Rebase.

[official-gcc.git] / libgo / go / net / dnsmsg.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.

// DNS packet assembly.  See RFC 1035.
//
// This is intended to support name resolution during Dial.
// It doesn't have to be blazing fast.
//
// Each message structure has a Walk method that is used by
// a generic pack/unpack routine. Thus, if in the future we need
// to define new message structs, no new pack/unpack/printing code
// needs to be written.
//
// The first half of this file defines the DNS message formats.
// The second half implements the conversion to and from wire format.
// A few of the structure elements have string tags to aid the
// generic pack/unpack routines.
//
// TODO(rsc):  There are enough names defined in this file that they're all
// prefixed with dns.  Perhaps put this in its own package later.

package net

// Packet formats

// Wire constants.
const (
        // valid dnsRR_Header.Rrtype and dnsQuestion.qtype
        dnsTypeA     = 1
        dnsTypeNS    = 2
        dnsTypeMD    = 3
        dnsTypeMF    = 4
        dnsTypeCNAME = 5
        dnsTypeSOA   = 6
        dnsTypeMB    = 7
        dnsTypeMG    = 8
        dnsTypeMR    = 9
        dnsTypeNULL  = 10
        dnsTypeWKS   = 11
        dnsTypePTR   = 12
        dnsTypeHINFO = 13
        dnsTypeMINFO = 14
        dnsTypeMX    = 15
        dnsTypeTXT   = 16
        dnsTypeAAAA  = 28
        dnsTypeSRV   = 33

        // valid dnsQuestion.qtype only
        dnsTypeAXFR  = 252
        dnsTypeMAILB = 253
        dnsTypeMAILA = 254
        dnsTypeALL   = 255

        // valid dnsQuestion.qclass
        dnsClassINET   = 1
        dnsClassCSNET  = 2
        dnsClassCHAOS  = 3
        dnsClassHESIOD = 4
        dnsClassANY    = 255

        // dnsMsg.rcode
        dnsRcodeSuccess        = 0
        dnsRcodeFormatError    = 1
        dnsRcodeServerFailure  = 2
        dnsRcodeNameError      = 3
        dnsRcodeNotImplemented = 4
        dnsRcodeRefused        = 5
)

// A dnsStruct describes how to iterate over its fields to emulate
// reflective marshalling.
type dnsStruct interface {
        // Walk iterates over fields of a structure and calls f
        // with a reference to that field, the name of the field
        // and a tag ("", "domain", "ipv4", "ipv6") specifying
        // particular encodings. Possible concrete types
        // for v are *uint16, *uint32, *string, or []byte, and
        // *int, *bool in the case of dnsMsgHdr.
        // Whenever f returns false, Walk must stop and return
        // false, and otherwise return true.
        Walk(f func(v interface{}, name, tag string) (ok bool)) (ok bool)
}

// The wire format for the DNS packet header.
type dnsHeader struct {
        Id                                 uint16
        Bits                               uint16
        Qdcount, Ancount, Nscount, Arcount uint16
}

func (h *dnsHeader) Walk(f func(v interface{}, name, tag string) bool) bool {
        return f(&h.Id, "Id", "") &&
                f(&h.Bits, "Bits", "") &&
                f(&h.Qdcount, "Qdcount", "") &&
                f(&h.Ancount, "Ancount", "") &&
                f(&h.Nscount, "Nscount", "") &&
                f(&h.Arcount, "Arcount", "")
}

const (
        // dnsHeader.Bits
        _QR = 1 << 15 // query/response (response=1)
        _AA = 1 << 10 // authoritative
        _TC = 1 << 9  // truncated
        _RD = 1 << 8  // recursion desired
        _RA = 1 << 7  // recursion available
)

// DNS queries.
type dnsQuestion struct {
        Name   string `net:"domain-name"` // `net:"domain-name"` specifies encoding; see packers below
        Qtype  uint16
        Qclass uint16
}

func (q *dnsQuestion) Walk(f func(v interface{}, name, tag string) bool) bool {
        return f(&q.Name, "Name", "domain") &&
                f(&q.Qtype, "Qtype", "") &&
                f(&q.Qclass, "Qclass", "")
}

// DNS responses (resource records).
// There are many types of messages,
// but they all share the same header.
type dnsRR_Header struct {
        Name     string `net:"domain-name"`
        Rrtype   uint16
        Class    uint16
        Ttl      uint32
        Rdlength uint16 // length of data after header
}

func (h *dnsRR_Header) Header() *dnsRR_Header {
        return h
}

func (h *dnsRR_Header) Walk(f func(v interface{}, name, tag string) bool) bool {
        return f(&h.Name, "Name", "domain") &&
                f(&h.Rrtype, "Rrtype", "") &&
                f(&h.Class, "Class", "") &&
                f(&h.Ttl, "Ttl", "") &&
                f(&h.Rdlength, "Rdlength", "")
}

type dnsRR interface {
        dnsStruct
        Header() *dnsRR_Header
}

// Specific DNS RR formats for each query type.

type dnsRR_CNAME struct {
        Hdr   dnsRR_Header
        Cname string `net:"domain-name"`
}

func (rr *dnsRR_CNAME) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_CNAME) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) && f(&rr.Cname, "Cname", "domain")
}

type dnsRR_HINFO struct {
        Hdr dnsRR_Header
        Cpu string
        Os  string
}

func (rr *dnsRR_HINFO) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_HINFO) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) && f(&rr.Cpu, "Cpu", "") && f(&rr.Os, "Os", "")
}

type dnsRR_MB struct {
        Hdr dnsRR_Header
        Mb  string `net:"domain-name"`
}

func (rr *dnsRR_MB) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_MB) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) && f(&rr.Mb, "Mb", "domain")
}

type dnsRR_MG struct {
        Hdr dnsRR_Header
        Mg  string `net:"domain-name"`
}

func (rr *dnsRR_MG) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_MG) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) && f(&rr.Mg, "Mg", "domain")
}

type dnsRR_MINFO struct {
        Hdr   dnsRR_Header
        Rmail string `net:"domain-name"`
        Email string `net:"domain-name"`
}

func (rr *dnsRR_MINFO) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_MINFO) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) && f(&rr.Rmail, "Rmail", "domain") && f(&rr.Email, "Email", "domain")
}

type dnsRR_MR struct {
        Hdr dnsRR_Header
        Mr  string `net:"domain-name"`
}

func (rr *dnsRR_MR) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_MR) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) && f(&rr.Mr, "Mr", "domain")
}

type dnsRR_MX struct {
        Hdr  dnsRR_Header
        Pref uint16
        Mx   string `net:"domain-name"`
}

func (rr *dnsRR_MX) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_MX) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) && f(&rr.Pref, "Pref", "") && f(&rr.Mx, "Mx", "domain")
}

type dnsRR_NS struct {
        Hdr dnsRR_Header
        Ns  string `net:"domain-name"`
}

func (rr *dnsRR_NS) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_NS) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) && f(&rr.Ns, "Ns", "domain")
}

type dnsRR_PTR struct {
        Hdr dnsRR_Header
        Ptr string `net:"domain-name"`
}

func (rr *dnsRR_PTR) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_PTR) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) && f(&rr.Ptr, "Ptr", "domain")
}

type dnsRR_SOA struct {
        Hdr     dnsRR_Header
        Ns      string `net:"domain-name"`
        Mbox    string `net:"domain-name"`
        Serial  uint32
        Refresh uint32
        Retry   uint32
        Expire  uint32
        Minttl  uint32
}

func (rr *dnsRR_SOA) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_SOA) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) &&
                f(&rr.Ns, "Ns", "domain") &&
                f(&rr.Mbox, "Mbox", "domain") &&
                f(&rr.Serial, "Serial", "") &&
                f(&rr.Refresh, "Refresh", "") &&
                f(&rr.Retry, "Retry", "") &&
                f(&rr.Expire, "Expire", "") &&
                f(&rr.Minttl, "Minttl", "")
}

type dnsRR_TXT struct {
        Hdr dnsRR_Header
        Txt string // not domain name
}

func (rr *dnsRR_TXT) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_TXT) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) && f(&rr.Txt, "Txt", "")
}

type dnsRR_SRV struct {
        Hdr      dnsRR_Header
        Priority uint16
        Weight   uint16
        Port     uint16
        Target   string `net:"domain-name"`
}

func (rr *dnsRR_SRV) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_SRV) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) &&
                f(&rr.Priority, "Priority", "") &&
                f(&rr.Weight, "Weight", "") &&
                f(&rr.Port, "Port", "") &&
                f(&rr.Target, "Target", "domain")
}

type dnsRR_A struct {
        Hdr dnsRR_Header
        A   uint32 `net:"ipv4"`
}

func (rr *dnsRR_A) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_A) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) && f(&rr.A, "A", "ipv4")
}

type dnsRR_AAAA struct {
        Hdr  dnsRR_Header
        AAAA [16]byte `net:"ipv6"`
}

func (rr *dnsRR_AAAA) Header() *dnsRR_Header {
        return &rr.Hdr
}

func (rr *dnsRR_AAAA) Walk(f func(v interface{}, name, tag string) bool) bool {
        return rr.Hdr.Walk(f) && f(rr.AAAA[:], "AAAA", "ipv6")
}

// Packing and unpacking.
//
// 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.

// Map of constructors for each RR wire type.
var rr_mk = map[int]func() dnsRR{
        dnsTypeCNAME: func() dnsRR { return new(dnsRR_CNAME) },
        dnsTypeHINFO: func() dnsRR { return new(dnsRR_HINFO) },
        dnsTypeMB:    func() dnsRR { return new(dnsRR_MB) },
        dnsTypeMG:    func() dnsRR { return new(dnsRR_MG) },
        dnsTypeMINFO: func() dnsRR { return new(dnsRR_MINFO) },
        dnsTypeMR:    func() dnsRR { return new(dnsRR_MR) },
        dnsTypeMX:    func() dnsRR { return new(dnsRR_MX) },
        dnsTypeNS:    func() dnsRR { return new(dnsRR_NS) },
        dnsTypePTR:   func() dnsRR { return new(dnsRR_PTR) },
        dnsTypeSOA:   func() dnsRR { return new(dnsRR_SOA) },
        dnsTypeTXT:   func() dnsRR { return new(dnsRR_TXT) },
        dnsTypeSRV:   func() dnsRR { return new(dnsRR_SRV) },
        dnsTypeA:     func() dnsRR { return new(dnsRR_A) },
        dnsTypeAAAA:  func() dnsRR { return new(dnsRR_AAAA) },
}

// 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.
        // 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
        for i := 0; i < len(s); i++ {
                if s[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] = s[j]
                                off++
                        }
                        begin = i + 1
                }
        }
        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
                        }
                        s += string(msg[off:off+c]) + "."
                        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
}

// packStruct packs a structure into msg at specified offset off, and
// returns off1 such that msg[off:off1] is the encoded data.
func packStruct(any dnsStruct, msg []byte, off int) (off1 int, ok bool) {
        ok = any.Walk(func(field interface{}, name, tag string) bool {
                switch fv := field.(type) {
                default:
                        println("net: dns: unknown packing type")
                        return false
                case *uint16:
                        i := *fv
                        if off+2 > len(msg) {
                                return false
                        }
                        msg[off] = byte(i >> 8)
                        msg[off+1] = byte(i)
                        off += 2
                case *uint32:
                        i := *fv
                        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 []byte:
                        n := len(fv)
                        if off+n > len(msg) {
                                return false
                        }
                        copy(msg[off:off+n], fv)
                        off += n
                case *string:
                        s := *fv
                        switch tag {
                        default:
                                println("net: dns: unknown string tag", tag)
                                return false
                        case "domain":
                                off, ok = packDomainName(s, msg, off)
                                if !ok {
                                        return false
                                }
                        case "":
                                // Counted string: 1 byte length.
                                if len(s) > 255 || off+1+len(s) > len(msg) {
                                        return false
                                }
                                msg[off] = byte(len(s))
                                off++
                                off += copy(msg[off:], s)
                        }
                }
                return true
        })
        if !ok {
                return len(msg), false
        }
        return off, true
}

// unpackStruct decodes msg[off:] into the given structure, and
// returns off1 such that msg[off:off1] is the encoded data.
func unpackStruct(any dnsStruct, msg []byte, off int) (off1 int, ok bool) {
        ok = any.Walk(func(field interface{}, name, tag string) bool {
                switch fv := field.(type) {
                default:
                        println("net: dns: unknown packing type")
                        return false
                case *uint16:
                        if off+2 > len(msg) {
                                return false
                        }
                        *fv = uint16(msg[off])<<8 | uint16(msg[off+1])
                        off += 2
                case *uint32:
                        if off+4 > len(msg) {
                                return false
                        }
                        *fv = uint32(msg[off])<<24 | uint32(msg[off+1])<<16 |
                                uint32(msg[off+2])<<8 | uint32(msg[off+3])
                        off += 4
                case []byte:
                        n := len(fv)
                        if off+n > len(msg) {
                                return false
                        }
                        copy(fv, msg[off:off+n])
                        off += n
                case *string:
                        var s string
                        switch tag {
                        default:
                                println("net: dns: unknown string tag", tag)
                                return false
                        case "domain":
                                s, off, ok = unpackDomainName(msg, off)
                                if !ok {
                                        return false
                                }
                        case "":
                                if off >= len(msg) || off+1+int(msg[off]) > len(msg) {
                                        return 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 = s
                }
                return true
        })
        if !ok {
                return len(msg), false
        }
        return off, true
}

// Generic struct printer. Prints fields with tag "ipv4" or "ipv6"
// as IP addresses.
func printStruct(any dnsStruct) string {
        s := "{"
        i := 0
        any.Walk(func(val interface{}, name, tag string) bool {
                i++
                if i > 1 {
                        s += ", "
                }
                s += name + "="
                switch tag {
                case "ipv4":
                        i := *val.(*uint32)
                        s += IPv4(byte(i>>24), byte(i>>16), byte(i>>8), byte(i)).String()
                case "ipv6":
                        i := val.([]byte)
                        s += IP(i).String()
                default:
                        var i int64
                        switch v := val.(type) {
                        default:
                                // can't really happen.
                                s += "<unknown type>"
                                return true
                        case *string:
                                s += *v
                                return true
                        case []byte:
                                s += string(v)
                                return true
                        case *bool:
                                if *v {
                                        s += "true"
                                } else {
                                        s += "false"
                                }
                                return true
                        case *int:
                                i = int64(*v)
                        case *uint:
                                i = int64(*v)
                        case *uint8:
                                i = int64(*v)
                        case *uint16:
                                i = int64(*v)
                        case *uint32:
                                i = int64(*v)
                        case *uint64:
                                i = int64(*v)
                        case *uintptr:
                                i = int64(*v)
                        }
                        s += itoa(int(i))
                }
                return true
        })
        s += "}"
        return s
}

// Resource record packer.
func packRR(rr dnsRR, msg []byte, off int) (off2 int, ok bool) {
        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
        }
        // 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 dnsRR, off1 int, ok bool) {
        // unpack just the header, to find the rr type and length
        var h dnsRR_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 {
                return &h, end, true
        }
        rr = mk()
        off, ok = unpackStruct(rr, msg, off0)
        if off != end {
                return &h, end, true
        }
        return rr, off, ok
}

// Usable representation of a DNS packet.

// A manually-unpacked version of (id, bits).
// This is in its own struct for easy printing.
type dnsMsgHdr struct {
        id                  uint16
        response            bool
        opcode              int
        authoritative       bool
        truncated           bool
        recursion_desired   bool
        recursion_available bool
        rcode               int
}

func (h *dnsMsgHdr) Walk(f func(v interface{}, name, tag string) bool) bool {
        return f(&h.id, "id", "") &&
                f(&h.response, "response", "") &&
                f(&h.opcode, "opcode", "") &&
                f(&h.authoritative, "authoritative", "") &&
                f(&h.truncated, "truncated", "") &&
                f(&h.recursion_desired, "recursion_desired", "") &&
                f(&h.recursion_available, "recursion_available", "") &&
                f(&h.rcode, "rcode", "")
}

type dnsMsg struct {
        dnsMsgHdr
        question []dnsQuestion
        answer   []dnsRR
        ns       []dnsRR
        extra    []dnsRR
}

func (dns *dnsMsg) Pack() (msg []byte, ok bool) {
        var dh dnsHeader

        // Convert convenient dnsMsg into wire-like dnsHeader.
        dh.Id = dns.id
        dh.Bits = uint16(dns.opcode)<<11 | uint16(dns.rcode)
        if dns.recursion_available {
                dh.Bits |= _RA
        }
        if dns.recursion_desired {
                dh.Bits |= _RD
        }
        if dns.truncated {
                dh.Bits |= _TC
        }
        if dns.authoritative {
                dh.Bits |= _AA
        }
        if dns.response {
                dh.Bits |= _QR
        }

        // 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, 2000)

        // 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[0:off], true
}

func (dns *dnsMsg) Unpack(msg []byte) bool {
        // Header.
        var dh dnsHeader
        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.recursion_desired = (dh.Bits & _RD) != 0
        dns.recursion_available = (dh.Bits & _RA) != 0
        dns.rcode = int(dh.Bits & 0xF)

        // Arrays.
        dns.question = make([]dnsQuestion, dh.Qdcount)
        dns.answer = make([]dnsRR, 0, dh.Ancount)
        dns.ns = make([]dnsRR, 0, dh.Nscount)
        dns.extra = make([]dnsRR, 0, dh.Arcount)

        var rec dnsRR

        for i := 0; i < len(dns.question); i++ {
                off, ok = unpackStruct(&dns.question[i], msg, off)
        }
        for i := 0; i < int(dh.Ancount); i++ {
                rec, off, ok = unpackRR(msg, off)
                if !ok {
                        return false
                }
                dns.answer = append(dns.answer, rec)
        }
        for i := 0; i < int(dh.Nscount); i++ {
                rec, off, ok = unpackRR(msg, off)
                if !ok {
                        return false
                }
                dns.ns = append(dns.ns, rec)
        }
        for i := 0; i < int(dh.Arcount); i++ {
                rec, off, ok = unpackRR(msg, off)
                if !ok {
                        return false
                }
                dns.extra = append(dns.extra, rec)
        }
        //      if off != len(msg) {
        //              println("extra bytes in dns packet", off, "<", len(msg));
        //      }
        return true
}

func (dns *dnsMsg) String() string {
        s := "DNS: " + printStruct(&dns.dnsMsgHdr) + "\n"
        if len(dns.question) > 0 {
                s += "-- Questions\n"
                for i := 0; i < len(dns.question); i++ {
                        s += printStruct(&dns.question[i]) + "\n"
                }
        }
        if len(dns.answer) > 0 {
                s += "-- Answers\n"
                for i := 0; i < len(dns.answer); i++ {
                        s += printStruct(dns.answer[i]) + "\n"
                }
        }
        if len(dns.ns) > 0 {
                s += "-- Name servers\n"
                for i := 0; i < len(dns.ns); i++ {
                        s += printStruct(dns.ns[i]) + "\n"
                }
        }
        if len(dns.extra) > 0 {
                s += "-- Extra\n"
                for i := 0; i < len(dns.extra); i++ {
                        s += printStruct(dns.extra[i]) + "\n"
                }
        }
        return s
}