msg.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This is not (yet) optimized for speed.

// DNS packet assembly, see RFC 1035. Converting from - Unpack() -
// and to - Pack() - wire format.
// All the packers and unpackers take a (msg []byte, off int)
// and return (off1 int, ok bool).  If they return ok==false, they
// also return off1==len(msg), so that the next unpacker will
// also fail.  This lets us avoid checks of ok until the end of a
// packing sequence.

package dns

import (
	"os"
	//        "fmt"
	"reflect"
	"net"
	"rand"
	"time"
	"strconv"
	"encoding/base64"
	"encoding/base32"
	"encoding/hex"
)

var (
	ErrUnpack    os.Error = &Error{Error: "unpacking failed"}
	ErrPack      os.Error = &Error{Error: "packing failed"}
	ErrId        os.Error = &Error{Error: "id mismatch"}
	ErrShortRead os.Error = &Error{Error: "short read"}
	ErrConn      os.Error = &Error{Error: "conn holds both UDP and TCP connection"}
	ErrConnEmpy  os.Error = &Error{Error: "conn has no connection"}
	ErrServ      os.Error = &Error{Error: "no servers could be reached"}
	ErrKey       os.Error = &Error{Error: "bad key"}
	ErrPrivKey   os.Error = &Error{Error: "bad private key"}
	ErrKeySize   os.Error = &Error{Error: "bad key size"}
	ErrAlg       os.Error = &Error{Error: "bad algorithm"}
	ErrTime      os.Error = &Error{Error: "bad time"}
	ErrNoSig     os.Error = &Error{Error: "no signature found"}
	ErrSig       os.Error = &Error{Error: "bad signature"}
	ErrSigGen    os.Error = &Error{Error: "bad signature generation"}
	ErrAuth      os.Error = &Error{Error: "bad authentication"}
	ErrXfrSoa    os.Error = &Error{Error: "no SOA seen"}
	ErrHandle    os.Error = &Error{Error: "handle is nil"}
	ErrChan      os.Error = &Error{Error: "channel is nil"}
)

// A manually-unpacked version of (id, bits).
// This is in its own struct for easy printing.
type MsgHdr struct {
	Id                 uint16
	Response           bool
	Opcode             int
	Authoritative      bool
	Truncated          bool
	RecursionDesired   bool
	RecursionAvailable bool
	Zero               bool
	AuthenticatedData  bool
	CheckingDisabled   bool
	Rcode              int
}

// The layout of a DNS message.
type Msg struct {
	MsgHdr
	Question []Question
	Answer   []RR
	Ns       []RR
	Extra    []RR
}

// Map of strings for each RR wire type.
var Rr_str = map[uint16]string{
	TypeCNAME:      "CNAME",
	TypeHINFO:      "HINFO",
	TypeMB:         "MB",
	TypeMG:         "MG",
	TypeMINFO:      "MINFO",
	TypeMR:         "MR",
	TypeMX:         "MX",
	TypeNS:         "NS",
	TypePTR:        "PTR",
	TypeSOA:        "SOA",
	TypeTXT:        "TXT",
	TypeSRV:        "SRV",
	TypeNAPTR:      "NAPTR",
	TypeKX:         "KX",
	TypeCERT:       "CERT",
	TypeDNAME:      "DNAME",
	TypeA:          "A",
	TypeAAAA:       "AAAA",
	TypeLOC:        "LOC",
	TypeOPT:        "OPT",
	TypeDS:         "DS",
	TypeDHCID:      "DHCID",
	TypeIPSECKEY:   "IPSECKEY",
	TypeSSHFP:      "SSHFP",
	TypeRRSIG:      "RRSIG",
	TypeNSEC:       "NSEC",
	TypeDNSKEY:     "DNSKEY",
	TypeNSEC3:      "NSEC3",
	TypeNSEC3PARAM: "NSEC3PARAM",
	TypeTALINK:     "TALINK",
	TypeSPF:        "SPF",
	TypeTKEY:       "TKEY", // Meta RR
	TypeTSIG:       "TSIG", // Meta RR
	TypeAXFR:       "AXFR", // Meta RR
	TypeIXFR:       "IXFR", // Meta RR
	TypeALL:        "ANY",  // Meta RR
	TypeURI:        "URI",
	TypeTA:         "TA",
	TypeDLV:        "DLV",
}

// Reverse of Rr_str (needed for string parsing).
var Str_rr = reverse(Rr_str)

// Map of strings for each CLASS wire type.
var Class_str = map[uint16]string{
	ClassINET:   "IN",
	ClassCSNET:  "CS",
	ClassCHAOS:  "CH",
	ClassHESIOD: "HS",
	ClassANY:    "ANY",
}

// Map of strings for opcodes.
var opcode_str = map[int]string{
	OpcodeQuery:  "QUERY",
	OpcodeIQuery: "IQUERY",
	OpcodeStatus: "STATUS",
	OpcodeNotify: "NOTIFY",
	OpcodeUpdate: "UPDATE",
}

// Map of strings for rcodes.
var rcode_str = map[int]string{
	RcodeSuccess:        "NOERROR",
	RcodeFormatError:    "FORMERR",
	RcodeServerFailure:  "SERVFAIL",
	RcodeNameError:      "NXDOMAIN",
	RcodeNotImplemented: "NOTIMPL",
	RcodeRefused:        "REFUSED",
	RcodeYXDomain:       "YXDOMAIN", // From RFC 2136
	RcodeYXRrset:        "YXRRSET",
	RcodeNXRrset:        "NXRRSET",
	RcodeNotAuth:        "NOTAUTH",
	RcodeNotZone:        "NOTZONE",
}

// Rather than write the usual handful of routines to pack and
// unpack every message that can appear on the wire, we use
// reflection to write a generic pack/unpack for structs and then
// use it. Thus, if in the future we need to define new message
// structs, no new pack/unpack/printing code needs to be written.

// Pack a domain name s into msg[off:].
// Domain names are a sequence of counted strings
// split at the dots.  They end with a zero-length string.
func packDomainName(s string, msg []byte, off int) (off1 int, ok bool) {
	// Add trailing dot to canonicalize name.
	if n := len(s); n == 0 || s[n-1] != '.' {
		s += "."
	}

	// Each dot ends a segment of the name.
	// We trade each dot byte for a length byte.
	// Except for escaped dots (\.), which are normal dots.
	// There is also a trailing zero.
	// Check that we have all the space we need.
	tot := len(s) + 1
	if off+tot > len(msg) {
		return len(msg), false
	}

	// Emit sequence of counted strings, chopping at dots.
	begin := 0
	bs := []byte(s)
	ls := len(bs)
	for i := 0; i < ls; i++ {
		if bs[i] == '\\' {
			for j := i; j < len(s)-1; j++ {
				bs[j] = bs[j+1]
			}
			ls--
			continue
		}

		if bs[i] == '.' {
			if i-begin >= 1<<6 { // top two bits of length must be clear
				return len(msg), false
			}
			msg[off] = byte(i - begin)
			off++
			for j := begin; j < i; j++ {
				msg[off] = bs[j]
				off++
			}
			begin = i + 1
		}
	}
	// Root label is special
	if string(bs) == "." {
		return off, true
	}
	msg[off] = 0
	off++
	return off, true
}

// Unpack a domain name.
// In addition to the simple sequences of counted strings above,
// domain names are allowed to refer to strings elsewhere in the
// packet, to avoid repeating common suffixes when returning
// many entries in a single domain.  The pointers are marked
// by a length byte with the top two bits set.  Ignoring those
// two bits, that byte and the next give a 14 bit offset from msg[0]
// where we should pick up the trail.
// Note that if we jump elsewhere in the packet,
// we return off1 == the offset after the first pointer we found,
// which is where the next record will start.
// In theory, the pointers are only allowed to jump backward.
// We let them jump anywhere and stop jumping after a while.
func unpackDomainName(msg []byte, off int) (s string, off1 int, ok bool) {
	s = ""
	ptr := 0 // number of pointers followed
Loop:
	for {
		if off >= len(msg) {
			return "", len(msg), false
		}
		c := int(msg[off])
		off++
		switch c & 0xC0 {
		case 0x00:
			if c == 0x00 {
				// end of name
				break Loop
			}
			// literal string
			if off+c > len(msg) {
				return "", len(msg), false
			}
			for j := off; j < off+c; j++ {
				if msg[j] == '.' {
					// literal dot, escape it
					s += "\\."
				} else {
					s += string(msg[j])
				}
			}
			s += "."
			off += c
		case 0xC0:
			// pointer to somewhere else in msg.
			// remember location after first ptr,
			// since that's how many bytes we consumed.
			// also, don't follow too many pointers --
			// maybe there's a loop.
			if off >= len(msg) {
				return "", len(msg), false
			}
			c1 := msg[off]
			off++
			if ptr == 0 {
				off1 = off
			}
			if ptr++; ptr > 10 {
				return "", len(msg), false
			}
			off = (c^0xC0)<<8 | int(c1)
		default:
			// 0x80 and 0x40 are reserved
			return "", len(msg), false
		}
	}
	if ptr == 0 {
		off1 = off
	}
	return s, off1, true
}

// Pack a reflect.StructValue into msg.  Struct members can only be uint8, uint16, uint32, string,
// slices and other (often anonymous) structs.
func packStructValue(val reflect.Value, msg []byte, off int) (off1 int, ok bool) {
	for i := 0; i < val.NumField(); i++ {
		f := val.Type().Field(i)
		switch fv := val.Field(i); fv.Kind() {
		default:
		BadType:
			//fmt.Fprintf(os.Stderr, "dns: unknown packing type %v\n", f.Type)
			return len(msg), false
		case reflect.Slice:
			switch f.Tag {
			default:
				//fmt.Fprintf(os.Stderr, "dns: unknown packing slice tag %v\n", f.Tag)
				return len(msg), false
			case "OPT": // edns
				for j := 0; j < val.Field(i).Len(); j++ {
					element := val.Field(i).Index(j)
					code := uint16(element.Field(0).Uint())
					// for each code we should do something else
					h, e := hex.DecodeString(string(element.Field(1).String()))
					if e != nil {
						//fmt.Fprintf(os.Stderr, "dns: failure packing OTP")
						return len(msg), false
					}
					data := string(h)
					// Option Code
					msg[off] = byte(code >> 8)
					msg[off+1] = byte(code)
					// Length
					msg[off+2] = byte(len(data) >> 8)
					msg[off+3] = byte(len(data))
					off += 4
					copy(msg[off:off+len(data)], []byte(data))
					off += len(data)
				}
			case "A":
				// It must be a slice of 4, even if it is 16, we encode
				// only the first 4
				if off+net.IPv4len > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow packing A")
					return len(msg), false
				}
				if fv.Len() == net.IPv6len {
					msg[off] = byte(fv.Index(12).Uint())
					msg[off+1] = byte(fv.Index(13).Uint())
					msg[off+2] = byte(fv.Index(14).Uint())
					msg[off+3] = byte(fv.Index(15).Uint())
				} else {
					msg[off] = byte(fv.Index(0).Uint())
					msg[off+1] = byte(fv.Index(1).Uint())
					msg[off+2] = byte(fv.Index(2).Uint())
					msg[off+3] = byte(fv.Index(3).Uint())
				}
				off += net.IPv4len
			case "AAAA":
				if fv.Len() > net.IPv6len || off+fv.Len() > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow packing AAAA")
					return len(msg), false
				}
				for j := 0; j < net.IPv6len; j++ {
					msg[off] = byte(fv.Index(j).Uint())
					off++
				}
			case "NSEC": // NSEC/NSEC3
				for j := 0; j < val.Field(i).Len(); j++ {
					var _ = byte(fv.Index(j).Uint())
				}
				// handle type bit maps
				// TODO(mg)
			}
		case reflect.Struct:
			off, ok = packStructValue(fv, msg, off)
		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
			i := fv.Uint()
			switch fv.Type().Kind() {
			default:
				goto BadType
			case reflect.Uint8:
				if off+1 > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow packing uint8")
					return len(msg), false
				}
				msg[off] = byte(i)
				off++
			case reflect.Uint16:
				if off+2 > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow packing uint16")
					return len(msg), false
				}
				msg[off] = byte(i >> 8)
				msg[off+1] = byte(i)
				off += 2
			case reflect.Uint32:
				if off+4 > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow packing uint32")
					return len(msg), false
				}
				msg[off] = byte(i >> 24)
				msg[off+1] = byte(i >> 16)
				msg[off+2] = byte(i >> 8)
				msg[off+3] = byte(i)
				off += 4
			case reflect.Uint64:
				// Only used in TSIG, where it stops at 48 bits, so we discard the upper 16
				if off+6 > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow packing uint64")
					return len(msg), false
				}
				msg[off] = byte(i >> 40)
				msg[off+1] = byte(i >> 32)
				msg[off+2] = byte(i >> 24)
				msg[off+3] = byte(i >> 16)
				msg[off+4] = byte(i >> 8)
				msg[off+5] = byte(i)
				off += 6
			}
		case reflect.String:
			// There are multiple string encodings.
			// The tag distinguishes ordinary strings from domain names.
			s := fv.String()
			switch f.Tag {
			default:
				//fmt.Fprintf(os.Stderr, "dns: unknown packing string tag %v", f.Tag)
				return len(msg), false
			case "base32":
				b32, err := packBase32([]byte(s))
				if err != nil {
					//fmt.Fprintf(os.Stderr, "dns: overflow packing base32")
					return len(msg), false
				}
				copy(msg[off:off+len(b32)], b32)
				off += len(b32)
			case "base64":
				b64, err := packBase64([]byte(s))
				if err != nil {
					//fmt.Fprintf(os.Stderr, "dns: overflow packing base64")
					return len(msg), false
				}
				copy(msg[off:off+len(b64)], b64)
				off += len(b64)
			case "domain-name":
				off, ok = packDomainName(s, msg, off)
				if !ok {
					//fmt.Fprintf(os.Stderr, "dns: overflow packing domain-name")
					return len(msg), false
				}
			case "size-hex":
				fallthrough
			case "hex":
				// There is no length encoded here
				h, e := hex.DecodeString(s)
				if e != nil {
					//fmt.Fprintf(os.Stderr, "dns: overflow packing (size-)hex string")
					return len(msg), false
				}
				copy(msg[off:off+hex.DecodedLen(len(s))], h)
				off += hex.DecodedLen(len(s))
			case "size":
				// the size is already encoded in the RR, we can safely use the 
				// length of string. String is RAW (not encoded in hex, nor base64)
				copy(msg[off:off+len(s)], s)
				off += len(s)
			case "txt":
				// Counted string: 1 byte length, but the string may be longer
				// than 255, in that case it should be multiple strings, for now:
				fallthrough
			case "":
				// Counted string: 1 byte length.
				if len(s) > 255 || off+1+len(s) > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow packing string")
					return len(msg), false
				}
				msg[off] = byte(len(s))
				off++
				for i := 0; i < len(s); i++ {
					msg[off+i] = s[i]
				}
				off += len(s)
			}
		}
	}
	return off, true
}

func structValue(any interface{}) reflect.Value {
	return reflect.ValueOf(any).Elem()
}

func packStruct(any interface{}, msg []byte, off int) (off1 int, ok bool) {
	off, ok = packStructValue(structValue(any), msg, off)
	return off, ok
}

// Unpack a reflect.StructValue from msg.
// Same restrictions as packStructValue.
func unpackStructValue(val reflect.Value, msg []byte, off int) (off1 int, ok bool) {
	for i := 0; i < val.NumField(); i++ {
		f := val.Type().Field(i)
		switch fv := val.Field(i); fv.Kind() {
		default:
		BadType:
			//fmt.Fprintf(os.Stderr, "dns: unknown unpacking type %v", f.Type)
			return len(msg), false
		case reflect.Slice:
			switch f.Tag {
			default:
				//fmt.Fprintf(os.Stderr, "dns: unknown unpacking slice tag %v", f.Tag)
				return len(msg), false
			case "A":
				if off+net.IPv4len > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow unpacking A")
					return len(msg), false
				}
				b := net.IPv4(msg[off], msg[off+1], msg[off+2], msg[off+3])
				fv.Set(reflect.ValueOf(b))
				off += net.IPv4len
			case "AAAA":
				if off+net.IPv6len > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow unpacking AAAA")
					return len(msg), false
				}
				p := make(net.IP, net.IPv6len)
				copy(p, msg[off:off+net.IPv6len])
				b := net.IP(p)
				fv.Set(reflect.ValueOf(b))
				off += net.IPv6len
			case "OPT": // EDNS
				if off+2 > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow unpacking OPT")
					// No room for anything else
					break
				}
				opt := make([]Option, 1)
				opt[0].Code, off = unpackUint16(msg, off)
				optlen, off1 := unpackUint16(msg, off)
				if off1+int(optlen) > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow unpacking OPT")
					return len(msg), false
				}
				opt[0].Data = hex.EncodeToString(msg[off1 : off1+int(optlen)])
				fv.Set(reflect.ValueOf(opt))
				off = off1 + int(optlen)
			case "NSEC": // NSEC/NSEC3
				if off+1 > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow unpacking NSEC")
					return len(msg), false
				}
				// Fix multple windows TODO(mg)
				nsec := make([]uint16, 256) // use append TODO(mg)
				ni := 0
				window := int(msg[off])
				blocks := int(msg[off+1])
				if off+blocks > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow unpacking NSEC")
					return len(msg), false
				}
				if blocks == 0 {
					// Nothing encoded in this window
					// Kinda lame to alloc above and to clear it here
					nsec = nsec[:ni]
					fv.Set(reflect.ValueOf(nsec))
					break
				}

				off += 2
				for j := 0; j < blocks; j++ {
					b := msg[off+j]
					// Check the bits one by one, and set the type
					if b&0x80 == 0x80 {
						nsec[ni] = uint16(window*256 + j*8 + 0)
						ni++
					}
					if b&0x40 == 0x40 {
						nsec[ni] = uint16(window*256 + j*8 + 1)
						ni++
					}
					if b&0x20 == 0x20 {
						nsec[ni] = uint16(window*256 + j*8 + 2)
						ni++
					}
					if b&0x10 == 0x10 {
						nsec[ni] = uint16(window*256 + j*8 + 3)
						ni++
					}
					if b&0x8 == 0x8 {
						nsec[ni] = uint16(window*256 + j*8 + 4)
						ni++
					}
					if b&0x4 == 0x4 {
						nsec[ni] = uint16(window*256 + j*8 + 5)
						ni++
					}
					if b&0x2 == 0x2 {
						nsec[ni] = uint16(window*256 + j*8 + 6)
						ni++
					}
					if b&0x1 == 0x1 {
						nsec[ni] = uint16(window*256 + j*8 + 7)
						ni++
					}
				}
				nsec = nsec[:ni]
				fv.Set(reflect.ValueOf(nsec))
				off += blocks
			}
		case reflect.Struct:
			off, ok = unpackStructValue(fv, msg, off)
		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
			switch fv.Type().Kind() {
			default:
				goto BadType
			case reflect.Uint8:
				if off+1 > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow unpacking uint8")
					return len(msg), false
				}
				i := uint8(msg[off])
				fv.SetUint(uint64(i))
				off++
			case reflect.Uint16:
				var i uint16
				if off+2 > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow unpacking uint16")
					return len(msg), false
				}
				i, off = unpackUint16(msg, off)
				fv.SetUint(uint64(i))
			case reflect.Uint32:
				if off+4 > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow unpacking uint32")
					return len(msg), false
				}
				i := uint32(msg[off])<<24 | uint32(msg[off+1])<<16 | uint32(msg[off+2])<<8 | uint32(msg[off+3])
				fv.SetUint(uint64(i))
				off += 4
			case reflect.Uint64:
				// This is *only* used in TSIG where the last 48 bits are occupied
				// So for now, assume a uint48 (6 bytes)
				if off+6 > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: overflow unpacking uint64")
					return len(msg), false
				}
				i := uint64(msg[off])<<40 | uint64(msg[off+1])<<32 | uint64(msg[off+2])<<24 | uint64(msg[off+3])<<16 |
					uint64(msg[off+4])<<8 | uint64(msg[off+5])
				fv.SetUint(uint64(i))
				off += 6
			}
		case reflect.String:
			var s string
			switch f.Tag {
			default:
				//fmt.Fprintf(os.Stderr, "dns: unknown unpacking string tag %v", f.Tag)
				return len(msg), false
			case "hex":
				// Rest of the RR is hex encoded, network order an issue here?
				rdlength := int(val.FieldByName("Hdr").FieldByName("Rdlength").Uint())
				var consumed int
				switch val.Type().Name() {
				case "RR_DS":
					consumed = 4 // KeyTag(2) + Algorithm(1) + DigestType(1)
				case "RR_SSHFP":
					consumed = 2 // Algorithm(1) + Type(1)
				case "RR_NSEC3PARAM":
					consumed = 5 // Hash(1) + Flags(1) + Iterations(2) + SaltLength(1)
				case "RR_RFC3597":
					fallthrough // Rest is the unknown data
				default:
					consumed = 0 // return len(msg), false?
				}
				s = hex.EncodeToString(msg[off : off+rdlength-consumed])
				off += rdlength - consumed
			case "base64":
				// Rest of the RR is base64 encoded value
				rdlength := int(val.FieldByName("Hdr").FieldByName("Rdlength").Uint())
				// Need to know how much of rdlength is already consumed, in this packet
				var consumed int
				switch val.Type().Name() {
				case "RR_DNSKEY":
					consumed = 4 // Flags(2) + Protocol(1) + Algorithm(1)
				case "RR_RRSIG":
					consumed = 18 // TypeCovered(2) + Algorithm(1) + Labels(1) +
					// OrigTTL(4) + SigExpir(4) + SigIncep(4) + KeyTag(2) + len(signername)
					// Should already be set in the sequence of parsing (comes before)
					// Work because of rfc4034, section 3.17
					consumed += len(val.FieldByName("SignerName").String()) + 1
				default:
					consumed = 0 // TODO
				}
				s = unpackBase64(msg[off : off+rdlength-consumed])
				off += rdlength - consumed
			case "domain-name":
				s, off, ok = unpackDomainName(msg, off)
				if !ok {
					//fmt.Fprintf(os.Stderr, "dns: failure unpacking domain-name")
					return len(msg), false
				}
			case "size-base32":
				var size int
				switch val.Type().Name() {
				case "RR_NSEC3":
					switch f.Name {
					case "NextDomain":
						name := val.FieldByName("HashLength")
						size = int(name.Uint())
					}
				}
				if off+size > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: failure unpacking size-base32 string")
					return len(msg), false
				}
				s = unpackBase32(msg[off : off+size])
				off += size
			case "size-hex":
				// a "size" string, but it must be encoded in hex in the string
				var size int
				switch val.Type().Name() {
				case "RR_NSEC3":
					switch f.Name {
					case "Salt":
						name := val.FieldByName("SaltLength")
						size = int(name.Uint())
					case "NextDomain":
						name := val.FieldByName("HashLength")
						size = int(name.Uint())
					}
				case "RR_TSIG":
					switch f.Name {
					case "MAC":
						name := val.FieldByName("MACSize")
						size = int(name.Uint())
					case "OtherData":
						name := val.FieldByName("OtherLen")
						size = int(name.Uint())
					}
				}
				if off+size > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: failure unpacking size-hex string")
					return len(msg), false
				}
				s = hex.EncodeToString(msg[off : off+size])
				off += size
			case "txt":
				// 1 or multiple txt pieces
				rdlength := int(val.FieldByName("Hdr").FieldByName("Rdlength").Uint())
			Txt:
				if off >= len(msg) || off+1+int(msg[off]) > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: failure unpacking txt string")
					return len(msg), false
				}
				n := int(msg[off])
				off++
				b := make([]byte, n)
				for i := 0; i < n; i++ {
					b[i] = msg[off+i]
				}
				off += n
				s += string(b)
				if off < rdlength {
					// More to come
					goto Txt
				}
			case "":
				if off >= len(msg) || off+1+int(msg[off]) > len(msg) {
					//fmt.Fprintf(os.Stderr, "dns: failure unpacking string")
					return len(msg), false
				}
				n := int(msg[off])
				off++
				b := make([]byte, n)
				for i := 0; i < n; i++ {
					b[i] = msg[off+i]
				}
				off += n
				s = string(b)
			}
			fv.SetString(s)
		}
	}
	return off, true
}

// Helper function for unpacking
func unpackUint16(msg []byte, off int) (v uint16, off1 int) {
	v = uint16(msg[off])<<8 | uint16(msg[off+1])
	off1 = off + 2
	return
}

func unpackStruct(any interface{}, msg []byte, off int) (off1 int, ok bool) {
	off, ok = unpackStructValue(structValue(any), msg, off)
	return off, ok
}

func unpackBase32(b []byte) string {
	b32 := make([]byte, base32.HexEncoding.EncodedLen(len(b)))
	base32.HexEncoding.Encode(b32, b)
	return string(b32)
}

func unpackBase64(b []byte) string {
	b64 := make([]byte, base64.StdEncoding.EncodedLen(len(b)))
	base64.StdEncoding.Encode(b64, b)
	return string(b64)
}

// Helper function for packing, mostly used in dnssec.go
func packUint16(i uint16) (byte, byte) {
	return byte(i >> 8), byte(i)
}

func packBase64(s []byte) ([]byte, os.Error) {
	b64len := base64.StdEncoding.DecodedLen(len(s))
	buf := make([]byte, b64len)
	n, err := base64.StdEncoding.Decode(buf, []byte(s))
	if err != nil {
		return nil, err
	}
	buf = buf[:n]
	return buf, nil
}

// Helper function for packing, mostly used in dnssec.go
func packBase32(s []byte) ([]byte, os.Error) {
	b32len := base32.HexEncoding.DecodedLen(len(s))
	buf := make([]byte, b32len)
	n, err := base32.HexEncoding.Decode(buf, []byte(s))
	if err != nil {
		return nil, err
	}
	buf = buf[:n]
	return buf, nil
}

// Resource record packer.
func packRR(rr RR, msg []byte, off int) (off2 int, ok bool) {
	if rr == nil {
		return len(msg), false
	}

	var off1 int
	// pack twice, once to find end of header
	// and again to find end of packet.
	// a bit inefficient but this doesn't need to be fast.
	// off1 is end of header
	// off2 is end of rr
	off1, ok = packStruct(rr.Header(), msg, off)
	off2, ok = packStruct(rr, msg, off)
	if !ok {
		return len(msg), false
	}

	// TODO make this quicker?
	// pack a third time; redo header with correct data length
	rr.Header().Rdlength = uint16(off2 - off1)
	packStruct(rr.Header(), msg, off)
	return off2, true
}

// Resource record unpacker.
func unpackRR(msg []byte, off int) (rr RR, off1 int, ok bool) {
	// unpack just the header, to find the rr type and length
	var h RR_Header
	off0 := off
	if off, ok = unpackStruct(&h, msg, off); !ok {
		return nil, len(msg), false
	}
	end := off + int(h.Rdlength)

	// make an rr of that type and re-unpack.
	// again inefficient but doesn't need to be fast.
	mk, known := rr_mk[int(h.Rrtype)]
	if !known {
		rr = new(RR_RFC3597)
	} else {
		rr = mk()
	}
	off, ok = unpackStruct(rr, msg, off0)
	if off != end {
		return &h, end, true
	}
	return rr, off, ok
}

// Reverse a map
func reverse(m map[uint16]string) map[string]uint16 {
	n := make(map[string]uint16)
	for u, s := range m {
		n[s] = u
	}
	return n
}

// Convert a MsgHdr to a string, mimic the way Dig displays headers:
//;; opcode: QUERY, status: NOERROR, id: 48404
//;; flags: qr aa rd ra;
func (h *MsgHdr) String() string {
	if h == nil {
		return "<nil> MsgHdr"
	}

	s := ";; opcode: " + opcode_str[h.Opcode]
	s += ", status: " + rcode_str[h.Rcode]
	s += ", id: " + strconv.Itoa(int(h.Id)) + "\n"

	s += ";; flags:"
	if h.Response {
		s += " qr"
	}
	if h.Authoritative {
		s += " aa"
	}
	if h.Truncated {
		s += " tc"
	}
	if h.RecursionDesired {
		s += " rd"
	}
	if h.RecursionAvailable {
		s += " ra"
	}
	if h.Zero { // Hmm
		s += " z"
	}
	if h.AuthenticatedData {
		s += " ad"
	}
	if h.CheckingDisabled {
		s += " cd"
	}

	s += ";"
	return s
}

// Pack a msg: convert it to wire format.
func (dns *Msg) Pack() (msg []byte, ok bool) {
	var dh Header

	// Convert convenient Msg into wire-like Header.
	dh.Id = dns.Id
	dh.Bits = uint16(dns.Opcode)<<11 | uint16(dns.Rcode)
	if dns.Response {
		dh.Bits |= _QR
	}
	if dns.Authoritative {
		dh.Bits |= _AA
	}
	if dns.Truncated {
		dh.Bits |= _TC
	}
	if dns.RecursionDesired {
		dh.Bits |= _RD
	}
	if dns.RecursionAvailable {
		dh.Bits |= _RA
	}
	if dns.Zero {
		dh.Bits |= _Z
	}
	if dns.AuthenticatedData {
		dh.Bits |= _AD
	}
	if dns.CheckingDisabled {
		dh.Bits |= _CD
	}

	// Prepare variable sized arrays.
	question := dns.Question
	answer := dns.Answer
	ns := dns.Ns
	extra := dns.Extra

	dh.Qdcount = uint16(len(question))
	dh.Ancount = uint16(len(answer))
	dh.Nscount = uint16(len(ns))
	dh.Arcount = uint16(len(extra))

	// Could work harder to calculate message size,
	// but this is far more than we need and not
	// big enough to hurt the allocator.
	msg = make([]byte, DefaultMsgSize) // TODO, calculate REAL size

	// Pack it in: header and then the pieces.
	off := 0
	off, ok = packStruct(&dh, msg, off)
	for i := 0; i < len(question); i++ {
		off, ok = packStruct(&question[i], msg, off)
	}
	for i := 0; i < len(answer); i++ {
		off, ok = packRR(answer[i], msg, off)
	}
	for i := 0; i < len(ns); i++ {
		off, ok = packRR(ns[i], msg, off)
	}
	for i := 0; i < len(extra); i++ {
		off, ok = packRR(extra[i], msg, off)
	}
	if !ok {
		return nil, false
	}
	return msg[:off], true
}

// Unpack a binary message to a Msg structure.
func (dns *Msg) Unpack(msg []byte) bool {
	// Header.
	var dh Header
	off := 0
	var ok bool
	if off, ok = unpackStruct(&dh, msg, off); !ok {
		return false
	}
	dns.Id = dh.Id
	dns.Response = (dh.Bits & _QR) != 0
	dns.Opcode = int(dh.Bits>>11) & 0xF
	dns.Authoritative = (dh.Bits & _AA) != 0
	dns.Truncated = (dh.Bits & _TC) != 0
	dns.RecursionDesired = (dh.Bits & _RD) != 0
	dns.RecursionAvailable = (dh.Bits & _RA) != 0
	dns.Rcode = int(dh.Bits & 0xF)

	// Arrays.
	dns.Question = make([]Question, dh.Qdcount)
	dns.Answer = make([]RR, dh.Ancount)
	dns.Ns = make([]RR, dh.Nscount)
	dns.Extra = make([]RR, dh.Arcount)

	for i := 0; i < len(dns.Question); i++ {
		off, ok = unpackStruct(&dns.Question[i], msg, off)
	}
	for i := 0; i < len(dns.Answer); i++ {
		dns.Answer[i], off, ok = unpackRR(msg, off)
	}
	for i := 0; i < len(dns.Ns); i++ {
		dns.Ns[i], off, ok = unpackRR(msg, off)
	}
	for i := 0; i < len(dns.Extra); i++ {
		dns.Extra[i], off, ok = unpackRR(msg, off)
	}
	if !ok {
		return false
	}
	if off != len(msg) {
		// TODO(mg) remove eventually
		println("extra bytes in dns packet", off, "<", len(msg))
	}
	return true
}

// Convert a complete message to a string with dig-like output.
func (dns *Msg) String() string {
	if dns == nil {
		return "<nil> MsgHdr"
	}
	s := dns.MsgHdr.String() + " "
	s += "QUERY: " + strconv.Itoa(len(dns.Question)) + ", "
	s += "ANSWER: " + strconv.Itoa(len(dns.Answer)) + ", "
	s += "AUTHORITY: " + strconv.Itoa(len(dns.Ns)) + ", "
	s += "ADDITIONAL: " + strconv.Itoa(len(dns.Extra)) + "\n"
	if len(dns.Question) > 0 {
		s += "\n;; QUESTION SECTION:\n"
		for i := 0; i < len(dns.Question); i++ {
			// Need check if it exists? TODO(mg)
			s += dns.Question[i].String() + "\n"
		}
	}
	if len(dns.Answer) > 0 {
		s += "\n;; ANSWER SECTION:\n"
		for i := 0; i < len(dns.Answer); i++ {
			if dns.Answer[i] != nil {
				s += dns.Answer[i].String() + "\n"
			}
		}
	}
	if len(dns.Ns) > 0 {
		s += "\n;; AUTHORITY SECTION:\n"
		for i := 0; i < len(dns.Ns); i++ {
			if dns.Ns[i] != nil {
				s += dns.Ns[i].String() + "\n"
			}
		}
	}
	if len(dns.Extra) > 0 {
		s += "\n;; ADDITIONAL SECTION:\n"
		for i := 0; i < len(dns.Extra); i++ {
			if dns.Extra[i] != nil {
				s += dns.Extra[i].String() + "\n"
			}
		}
	}
	return s
}

// Return a 16 bits random number to be used as a
// message id. The random provided should be good enough.
func Id() uint16 {
	return uint16(rand.Int()) ^ uint16(time.Nanoseconds())
}