// 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 marshaling.
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
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
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
}
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_MX struct {
Hdr dnsRR_Header
Pref uint16
Mx string
}
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
}
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
}
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
Mbox string
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 {
if !rr.Hdr.Walk(f) {
return false
}
var n uint16 = 0
for n < rr.Hdr.Rdlength {
var txt string
if !f(&txt, "Txt", "") {
return false
}
// more bytes than rr.Hdr.Rdlength said there would be
if rr.Hdr.Rdlength-n < uint16(len(txt))+1 {
return false
}
n += uint16(len(txt)) + 1
rr.Txt += txt
}
return true
}
type dnsRR_SRV struct {
Hdr dnsRR_Header
Priority uint16
Weight uint16
Port uint16
Target string
}
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
}
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
}
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) },
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 += "."
}
// Allow root domain.
if s == "." {
msg[off] = 0
off++
return off, true
}
// 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
}
if i-begin == 0 {
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 len(s) == 0 {
s = "."
}
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)
if !ok {
return len(msg), false
}
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)
if !ok {
return nil, false
}
for i := 0; i < len(question); i++ {
off, ok = packStruct(&question[i], msg, off)
if !ok {
return nil, false
}
}
for i := 0; i < len(answer); i++ {
off, ok = packRR(answer[i], msg, off)
if !ok {
return nil, false
}
}
for i := 0; i < len(ns); i++ {
off, ok = packRR(ns[i], msg, off)
if !ok {
return nil, false
}
}
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)
if !ok {
return false
}
}
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
}
// IsResponseTo reports whether m is an acceptable response to query.
func (m *dnsMsg) IsResponseTo(query *dnsMsg) bool {
if !m.response {
return false
}
if m.id != query.id {
return false
}
if len(m.question) != len(query.question) {
return false
}
for i, q := range m.question {
q2 := query.question[i]
if !equalASCIILabel(q.Name, q2.Name) || q.Qtype != q2.Qtype || q.Qclass != q2.Qclass {
return false
}
}
return true
}