllAnswers(); err != nil {
  1037  		return err
  1038  	}
  1039  	if m.Authorities, err = p.AllAuthorities(); err != nil {
  1040  		return err
  1041  	}
  1042  	if m.Additionals, err = p.AllAdditionals(); err != nil {
  1043  		return err
  1044  	}
  1045  	return nil
  1046  }
  1047  
  1048  // Pack packs a full Message.
  1049  func (m *Message) Pack() ([]byte, error) {
  1050  	return m.AppendPack(make([]byte, 0, packStartingCap))
  1051  }
  1052  
  1053  // AppendPack is like Pack but appends the full Message to b and returns the
  1054  // extended buffer.
  1055  func (m *Message) AppendPack(b []byte) ([]byte, error) {
  1056  	// Validate the lengths. It is very unlikely that anyone will try to
  1057  	// pack more than 65535 of any particular type, but it is possible and
  1058  	// we should fail gracefully.
  1059  	if len(m.Questions) > int(^uint16(0)) {
  1060  		return nil, errTooManyQuestions
  1061  	}
  1062  	if len(m.Answers) > int(^uint16(0)) {
  1063  		return nil, errTooManyAnswers
  1064  	}
  1065  	if len(m.Authorities) > int(^uint16(0)) {
  1066  		return nil, errTooManyAuthorities
  1067  	}
  1068  	if len(m.Additionals) > int(^uint16(0)) {
  1069  		return nil, errTooManyAdditionals
  1070  	}
  1071  
  1072  	var h header
  1073  	h.id, h.bits = m.Header.pack()
  1074  
  1075  	h.questions = uint16(len(m.Questions))
  1076  	h.answers = uint16(len(m.Answers))
  1077  	h.authorities = uint16(len(m.Authorities))
  1078  	h.additionals = uint16(len(m.Additionals))
  1079  
  1080  	compressionOff := len(b)
  1081  	msg := h.pack(b)
  1082  
  1083  	// RFC 1035 allows (but does not require) compression for packing. RFC
  1084  	// 1035 requires unpacking implementations to support compression, so
  1085  	// unconditionally enabling it is fine.
  1086  	//
  1087  	// DNS lookups are typically done over UDP, and RFC 1035 states that UDP
  1088  	// DNS messages can be a maximum of 512 bytes long. Without compression,
  1089  	// many DNS response messages are over this limit, so enabling
  1090  	// compression will help ensure compliance.
  1091  	compression := map[string]int{}
  1092  
  1093  	for i := range m.Questions {
  1094  		var err error
  1095  		if msg, err = m.Questions[i].pack(msg, compression, compressionOff); err != nil {
  1096  			return nil, &nestedError{"packing Question", err}
  1097  		}
  1098  	}
  1099  	for i := range m.Answers {
  1100  		var err error
  1101  		if msg, err = m.Answers[i].pack(msg, compression, compressionOff); err != nil {
  1102  			return nil, &nestedError{"packing Answer", err}
  1103  		}
  1104  	}
  1105  	for i := range m.Authorities {
  1106  		var err error
  1107  		if msg, err = m.Authorities[i].pack(msg, compression, compressionOff); err != nil {
  1108  			return nil, &nestedError{"packing Authority", err}
  1109  		}
  1110  	}
  1111  	for i := range m.Additionals {
  1112  		var err error
  1113  		if msg, err = m.Additionals[i].pack(msg, compression, compressionOff); err != nil {
  1114  			return nil, &nestedError{"packing Additional", err}
  1115  		}
  1116  	}
  1117  
  1118  	return msg, nil
  1119  }
  1120  
  1121  // GoString implements fmt.GoStringer.GoString.
  1122  func (m *Message) GoString() string {
  1123  	s := "dnsmessage.Message{Header: " + m.Header.GoString() + ", " +
  1124  		"Questions: []dnsmessage.Question{"
  1125  	if len(m.Questions) > 0 {
  1126  		s += m.Questions[0].GoString()
  1127  		for _, q := range m.Questions[1:] {
  1128  			s += ", " + q.GoString()
  1129  		}
  1130  	}
  1131  	s += "}, Answers: []dnsmessage.Resource{"
  1132  	if len(m.Answers) > 0 {
  1133  		s += m.Answers[0].GoString()
  1134  		for _, a := range m.Answers[1:] {
  1135  			s += ", " + a.GoString()
  1136  		}
  1137  	}
  1138  	s += "}, Authorities: []dnsmessage.Resource{"
  1139  	if len(m.Authorities) > 0 {
  1140  		s += m.Authorities[0].GoString()
  1141  		for _, a := range m.Authorities[1:] {
  1142  			s += ", " + a.GoString()
  1143  		}
  1144  	}
  1145  	s += "}, Additionals: []dnsmessage.Resource{"
  1146  	if len(m.Additionals) > 0 {
  1147  		s += m.Additionals[0].GoString()
  1148  		for _, a := range m.Additionals[1:] {
  1149  			s += ", " + a.GoString()
  1150  		}
  1151  	}
  1152  	return s + "}}"
  1153  }
  1154  
  1155  // A Builder allows incrementally packing a DNS message.
  1156  //
  1157  // Example usage:
  1158  //	buf := make([]byte, 2, 514)
  1159  //	b := NewBuilder(buf, Header{...})
  1160  //	b.EnableCompression()
  1161  //	// Optionally start a section and add things to that section.
  1162  //	// Repeat adding sections as necessary.
  1163  //	buf, err := b.Finish()
  1164  //	// If err is nil, buf[2:] will contain the built bytes.
  1165  type Builder struct {
  1166  	// msg is the storage for the message being built.
  1167  	msg []byte
  1168  
  1169  	// section keeps track of the current section being built.
  1170  	section section
  1171  
  1172  	// header keeps track of what should go in the header when Finish is
  1173  	// called.
  1174  	header header
  1175  
  1176  	// start is the starting index of the bytes allocated in msg for header.
  1177  	start int
  1178  
  1179  	// compression is a mapping from name suffixes to their starting index
  1180  	// in msg.
  1181  	compression map[string]int
  1182  }
  1183  
  1184  // NewBuilder creates a new builder with compression disabled.
  1185  //
  1186  // Note: Most users will want to immediately enable compression with the
  1187  // EnableCompression method. See that method's comment for why you may or may
  1188  // not want to enable compression.
  1189  //
  1190  // The DNS message is appended to the provided initial buffer buf (which may be
  1191  // nil) as it is built. The final message is returned by the (*Builder).Finish
  1192  // method, which may return the same underlying array if there was sufficient
  1193  // capacity in the slice.
  1194  func NewBuilder(buf []byte, h Header) Builder {
  1195  	if buf == nil {
  1196  		buf = make([]byte, 0, packStartingCap)
  1197  	}
  1198  	b := Builder{msg: buf, start: len(buf)}
  1199  	b.header.id, b.header.bits = h.pack()
  1200  	var hb [headerLen]byte
  1201  	b.msg = append(b.msg, hb[:]...)
  1202  	b.section = sectionHeader
  1203  	return b
  1204  }
  1205  
  1206  // EnableCompression enables compression in the Builder.
  1207  //
  1208  // Leaving compression disabled avoids compression related allocations, but can
  1209  // result in larger message sizes. Be careful with this mode as it can cause
  1210  // messages to exceed the UDP size limit.
  1211  //
  1212  // According to RFC 1035, section 4.1.4, the use of compression is optional, but
  1213  // all implementations must accept both compressed and uncompressed DNS
  1214  // messages.
  1215  //
  1216  // Compression should be enabled before any sections are added for best results.
  1217  func (b *Builder) EnableCompression() {
  1218  	b.compression = map[string]int{}
  1219  }
  1220  
  1221  func (b *Builder) startCheck(s section) error {
  1222  	if b.section <= sectionNotStarted {
  1223  		return ErrNotStarted
  1224  	}
  1225  	if b.section > s {
  1226  		return ErrSectionDone
  1227  	}
  1228  	return nil
  1229  }
  1230  
  1231  // StartQuestions prepares the builder for packing Questions.
  1232  func (b *Builder) StartQuestions() error {
  1233  	if err := b.startCheck(sectionQuestions); err != nil {
  1234  		return err
  1235  	}
  1236  	b.section = sectionQuestions
  1237  	return nil
  1238  }
  1239  
  1240  // StartAnswers prepares the builder for packing Answers.
  1241  func (b *Builder) StartAnswers() error {
  1242  	if err := b.startCheck(sectionAnswers); err != nil {
  1243  		return err
  1244  	}
  1245  	b.section = sectionAnswers
  1246  	return nil
  1247  }
  1248  
  1249  // StartAuthorities prepares the builder for packing Authorities.
  1250  func (b *Builder) StartAuthorities() error {
  1251  	if err := b.startCheck(sectionAuthorities); err != nil {
  1252  		return err
  1253  	}
  1254  	b.section = sectionAuthorities
  1255  	return nil
  1256  }
  1257  
  1258  // StartAdditionals prepares the builder for packing Additionals.
  1259  func (b *Builder) StartAdditionals() error {
  1260  	if err := b.startCheck(sectionAdditionals); err != nil {
  1261  		return err
  1262  	}
  1263  	b.section = sectionAdditionals
  1264  	return nil
  1265  }
  1266  
  1267  func (b *Builder) incrementSectionCount() error {
  1268  	var count *uint16
  1269  	var err error
  1270  	switch b.section {
  1271  	case sectionQuestions:
  1272  		count = &b.header.questions
  1273  		err = errTooManyQuestions
  1274  	case sectionAnswers:
  1275  		count = &b.header.answers
  1276  		err = errTooManyAnswers
  1277  	case sectionAuthorities:
  1278  		count = &b.header.authorities
  1279  		err = errTooManyAuthorities
  1280  	case sectionAdditionals:
  1281  		count = &b.header.additionals
  1282  		err = errTooManyAdditionals
  1283  	}
  1284  	if *count == ^uint16(0) {
  1285  		return err
  1286  	}
  1287  	*count++
  1288  	return nil
  1289  }
  1290  
  1291  // Question adds a single Question.
  1292  func (b *Builder) Question(q Question) error {
  1293  	if b.section < sectionQuestions {
  1294  		return ErrNotStarted
  1295  	}
  1296  	if b.section > sectionQuestions {
  1297  		return ErrSectionDone
  1298  	}
  1299  	msg, err := q.pack(b.msg, b.compression, b.start)
  1300  	if err != nil {
  1301  		return err
  1302  	}
  1303  	if err := b.incrementSectionCount(); err != nil {
  1304  		return err
  1305  	}
  1306  	b.msg = msg
  1307  	return nil
  1308  }
  1309  
  1310  func (b *Builder) checkResourceSection() error {
  1311  	if b.section < sectionAnswers {
  1312  		return ErrNotStarted
  1313  	}
  1314  	if b.section > sectionAdditionals {
  1315  		return ErrSectionDone
  1316  	}
  1317  	return nil
  1318  }
  1319  
  1320  // CNAMEResource adds a single CNAMEResource.
  1321  func (b *Builder) CNAMEResource(h ResourceHeader, r CNAMEResource) error {
  1322  	if err := b.checkResourceSection(); err != nil {
  1323  		return err
  1324  	}
  1325  	h.Type = r.realType()
  1326  	msg, lenOff, err := h.pack(b.msg, b.compression, b.start)
  1327  	if err != nil {
  1328  		return &nestedError{"ResourceHeader", err}
  1329  	}
  1330  	preLen := len(msg)
  1331  	if msg, err = r.pack(msg, b.compression, b.start); err != nil {
  1332  		return &nestedError{"CNAMEResource body", err}
  1333  	}
  1334  	if err := h.fixLen(msg, lenOff, preLen); err != nil {
  1335  		return err
  1336  	}
  1337  	if err := b.incrementSectionCount(); err != nil {
  1338  		return err
  1339  	}
  1340  	b.msg = msg
  1341  	return nil
  1342  }
  1343  
  1344  // MXResource adds a single MXResource.
  1345  func (b *Builder) MXResource(h ResourceHeader, r MXResource) error {
  1346  	if err := b.checkResourceSection(); err != nil {
  1347  		return err
  1348  	}
  1349  	h.Type = r.realType()
  1350  	msg, lenOff, err := h.pack(b.msg, b.compression, b.start)
  1351  	if err != nil {
  1352  		return &nestedError{"ResourceHeader", err}
  1353  	}
  1354  	preLen := len(msg)
  1355  	if msg, err = r.pack(msg, b.compression, b.start); err != nil {
  1356  		return &nestedError{"MXResource body", err}
  1357  	}
  1358  	if err := h.fixLen(msg, lenOff, preLen); err != nil {
  1359  		return err
  1360  	}
  1361  	if err := b.incrementSectionCount(); err != nil {
  1362  		return err
  1363  	}
  1364  	b.msg = msg
  1365  	return nil
  1366  }
  1367  
  1368  // NSResource adds a single NSResource.
  1369  func (b *Builder) NSResource(h ResourceHeader, r NSResource) error {
  1370  	if err := b.checkResourceSection(); err != nil {
  1371  		return err
  1372  	}
  1373  	h.Type = r.realType()
  1374  	msg, lenOff, err := h.pack(b.msg, b.compression, b.start)
  1375  	if err != nil {
  1376  		return &nestedError{"ResourceHeader", err}
  1377  	}
  1378  	preLen := len(msg)
  1379  	if msg, err = r.pack(msg, b.compression, b.start); err != nil {
  1380  		return &nestedError{"NSResource body", err}
  1381  	}
  1382  	if err := h.fixLen(msg, lenOff, preLen); err != nil {
  1383  		return err
  1384  	}
  1385  	if err := b.incrementSectionCount(); err != nil {
  1386  		return err
  1387  	}
  1388  	b.msg = msg
  1389  	return nil
  1390  }
  1391  
  1392  // PTRResource adds a single PTRResource.
  1393  func (b *Builder) PTRResource(h ResourceHeader, r PTRResource) error {
  1394  	if err := b.checkResourceSection(); err != nil {
  1395  		return err
  1396  	}
  1397  	h.Type = r.realType()
  1398  	msg, lenOff, err := h.pack(b.msg, b.compression, b.start)
  1399  	if err != nil {
  1400  		return &nestedError{"ResourceHeader", err}
  1401  	}
  1402  	preLen := len(msg)
  1403  	if msg, err = r.pack(msg, b.compression, b.start); err != nil {
  1404  		return &nestedError{"PTRResource body", err}
  1405  	}
  1406  	if err := h.fixLen(msg, lenOff, preLen); err != nil {
  1407  		return err
  1408  	}
  1409  	if err := b.incrementSectionCount(); err != nil {
  1410  		return err
  1411  	}
  1412  	b.msg = msg
  1413  	return nil
  1414  }
  1415  
  1416  // SOAResource adds a single SOAResource.
  1417  func (b *Builder) SOAResource(h ResourceHeader, r SOAResource) error {
  1418  	if err := b.checkResourceSection(); err != nil {
  1419  		return err
  1420  	}
  1421  	h.Type = r.realType()
  1422  	msg, lenOff, err := h.pack(b.msg, b.compression, b.start)
  1423  	if err != nil {
  1424  		return &nestedError{"ResourceHeader", err}
  1425  	}
  1426  	preLen := len(msg)
  1427  	if msg, err = r.pack(msg, b.compression, b.start); err != nil {
  1428  		return &nestedError{"SOAResource body", err}
  1429  	}
  1430  	if err := h.fixLen(msg, lenOff, preLen); err != nil {
  1431  		return err
  1432  	}
  1433  	if err := b.incrementSectionCount(); err != nil {
  1434  		return err
  1435  	}
  1436  	b.msg = msg
  1437  	return nil
  1438  }
  1439  
  1440  // TXTResource adds a single TXTResource.
  1441  func (b *Builder) TXTResource(h ResourceHeader, r TXTResource) error {
  1442  	if err := b.checkResourceSection(); err != nil {
  1443  		return err
  1444  	}
  1445  	h.Type = r.realType()
  1446  	msg, lenOff, err := h.pack(b.msg, b.compression, b.start)
  1447  	if err != nil {
  1448  		return &nestedError{"ResourceHeader", err}
  1449  	}
  1450  	preLen := len(msg)
  1451  	if msg, err = r.pack(msg, b.compression, b.start); err != nil {
  1452  		return &nestedError{"TXTResource body", err}
  1453  	}
  1454  	if err := h.fixLen(msg, lenOff, preLen); err != nil {
  1455  		return err
  1456  	}
  1457  	if err := b.incrementSectionCount(); err != nil {
  1458  		return err
  1459  	}
  1460  	b.msg = msg
  1461  	return nil
  1462  }
  1463  
  1464  // SRVResource adds a single SRVResource.
  1465  func (b *Builder) SRVResource(h ResourceHeader, r SRVResource) error {
  1466  	if err := b.checkResourceSection(); err != nil {
  1467  		return err
  1468  	}
  1469  	h.Type = r.realType()
  1470  	msg, lenOff, err := h.pack(b.msg, b.compression, b.start)
  1471  	if err != nil {
  1472  		return &nestedError{"ResourceHeader", err}
  1473  	}
  1474  	preLen := len(msg)
  1475  	if msg, err = r.pack(msg, b.compression, b.start); err != nil {
  1476  		return &nestedError{"SRVResource body", err}
  1477  	}
  1478  	if err := h.fixLen(msg, lenOff, preLen); err != nil {
  1479  		return err
  1480  	}
  1481  	if err := b.incrementSectionCount(); err != nil {
  1482  		return err
  1483  	}
  1484  	b.msg = msg
  1485  	return nil
  1486  }
  1487  
  1488  // AResource adds a single AResource.
  1489  func (b *Builder) AResource(h ResourceHeader, r AResource) error {
  1490  	if err := b.checkResourceSection(); err != nil {
  1491  		return err
  1492  	}
  1493  	h.Type = r.realType()
  1494  	msg, lenOff, err := h.pack(b.msg, b.compression, b.start)
  1495  	if err != nil {
  1496  		return &nestedError{"ResourceHeader", err}
  1497  	}
  1498  	preLen := len(msg)
  1499  	if msg, err = r.pack(msg, b.compression, b.start); err != nil {
  1500  		return &nestedError{"AResource body", err}
  1501  	}
  1502  	if err := h.fixLen(msg, lenOff, preLen); err != nil {
  1503  		return err
  1504  	}
  1505  	if err := b.incrementSectionCount(); err != nil {
  1506  		return err
  1507  	}
  1508  	b.msg = msg
  1509  	return nil
  1510  }
  1511  
  1512  // AAAAResource adds a single AAAAResource.
  1513  func (b *Builder) AAAAResource(h ResourceHeader, r AAAAResource) error {
  1514  	if err := b.checkResourceSection(); err != nil {
  1515  		return err
  1516  	}
  1517  	h.Type = r.realType()
  1518  	msg, lenOff, err := h.pack(b.msg, b.compression, b.start)
  1519  	if err != nil {
  1520  		return &nestedError{"ResourceHeader", err}
  1521  	}
  1522  	preLen := len(msg)
  1523  	if msg, err = r.pack(msg, b.compression, b.start); err != nil {
  1524  		return &nestedError{"AAAAResource body", err}
  1525  	}
  1526  	if err := h.fixLen(msg, lenOff, preLen); err != nil {
  1527  		return err
  1528  	}
  1529  	if err := b.incrementSectionCount(); err != nil {
  1530  		return err
  1531  	}
  1532  	b.msg = msg
  1533  	return nil
  1534  }
  1535  
  1536  // OPTResource adds a single OPTResource.
  1537  func (b *Builder) OPTResource(h ResourceHeader, r OPTResource) error {
  1538  	if err := b.checkResourceSection(); err != nil {
  1539  		return err
  1540  	}
  1541  	h.Type = r.realType()
  1542  	msg, lenOff, err := h.pack(b.msg, b.compression, b.start)
  1543  	if err != nil {
  1544  		return &nestedError{"ResourceHeader", err}
  1545  	}
  1546  	preLen := len(msg)
  1547  	if msg, err = r.pack(msg, b.compression, b.start); err != nil {
  1548  		return &nestedError{"OPTResource body", err}
  1549  	}
  1550  	if err := h.fixLen(msg, lenOff, preLen); err != nil {
  1551  		return err
  1552  	}
  1553  	if err := b.incrementSectionCount(); err != nil {
  1554  		return err
  1555  	}
  1556  	b.msg = msg
  1557  	return nil
  1558  }
  1559  
  1560  // Finish ends message building and generates a binary message.
  1561  func (b *Builder) Finish() ([]byte, error) {
  1562  	if b.section < sectionHeader {
  1563  		return nil, ErrNotStarted
  1564  	}
  1565  	b.section = sectionDone
  1566  	// Space for the header was allocated in NewBuilder.
  1567  	b.header.pack(b.msg[b.start:b.start])
  1568  	return b.msg, nil
  1569  }
  1570  
  1571  // A ResourceHeader is the header of a DNS resource record. There are
  1572  // many types of DNS resource records, but they all share the same header.
  1573  type ResourceHeader struct {
  1574  	// Name is the domain name for which this resource record pertains.
  1575  	Name Name
  1576  
  1577  	// Type is the type of DNS resource record.
  1578  	//
  1579  	// This field will be set automatically during packing.
  1580  	Type Type
  1581  
  1582  	// Class is the class of network to which this DNS resource record
  1583  	// pertains.
  1584  	Class Class
  1585  
  1586  	// TTL is the length of time (measured in seconds) which this resource
  1587  	// record is valid for (time to live). All Resources in a set should
  1588  	// have the same TTL (RFC 2181 Section 5.2).
  1589  	TTL uint32
  1590  
  1591  	// Length is the length of data in the resource record after the header.
  1592  	//
  1593  	// This field will be set automatically during packing.
  1594  	Length uint16
  1595  }
  1596  
  1597  // GoString implements fmt.GoStringer.GoString.
  1598  func (h *ResourceHeader) GoString() string {
  1599  	return "dnsmessage.ResourceHeader{" +
  1600  		"Name: " + h.Name.GoString() + ", " +
  1601  		"Type: " + h.Type.GoString() + ", " +
  1602  		"Class: " + h.Class.GoString() + ", " +
  1603  		"TTL: " + printUint32(h.TTL) + ", " +
  1604  		"Length: " + printUint16(h.Length) + "}"
  1605  }
  1606  
  1607  // pack appends the wire format of the ResourceHeader to oldMsg.
  1608  //
  1609  // lenOff is the offset in msg where the Length field was packed.
  1610  func (h *ResourceHeader) pack(oldMsg []byte, compression map[string]int, compressionOff int) (msg []byte, lenOff int, err error) {
  1611  	msg = oldMsg
  1612  	if msg, err = h.Name.pack(msg, compression, compressionOff); err != nil {
  1613  		return oldMsg, 0, &nestedError{"Name", err}
  1614  	}
  1615  	msg = packType(msg, h.Type)
  1616  	msg = packClass(msg, h.Class)
  1617  	msg = packUint32(msg, h.TTL)
  1618  	lenOff = len(msg)
  1619  	msg = packUint16(msg, h.Length)
  1620  	return msg, lenOff, nil
  1621  }
  1622  
  1623  func (h *ResourceHeader) unpack(msg []byte, off int) (int, error) {
  1624  	newOff := off
  1625  	var err error
  1626  	if newOff, err = h.Name.unpack(msg, newOff); err != nil {
  1627  		return off, &nestedError{"Name", err}
  1628  	}
  1629  	if h.Type, newOff, err = unpackType(msg, newOff); err != nil {
  1630  		return off, &nestedError{"Type", err}
  1631  	}
  1632  	if h.Class, newOff, err = unpackClass(msg, newOff); err != nil {
  1633  		return off, &nestedError{"Class", err}
  1634  	}
  1635  	if h.TTL, newOff, err = unpackUint32(msg, newOff); err != nil {
  1636  		return off, &nestedError{"TTL", err}
  1637  	}
  1638  	if h.Length, newOff, err = unpackUint16(msg, newOff); err != nil {
  1639  		return off, &nestedError{"Length", err}
  1640  	}
  1641  	return newOff, nil
  1642  }
  1643  
  1644  // fixLen updates a packed ResourceHeader to include the length of the
  1645  // ResourceBody.
  1646  //
  1647  // lenOff is the offset of the ResourceHeader.Length field in msg.
  1648  //
  1649  // preLen is the length that msg was before the ResourceBody was packed.
  1650  func (h *ResourceHeader) fixLen(msg []byte, lenOff int, preLen int) error {
  1651  	conLen := len(msg) - preLen
  1652  	if conLen > int(^uint16(0)) {
  1653  		return errResTooLong
  1654  	}
  1655  
  1656  	// Fill in the length now that we know how long the content is.
  1657  	packUint16(msg[lenOff:lenOff], uint16(conLen))
  1658  	h.Length = uint16(conLen)
  1659  
  1660  	return nil
  1661  }
  1662  
  1663  // EDNS(0) wire constants.
  1664  const (
  1665  	edns0Version = 0
  1666  
  1667  	edns0DNSSECOK     = 0x00008000
  1668  	ednsVersionMask   = 0x00ff0000
  1669  	edns0DNSSECOKMask = 0x00ff8000
  1670  )
  1671  
  1672  // SetEDNS0 configures h for EDNS(0).
  1673  //
  1674  // The provided extRCode must be an extedned RCode.
  1675  func (h *ResourceHeader) SetEDNS0(udpPayloadLen int, extRCode RCode, dnssecOK bool) error {
  1676  	h.Name = Name{Data: [nameLen]byte{'.'}, Length: 1} // RFC 6891 section 6.1.2
  1677  	h.Type = TypeOPT
  1678  	h.Class = Class(udpPayloadLen)
  1679  	h.TTL = uint32(extRCode) >> 4 << 24
  1680  	if dnssecOK {
  1681  		h.TTL |= edns0DNSSECOK
  1682  	}
  1683  	return nil
  1684  }
  1685  
  1686  // DNSSECAllowed reports whether the DNSSEC OK bit is set.
  1687  func (h *ResourceHeader) DNSSECAllowed() bool {
  1688  	return h.TTL&edns0DNSSECOKMask == edns0DNSSECOK // RFC 6891 section 6.1.3
  1689  }
  1690  
  1691  // ExtendedRCode returns an extended RCode.
  1692  //
  1693  // The provided rcode must be the RCode in DNS message header.
  1694  func (h *ResourceHeader) ExtendedRCode(rcode RCode) RCode {
  1695  	if h.TTL&ednsVersionMask == edns0Version { // RFC 6891 section 6.1.3
  1696  		return RCode(h.TTL>>24<<4) | rcode
  1697  	}
  1698  	return rcode
  1699  }
  1700  
  1701  func skipResource(msg []byte, off int) (int, error) {
  1702  	newOff, err := skipName(msg, off)
  1703  	if err != nil {
  1704  		return off, &nestedError{"Name", err}
  1705  	}
  1706  	if newOff, err = skipType(msg, newOff); err != nil {
  1707  		return off, &nestedError{"Type", err}
  1708  	}
  1709  	if newOff, err = skipClass(msg, newOff); err != nil {
  1710  		return off, &nestedError{"Class", err}
  1711  	}
  1712  	if newOff, err = skipUint32(msg, newOff); err != nil {
  1713  		return off, &nestedError{"TTL", err}
  1714  	}
  1715  	length, newOff, err := unpackUint16(msg, newOff)
  1716  	if err != nil {
  1717  		return off, &nestedError{"Length", err}
  1718  	}
  1719  	if newOff += int(length); newOff > len(msg) {
  1720  		return off, errResourceLen
  1721  	}
  1722  	return newOff, nil
  1723  }
  1724  
  1725  // packUint16 appends the wire format of field to msg.
  1726  func packUint16(msg []byte, field uint16) []byte {
  1727  	return append(msg, byte(field>>8), byte(field))
  1728  }
  1729  
  1730  func unpackUint16(msg []byte, off int) (uint16, int, error) {
  1731  	if off+uint16Len > len(msg) {
  1732  		return 0, off, errBaseLen
  1733  	}
  1734  	return uint16(msg[off])<<8 | uint16(msg[off+1]), off + uint16Len, nil
  1735  }
  1736  
  1737  func skipUint16(msg []byte, off int) (int, error) {
  1738  	if off+uint16Len > len(msg) {
  1739  		return off, errBaseLen
  1740  	}
  1741  	return off + uint16Len, nil
  1742  }
  1743  
  1744  // packType appends the wire format of field to msg.
  1745  func packType(msg []byte, field Type) []byte {
  1746  	return packUint16(msg, uint16(field))
  1747  }
  1748  
  1749  func unpackType(msg []byte, off int) (Type, int, error) {
  1750  	t, o, err := unpackUint16(msg, off)
  1751  	return Type(t), o, err
  1752  }
  1753  
  1754  func skipType(msg []byte, off int) (int, error) {
  1755  	return skipUint16(msg, off)
  1756  }
  1757  
  1758  // packClass appends the wire format of field to msg.
  1759  func packClass(msg []byte, field Class) []byte {
  1760  	return packUint16(msg, uint16(field))
  1761  }
  1762  
  1763  func unpackClass(msg []byte, off int) (Class, int, error) {
  1764  	c, o, err := unpackUint16(msg, off)
  1765  	return Class(c), o, err
  1766  }
  1767  
  1768  func skipClass(msg []byte, off int) (int, error) {
  1769  	return skipUint16(msg, off)
  1770  }
  1771  
  1772  // packUint32 appends the wire format of field to msg.
  1773  func packUint32(msg []byte, field uint32) []byte {
  1774  	return append(
  1775  		msg,
  1776  		byte(field>>24),
  1777  		byte(field>>16),
  1778  		byte(field>>8),
  1779  		byte(field),
  1780  	)
  1781  }
  1782  
  1783  func unpackUint32(msg []byte, off int) (uint32, int, error) {
  1784  	if off+uint32Len > len(msg) {
  1785  		return 0, off, errBaseLen
  1786  	}
  1787  	v := uint32(msg[off])<<24 | uint32(msg[off+1])<<16 | uint32(msg[off+2])<<8 | uint32(msg[off+3])
  1788  	return v, off + uint32Len, nil
  1789  }
  1790  
  1791  func skipUint32(msg []byte, off int) (int, error) {
  1792  	if off+uint32Len > len(msg) {
  1793  		return off, errBaseLen
  1794  	}
  1795  	return off + uint32Len, nil
  1796  }
  1797  
  1798  // packText appends the wire format of field to msg.
  1799  func packText(msg []byte, field string) ([]byte, error) {
  1800  	l := len(field)
  1801  	if l > 255 {
  1802  		return nil, errStringTooLong
  1803  	}
  1804  	msg = append(msg, byte(l))
  1805  	msg = append(msg, field...)
  1806  
  1807  	return msg, nil
  1808  }
  1809  
  1810  func unpackText(msg []byte, off int) (string, int, error) {
  1811  	if off >= len(msg) {
  1812  		return "", off, errBaseLen
  1813  	}
  1814  	beginOff := off + 1
  1815  	endOff := beginOff + int(msg[off])
  1816  	if endOff > len(msg) {
  1817  		return "", off, errCalcLen
  1818  	}
  1819  	return string(msg[beginOff:endOff]), endOff, nil
  1820  }
  1821  
  1822  // packBytes appends the wire format of field to msg.
  1823  func packBytes(msg []byte, field []byte) []byte {
  1824  	return append(msg, field...)
  1825  }
  1826  
  1827  func unpackBytes(msg []byte, off int, field []byte) (int, error) {
  1828  	newOff := off + len(field)
  1829  	if newOff > len(msg) {
  1830  		return off, errBaseLen
  1831  	}
  1832  	copy(field, msg[off:newOff])
  1833  	return newOff, nil
  1834  }
  1835  
  1836  const nameLen = 255
  1837  
  1838  // A Name is a non-encoded domain name. It is used instead of strings to avoid
  1839  // allocations.
  1840  type Name struct {
  1841  	Data   [nameLen]byte
  1842  	Length uint8
  1843  }
  1844  
  1845  // NewName creates a new Name from a string.
  1846  func NewName(name string) (Name, error) {
  1847  	if len([]byte(name)) > nameLen {
  1848  		return Name{}, errCalcLen
  1849  	}
  1850  	n := Name{Length: uint8(len(name))}
  1851  	copy(n.Data[:], []byte(name))
  1852  	return n, nil
  1853  }
  1854  
  1855  // MustNewName creates a new Name from a string and panics on error.
  1856  func MustNewName(name string) Name {
  1857  	n, err := NewName(name)
  1858  	if err != nil {
  1859  		panic("creating name: " + err.Error())
  1860  	}
  1861  	return n
  1862  }
  1863  
  1864  // String implements fmt.Stringer.String.
  1865  func (n Name) String() string {
  1866  	return string(n.Data[:n.Length])
  1867  }
  1868  
  1869  // GoString implements fmt.GoStringer.GoString.
  1870  func (n *Name) GoString() string {
  1871  	return `dnsmessage.MustNewName("` + printString(n.Data[:n.Length]) + `")`
  1872  }
  1873  
  1874  // pack appends the wire format of the Name to msg.
  1875  //
  1876  // Domain names are a sequence of counted strings split at the dots. They end
  1877  // with a zero-length string. Compression can be used to reuse domain suffixes.
  1878  //
  1879  // The compression map will be updated with new domain suffixes. If compression
  1880  // is nil, compression will not be used.
  1881  func (n *Name) pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error) {
  1882  	oldMsg := msg
  1883  
  1884  	// Add a trailing dot to canonicalize name.
  1885  	if n.Length == 0 || n.Data[n.Length-1] != '.' {
  1886  		return oldMsg, errNonCanonicalName
  1887  	}
  1888  
  1889  	// Allow root domain.
  1890  	if n.Data[0] == '.' && n.Length == 1 {
  1891  		return append(msg, 0), nil
  1892  	}
  1893  
  1894  	// Emit sequence of counted strings, chopping at dots.
  1895  	for i, begin := 0, 0; i < int(n.Length); i++ {
  1896  		// Check for the end of the segment.
  1897  		if n.Data[i] == '.' {
  1898  			// The two most significant bits have special meaning.
  1899  			// It isn't allowed for segments to be long enough to
  1900  			// need them.
  1901  			if i-begin >= 1<<6 {
  1902  				return oldMsg, errSegTooLong
  1903  			}
  1904  
  1905  			// Segments must have a non-zero length.
  1906  			if i-begin == 0 {
  1907  				return oldMsg, errZeroSegLen
  1908  			}
  1909  
  1910  			msg = append(msg, byte(i-begin))
  1911  
  1912  			for j := begin; j < i; j++ {
  1913  				msg = append(msg, n.Data[j])
  1914  			}
  1915  
  1916  			begin = i + 1
  1917  			continue
  1918  		}
  1919  
  1920  		// We can only compress domain suffixes starting with a new
  1921  		// segment. A pointer is two bytes with the two most significant
  1922  		// bits set to 1 to indicate that it is a pointer.
  1923  		if (i == 0 || n.Data[i-1] == '.') && compression != nil {
  1924  			if ptr, ok := compression[string(n.Data[i:])]; ok {
  1925  				// Hit. Emit a pointer instead of the rest of
  1926  				// the domain.
  1927  				return append(msg, byte(ptr>>8|0xC0), byte(ptr)), nil
  1928  			}
  1929  
  1930  			// Miss. Add the suffix to the compression table if the
  1931  			// offset can be stored in the available 14 bytes.
  1932  			if len(msg) <= int(^uint16(0)>>2) {
  1933  				compression[string(n.Data[i:])] = len(msg) - compressionOff
  1934  			}
  1935  		}
  1936  	}
  1937  	return append(msg, 0), nil
  1938  }
  1939  
  1940  // unpack unpacks a domain name.
  1941  func (n *Name) unpack(msg []byte, off int) (int, error) {
  1942  	return n.unpackCompressed(msg, off, true /* allowCompression */)
  1943  }
  1944  
  1945  func (n *Name) unpackCompressed(msg []byte, off int, allowCompression bool) (int, error) {
  1946  	// currOff is the current working offset.
  1947  	currOff := off
  1948  
  1949  	// newOff is the offset where the next record will start. Pointers lead
  1950  	// to data that belongs to other names and thus doesn't count towards to
  1951  	// the usage of this name.
  1952  	newOff := off
  1953  
  1954  	// ptr is the number of pointers followed.
  1955  	var ptr int
  1956  
  1957  	// Name is a slice representation of the name data.
  1958  	name := n.Data[:0]
  1959  
  1960  Loop:
  1961  	for {
  1962  		if currOff >= len(msg) {
  1963  			return off, errBaseLen
  1964  		}
  1965  		c := int(msg[currOff])
  1966  		currOff++
  1967  		switch c & 0xC0 {
  1968  		case 0x00: // String segment
  1969  			if c == 0x00 {
  1970  				// A zero length signals the end of the name.
  1971  				break Loop
  1972  			}
  1973  			endOff := currOff + c
  1974  			if endOff > len(msg) {
  1975  				return off, errCalcLen
  1976  			}
  1977  			name = append(name, msg[currOff:endOff]...)
  1978  			name = append(name, '.')
  1979  			currOff = endOff
  1980  		case 0xC0: // Pointer
  1981  			if !allowCompression {
  1982  				return off, errCompressedSRV
  1983  			}
  1984  			if currOff >= len(msg) {
  1985  				return off, errInvalidPtr
  1986  			}
  1987  			c1 := msg[currOff]
  1988  			currOff++
  1989  			if ptr == 0 {
  1990  				newOff = currOff
  1991  			}
  1992  			// Don't follow too many pointers, maybe there's a loop.
  1993  			if ptr++; ptr > 10 {
  1994  				return off, errTooManyPtr
  1995  			}
  1996  			currOff = (c^0xC0)<<8 | int(c1)
  1997  		default:
  1998  			// Prefixes 0x80 and 0x40 are reserved.
  1999  			return off, errReserved
  2000  		}
  2001  	}
  2002  	if len(name) == 0 {
  2003  		name = append(name, '.')
  2004  	}
  2005  	if len(name) > len(n.Data) {
  2006  		return off, errCalcLen
  2007  	}
  2008  	n.Length = uint8(len(name))
  2009  	if ptr == 0 {
  2010  		newOff = currOff
  2011  	}
  2012  	return newOff, nil
  2013  }
  2014  
  2015  func skipName(msg []byte, off int) (int, error) {
  2016  	// newOff is the offset where the next record will start. Pointers lead
  2017  	// to data that belongs to other names and thus doesn't count towards to
  2018  	// the usage of this name.
  2019  	newOff := off
  2020  
  2021  Loop:
  2022  	for {
  2023  		if newOff >= len(msg) {
  2024  			return off, errBaseLen
  2025  		}
  2026  		c := int(msg[newOff])
  2027  		newOff++
  2028  		switch c & 0xC0 {
  2029  		case 0x00:
  2030  			if c == 0x00 {
  2031  				// A zero length signals the end of the name.
  2032  				break Loop
  2033  			}
  2034  			// literal string
  2035  			newOff += c
  2036  			if newOff > len(msg) {
  2037  				return off, errCalcLen
  2038  			}
  2039  		case 0xC0:
  2040  			// Pointer to somewhere else in msg.
  2041  
  2042  			// Pointers are two bytes.
  2043  			newOff++
  2044  
  2045  			// Don't follow the pointer as the data here has ended.
  2046  			break Loop
  2047  		default:
  2048  			// Prefixes 0x80 and 0x40 are reserved.
  2049  			return off, errReserved
  2050  		}
  2051  	}
  2052  
  2053  	return newOff, nil
  2054  }
  2055  
  2056  // A Question is a DNS query.
  2057  type Question struct {
  2058  	Name  Name
  2059  	Type  Type
  2060  	Class Class
  2061  }
  2062  
  2063  // pack appends the wire format of the Question to msg.
  2064  func (q *Question) pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error) {
  2065  	msg, err := q.Name.pack(msg, compression, compressionOff)
  2066  	if err != nil {
  2067  		return msg, &nestedError{"Name", err}
  2068  	}
  2069  	msg = packType(msg, q.Type)
  2070  	return packClass(msg, q.Class), nil
  2071  }
  2072  
  2073  // GoString implements fmt.GoStringer.GoString.
  2074  func (q *Question) GoString() string {
  2075  	return "dnsmessage.Question{" +
  2076  		"Name: " + q.Name.GoString() + ", " +
  2077  		"Type: " + q.Type.GoString() + ", " +
  2078  		"Class: " + q.Class.GoString() + "}"
  2079  }
  2080  
  2081  func unpackResourceBody(msg []byte, off int, hdr ResourceHeader) (ResourceBody, int, error) {
  2082  	var (
  2083  		r    ResourceBody
  2084  		err  error
  2085  		name string
  2086  	)
  2087  	switch hdr.Type {
  2088  	case TypeA:
  2089  		var rb AResource
  2090  		rb, err = unpackAResource(msg, off)
  2091  		r = &rb
  2092  		name = "A"
  2093  	case TypeNS:
  2094  		var rb NSResource
  2095  		rb, err = unpackNSResource(msg, off)
  2096  		r = &rb
  2097  		name = "NS"
  2098  	case TypeCNAME:
  2099  		var rb CNAMEResource
  2100  		rb, err = unpackCNAMEResource(msg, off)
  2101  		r = &rb
  2102  		name = "CNAME"
  2103  	case TypeSOA:
  2104  		var rb SOAResource
  2105  		rb, err = unpackSOAResource(msg, off)
  2106  		r = &rb
  2107  		name = "SOA"
  2108  	case TypePTR:
  2109  		var rb PTRResource
  2110  		rb, err = unpackPTRResource(msg, off)
  2111  		r = &rb
  2112  		name = "PTR"
  2113  	case TypeMX:
  2114  		var rb MXResource
  2115  		rb, err = unpackMXResource(msg, off)
  2116  		r = &rb
  2117  		name = "MX"
  2118  	case TypeTXT:
  2119  		var rb TXTResource
  2120  		rb, err = unpackTXTResource(msg, off, hdr.Length)
  2121  		r = &rb
  2122  		name = "TXT"
  2123  	case TypeAAAA:
  2124  		var rb AAAAResource
  2125  		rb, err = unpackAAAAResource(msg, off)
  2126  		r = &rb
  2127  		name = "AAAA"
  2128  	case TypeSRV:
  2129  		var rb SRVResource
  2130  		rb, err = unpackSRVResource(msg, off)
  2131  		r = &rb
  2132  		name = "SRV"
  2133  	case TypeOPT:
  2134  		var rb OPTResource
  2135  		rb, err = unpackOPTResource(msg, off, hdr.Length)
  2136  		r = &rb
  2137  		name = "OPT"
  2138  	}
  2139  	if err != nil {
  2140  		return nil, off, &nestedError{name + " record", err}
  2141  	}
  2142  	if r == nil {
  2143  		return nil, off, errors.New("invalid resource type: " + hdr.Type.String())
  2144  	}
  2145  	return r, off + int(hdr.Length), nil
  2146  }
  2147  
  2148  // A CNAMEResource is a CNAME Resource record.
  2149  type CNAMEResource struct {
  2150  	CNAME Name
  2151  }
  2152  
  2153  func (r *CNAMEResource) realType() Type {
  2154  	return TypeCNAME
  2155  }
  2156  
  2157  // pack appends the wire format of the CNAMEResource to msg.
  2158  func (r *CNAMEResource) pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error) {
  2159  	return r.CNAME.pack(msg, compression, compressionOff)
  2160  }
  2161  
  2162  // GoString implements fmt.GoStringer.GoString.
  2163  func (r *CNAMEResource) GoString() string {
  2164  	return "dnsmessage.CNAMEResource{CNAME: " + r.CNAME.GoString() + "}"
  2165  }
  2166  
  2167  func unpackCNAMEResource(msg []byte, off int) (CNAMEResource, error) {
  2168  	var cname Name
  2169  	if _, err := cname.unpack(msg, off); err != nil {
  2170  		return CNAMEResource{}, err
  2171  	}
  2172  	return CNAMEResource{cname}, nil
  2173  }
  2174  
  2175  // An MXResource is an MX Resource record.
  2176  type MXResource struct {
  2177  	Pref uint16
  2178  	MX   Name
  2179  }
  2180  
  2181  func (r *MXResource) realType() Type {
  2182  	return TypeMX
  2183  }
  2184  
  2185  // pack appends the wire format of the MXResource to msg.
  2186  func (r *MXResource) pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error) {
  2187  	oldMsg := msg
  2188  	msg = packUint16(msg, r.Pref)
  2189  	msg, err := r.MX.pack(msg, compression, compressionOff)
  2190  	if err != nil {
  2191  		return oldMsg, &nestedError{"MXResource.MX", err}
  2192  	}
  2193  	return msg, nil
  2194  }
  2195  
  2196  // GoString implements fmt.GoStringer.GoString.
  2197  func (r *MXResource) GoString() string {
  2198  	return "dnsmessage.MXResource{" +
  2199  		"Pref: " + printUint16(r.Pref) + ", " +
  2200  		"MX: " + r.MX.GoString() + "}"
  2201  }
  2202  
  2203  func unpackMXResource(msg []byte, off int) (MXResource, error) {
  2204  	pref, off, err := unpackUint16(msg, off)
  2205  	if err != nil {
  2206  		return MXResource{}, &nestedError{"Pref", err}
  2207  	}
  2208  	var mx Name
  2209  	if _, err := mx.unpack(msg, off); err != nil {
  2210  		return MXResource{}, &nestedError{"MX", err}
  2211  	}
  2212  	return MXResource{pref, mx}, nil
  2213  }
  2214  
  2215  // An NSResource is an NS Resource record.
  2216  type NSResource struct {
  2217  	NS Name
  2218  }
  2219  
  2220  func (r *NSResource) realType() Type {
  2221  	return TypeNS
  2222  }
  2223  
  2224  // pack appends the wire format of the NSResource to msg.
  2225  func (r *NSResource) pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error) {
  2226  	return r.NS.pack(msg, compression, compressionOff)
  2227  }
  2228  
  2229  // GoString implements fmt.GoStringer.GoString.
  2230  func (r *NSResource) GoString() string {
  2231  	return "dnsmessage.NSResource{NS: " + r.NS.GoString() + "}"
  2232  }
  2233  
  2234  func unpackNSResource(msg []byte, off int) (NSResource, error) {
  2235  	var ns Name
  2236  	if _, err := ns.unpack(msg, off); err != nil {
  2237  		return NSResource{}, err
  2238  	}
  2239  	return NSResource{ns}, nil
  2240  }
  2241  
  2242  // A PTRResource is a PTR Resource record.
  2243  type PTRResource struct {
  2244  	PTR Name
  2245  }
  2246  
  2247  func (r *PTRResource) realType() Type {
  2248  	return TypePTR
  2249  }
  2250  
  2251  // pack appends the wire format of the PTRResource to msg.
  2252  func (r *PTRResource) pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error) {
  2253  	return r.PTR.pack(msg, compression, compressionOff)
  2254  }
  2255  
  2256  // GoString implements fmt.GoStringer.GoString.
  2257  func (r *PTRResource) GoString() string {
  2258  	return "dnsmessage.PTRResource{PTR: " + r.PTR.GoString() + "}"
  2259  }
  2260  
  2261  func unpackPTRResource(msg []byte, off int) (PTRResource, error) {
  2262  	var ptr Name
  2263  	if _, err := ptr.unpack(msg, off); err != nil {
  2264  		return PTRResource{}, err
  2265  	}
  2266  	return PTRResource{ptr}, nil
  2267  }
  2268  
  2269  // An SOAResource is an SOA Resource record.
  2270  type SOAResource struct {
  2271  	NS      Name
  2272  	MBox    Name
  2273  	Serial  uint32
  2274  	Refresh uint32
  2275  	Retry   uint32
  2276  	Expire  uint32
  2277  
  2278  	// MinTTL the is the default TTL of Resources records which did not
  2279  	// contain a TTL value and the TTL of negative responses. (RFC 2308
  2280  	// Section 4)
  2281  	MinTTL uint32
  2282  }
  2283  
  2284  func (r *SOAResource) realType() Type {
  2285  	return TypeSOA
  2286  }
  2287  
  2288  // pack appends the wire format of the SOAResource to msg.
  2289  func (r *SOAResource) pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error) {
  2290  	oldMsg := msg
  2291  	msg, err := r.NS.pack(msg, compression, compressionOff)
  2292  	if err != nil {
  2293  		return oldMsg, &nestedError{"SOAResource.NS", err}
  2294  	}
  2295  	msg, err = r.MBox.pack(msg, compression, compressionOff)
  2296  	if err != nil {
  2297  		return oldMsg, &nestedError{"SOAResource.MBox", err}
  2298  	}
  2299  	msg = packUint32(msg, r.Serial)
  2300  	msg = packUint32(msg, r.Refresh)
  2301  	msg = packUint32(msg, r.Retry)
  2302  	msg = packUint32(msg, r.Expire)
  2303  	return packUint32(msg, r.MinTTL), nil
  2304  }
  2305  
  2306  // GoString implements fmt.GoStringer.GoString.
  2307  func (r *SOAResource) GoString() string {
  2308  	return "dnsmessage.SOAResource{" +
  2309  		"NS: " + r.NS.GoString() + ", " +
  2310  		"MBox: " + r.MBox.GoString() + ", " +
  2311  		"Serial: " + printUint32(r.Serial) + ", " +
  2312  		"Refresh: " + printUint32(r.Refresh) + ", " +
  2313  		"Retry: " + printUint32(r.Retry) + ", " +
  2314  		"Expire: " + printUint32(r.Expire) + ", " +
  2315  		"MinTTL: " + printUint32(r.MinTTL) + "}"
  2316  }
  2317  
  2318  func unpackSOAResource(msg []byte, off int) (SOAResource, error) {
  2319  	var ns Name
  2320  	off, err := ns.unpack(msg, off)
  2321  	if err != nil {
  2322  		return SOAResource{}, &nestedError{"NS", err}
  2323  	}
  2324  	var mbox Name
  2325  	if off, err = mbox.unpack(msg, off); err != nil {
  2326  		return SOAResource{}, &nestedError{"MBox", err}
  2327  	}
  2328  	serial, off, err := unpackUint32(msg, off)
  2329  	if err != nil {
  2330  		return SOAResource{}, &nestedError{"Serial", err}
  2331  	}
  2332  	refresh, off, err := unpackUint32(msg, off)
  2333  	if err != nil {
  2334  		return SOAResource{}, &nestedError{"Refresh", err}
  2335  	}
  2336  	retry, off, err := unpackUint32(msg, off)
  2337  	if err != nil {
  2338  		return SOAResource{}, &nestedError{"Retry", err}
  2339  	}
  2340  	expire, off, err := unpackUint32(msg, off)
  2341  	if err != nil {
  2342  		return SOAResource{}, &nestedError{"Expire", err}
  2343  	}
  2344  	minTTL, _, err := unpackUint32(msg, off)
  2345  	if err != nil {
  2346  		return SOAResource{}, &nestedError{"MinTTL", err}
  2347  	}
  2348  	return SOAResource{ns, mbox, serial, refresh, retry, expire, minTTL}, nil
  2349  }
  2350  
  2351  // A TXTResource is a TXT Resource record.
  2352  type TXTResource struct {
  2353  	TXT []string
  2354  }
  2355  
  2356  func (r *TXTResource) realType() Type {
  2357  	return TypeTXT
  2358  }
  2359  
  2360  // pack appends the wire format of the TXTResource to msg.
  2361  func (r *TXTResource) pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error) {
  2362  	oldMsg := msg
  2363  	for _, s := range r.TXT {
  2364  		var err error
  2365  		msg, err = packText(msg, s)
  2366  		if err != nil {
  2367  			return oldMsg, err
  2368  		}
  2369  	}
  2370  	return msg, nil
  2371  }
  2372  
  2373  // GoString implements fmt.GoStringer.GoString.
  2374  func (r *TXTResource) GoString() string {
  2375  	s := "dnsmessage.TXTResource{TXT: []string{"
  2376  	if len(r.TXT) == 0 {
  2377  		return s + "}}"
  2378  	}
  2379  	s += `"` + printString([]byte(r.TXT[0]))
  2380  	for _, t := range r.TXT[1:] {
  2381  		s += `", "` + printString([]byte(t))
  2382  	}
  2383  	return s + `"}}`
  2384  }
  2385  
  2386  func unpackTXTResource(msg []byte, off int, length uint16) (TXTResource, error) {
  2387  	txts := make([]string, 0, 1)
  2388  	for n := uint16(0); n < length; {
  2389  		var t string
  2390  		var err error
  2391  		if t, off, err = unpackText(msg, off); err != nil {
  2392  			return TXTResource{}, &nestedError{"text", err}
  2393  		}
  2394  		// Check if we got too many bytes.
  2395  		if length-n < uint16(len(t))+1 {
  2396  			return TXTResource{}, errCalcLen
  2397  		}
  2398  		n += uint16(len(t)) + 1
  2399  		txts = append(txts, t)
  2400  	}
  2401  	return TXTResource{txts}, nil
  2402  }
  2403  
  2404  // An SRVResource is an SRV Resource record.
  2405  type SRVResource struct {
  2406  	Priority uint16
  2407  	Weight   uint16
  2408  	Port     uint16
  2409  	Target   Name // Not compressed as per RFC 2782.
  2410  }
  2411  
  2412  func (r *SRVResource) realType() Type {
  2413  	return TypeSRV
  2414  }
  2415  
  2416  // pack appends the wire format of the SRVResource to msg.
  2417  func (r *SRVResource) pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error) {
  2418  	oldMsg := msg
  2419  	msg = packUint16(msg, r.Priority)
  2420  	msg = packUint16(msg, r.Weight)
  2421  	msg = packUint16(msg, r.Port)
  2422  	msg, err := r.Target.pack(msg, nil, compressionOff)
  2423  	if err != nil {
  2424  		return oldMsg, &nestedError{"SRVResource.Target", err}
  2425  	}
  2426  	return msg, nil
  2427  }
  2428  
  2429  // GoString implements fmt.GoStringer.GoString.
  2430  func (r *SRVResource) GoString() string {
  2431  	return "dnsmessage.SRVResource{" +
  2432  		"Priority: " + printUint16(r.Priority) + ", " +
  2433  		"Weight: " + printUint16(r.Weight) + ", " +
  2434  		"Port: " + printUint16(r.Port) + ", " +
  2435  		"Target: " + r.Target.GoString() + "}"
  2436  }
  2437  
  2438  func unpackSRVResource(msg []byte, off int) (SRVResource, error) {
  2439  	priority, off, err := unpackUint16(msg, off)
  2440  	if err != nil {
  2441  		return SRVResource{}, &nestedError{"Priority", err}
  2442  	}
  2443  	weight, off, err := unpackUint16(msg, off)
  2444  	if err != nil {
  2445  		return SRVResource{}, &nestedError{"Weight", err}
  2446  	}
  2447  	port, off, err := unpackUint16(msg, off)
  2448  	if err != nil {
  2449  		return SRVResource{}, &nestedError{"Port", err}
  2450  	}
  2451  	var target Name
  2452  	if _, err := target.unpackCompressed(msg, off, false /* allowCompression */); err != nil {
  2453  		return SRVResource{}, &nestedError{"Target", err}
  2454  	}
  2455  	return SRVResource{priority, weight, port, target}, nil
  2456  }
  2457  
  2458  // An AResource is an A Resource record.
  2459  type AResource struct {
  2460  	A [4]byte
  2461  }
  2462  
  2463  func (r *AResource) realType() Type {
  2464  	return TypeA
  2465  }
  2466  
  2467  // pack appends the wire format of the AResource to msg.
  2468  func (r *AResource) pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error) {
  2469  	return packBytes(msg, r.A[:]), nil
  2470  }
  2471  
  2472  // GoString implements fmt.GoStringer.GoString.
  2473  func (r *AResource) GoString() string {
  2474  	return "dnsmessage.AResource{" +
  2475  		"A: [4]byte{" + printByteSlice(r.A[:]) + "}}"
  2476  }
  2477  
  2478  func unpackAResource(msg []byte, off int) (AResource, error) {
  2479  	var a [4]byte
  2480  	if _, err := unpackBytes(msg, off, a[:]); err != nil {
  2481  		return AResource{}, err
  2482  	}
  2483  	return AResource{a}, nil
  2484  }
  2485  
  2486  // An AAAAResource is an AAAA Resource record.
  2487  type AAAAResource struct {
  2488  	AAAA [16]byte
  2489  }
  2490  
  2491  func (r *AAAAResource) realType() Type {
  2492  	return TypeAAAA
  2493  }
  2494  
  2495  // GoString implements fmt.GoStringer.GoString.
  2496  func (r *AAAAResource) GoString() string {
  2497  	return "dnsmessage.AAAAResource{" +
  2498  		"AAAA: [16]byte{" + printByteSlice(r.AAAA[:]) + "}}"
  2499  }
  2500  
  2501  // pack appends the wire format of the AAAAResource to msg.
  2502  func (r *AAAAResource) pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error) {
  2503  	return packBytes(msg, r.AAAA[:]), nil
  2504  }
  2505  
  2506  func unpackAAAAResource(msg []byte, off int) (AAAAResource, error) {
  2507  	var aaaa [16]byte
  2508  	if _, err := unpackBytes(msg, off, aaaa[:]); err != nil {
  2509  		return AAAAResource{}, err
  2510  	}
  2511  	return AAAAResource{aaaa}, nil
  2512  }
  2513  
  2514  // An OPTResource is an OPT pseudo Resource record.
  2515  //
  2516  // The pseudo resource record is part of the extension mechanisms for DNS
  2517  // as defined in RFC 6891.
  2518  type OPTResource struct {
  2519  	Options []Option
  2520  }
  2521  
  2522  // An Option represents a DNS message option within OPTResource.
  2523  //
  2524  // The message option is part of the extension mechanisms for DNS as
  2525  // defined in RFC 6891.
  2526  type Option struct {
  2527  	Code uint16 // option code
  2528  	Data []byte
  2529  }
  2530  
  2531  // GoString implements fmt.GoStringer.GoString.
  2532  func (o *Option) GoString() string {
  2533  	return "dnsmessage.Option{" +
  2534  		"Code: " + printUint16(o.Code) + ", " +
  2535  		"Data: []byte{" + printByteSlice(o.Data) + "}}"
  2536  }
  2537  
  2538  func (r *OPTResource) realType() Type {
  2539  	return TypeOPT
  2540  }
  2541  
  2542  func (r *OPTResource) pack(msg []byte, compression map[string]int, compressionOff int) ([]byte, error) {
  2543  	for _, opt := range r.Options {
  2544  		msg = packUint16(msg, opt.Code)
  2545  		l := uint16(len(opt.Data))
  2546  		msg = packUint16(msg, l)
  2547  		msg = packBytes(msg, opt.Data)
  2548  	}
  2549  	return msg, nil
  2550  }
  2551  
  2552  // GoString implements fmt.GoStringer.GoString.
  2553  func (r *OPTResource) GoString() string {
  2554  	s := "dnsmessage.OPTResource{Options: []dnsmessage.Option{"
  2555  	if len(r.Options) == 0 {
  2556  		return s + "}}"
  2557  	}
  2558  	s += r.Options[0].GoString()
  2559  	for _, o := range r.Options[1:] {
  2560  		s += ", " + o.GoString()
  2561  	}
  2562  	return s + "}}"
  2563  }
  2564  
  2565  func unpackOPTResource(msg []byte, off int, length uint16) (OPTResource, error) {
  2566  	var opts []Option
  2567  	for oldOff := off; off < oldOff+int(length); {
  2568  		var err error
  2569  		var o Option
  2570  		o.Code, off, err = unpackUint16(msg, off)
  2571  		if err != nil {
  2572  			return OPTResource{}, &nestedError{"Code", err}
  2573  		}
  2574  		var l uint16
  2575  		l, off, err = unpackUint16(msg, off)
  2576  		if err != nil {
  2577  			return OPTResource{}, &nestedError{"Data", err}
  2578  		}
  2579  		o.Data = make([]byte, l)
  2580  		if copy(o.Data, msg[off:]) != int(l) {
  2581  			return OPTResource{}, &nestedError{"Data", errCalcLen}
  2582  		}
  2583  		off += int(l)
  2584  		opts = append(opts, o)
  2585  	}
  2586  	return OPTResource{opts}, nil
  2587  }
  2588  

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