dns/tsig.go

package dns

// Implementation of TSIG: generation and validation
// RFC 2845 and RFC 4635
import (
	"io"
	"strconv"
	"strings"
	"crypto/hmac"
	"encoding/hex"
)

// HMAC hashing codes. These are transmitted as domain names.
const (
	HmacMD5    = "hmac-md5.sig-alg.reg.int."
	HmacSHA1   = "hmac-sha1."
	HmacSHA256 = "hmac-sha256."
)

type RR_TSIG struct {
	Hdr        RR_Header
	Algorithm  string "domain-name"
	TimeSigned uint64
	Fudge      uint16
	MACSize    uint16
	MAC        string "size-hex"
	OrigId     uint16
	Error      uint16
	OtherLen   uint16
	OtherData  string "size-hex"
}

func (rr *RR_TSIG) Header() *RR_Header {
	return &rr.Hdr
}

// move to defaults.go?
func (rr *RR_TSIG) SetDefaults() {
	rr.Header().Ttl = 0
	rr.Header().Class = ClassANY
	rr.Header().Rrtype = TypeTSIG
	rr.Fudge = 300
	rr.Algorithm = HmacMD5
}

// TSIG has no official presentation format, but this will suffice.
func (rr *RR_TSIG) String() string {
	return rr.Hdr.String() +
		" " + rr.Algorithm +
		" " + tsigTimeToDate(rr.TimeSigned) +
		" " + strconv.Itoa(int(rr.Fudge)) +
		" " + strconv.Itoa(int(rr.MACSize)) +
		" " + strings.ToUpper(rr.MAC) +
		" " + strconv.Itoa(int(rr.OrigId)) +
		" " + strconv.Itoa(int(rr.Error)) +
		" " + strconv.Itoa(int(rr.OtherLen)) +
		" " + rr.OtherData
}

// The following values must be put in wireformat, so that the MAC can be calculated.
// RFC 2845, section 3.4.2. TSIG Variables.
type tsigWireFmt struct {
	// From RR_HEADER
	Name  string "domain-name"
	Class uint16
	Ttl   uint32
	// Rdata of the TSIG
	Algorithm  string "domain-name"
	TimeSigned uint64
	Fudge      uint16
	// MACSize, MAC and OrigId excluded
	Error     uint16
	OtherLen  uint16
	OtherData string "size-hex"
}

// If we have the MAC use this type to convert it to wiredata.
// Section 3.4.3. Request MAC
type macWireFmt struct {
	MACSize uint16
	MAC     string "size-hex"
}

// 3.3. Time values used in TSIG calculations
type timerWireFmt struct {
	TimeSigned uint64
	Fudge      uint16
}

// Generate the HMAC for message. The TSIG RR is modified
// to include the MAC and MACSize. Note the the msg Id must
// already be set, otherwise the MAC will not be correct when
// the message is send.
// The string 'secret' must be encoded in base64.
func (t *RR_TSIG) Generate(m *Msg, secret string) bool {
	rawsecret, err := packBase64([]byte(secret))
	if err != nil {
		return false
	}
	t.OrigId = m.MsgHdr.Id

	msg, ok := m.Pack()
	if !ok {
		return false
	}
	buf, ok1 := tsigToBuf(t, msg, "", true)
	if !ok1 {
		return false
	}
	h := hmac.NewMD5([]byte(rawsecret))
	io.WriteString(h, string(buf))

	t.MAC = hex.EncodeToString(h.Sum())
	t.MACSize = uint16(len(h.Sum())) // Needs to be "on-the-wire" size.
	if !ok {
		return false
	}
	return true
}

// Verify a TSIG. The message should be the complete with
// the TSIG record still attached (as the last rr in the Additional
// section). Return true on success.
// The secret is a base64 encoded string with the secret.
func (t *RR_TSIG) Verify(msg []byte, secret, reqmac string, timers bool) bool {
	rawsecret, err := packBase64([]byte(secret))
	if err != nil {
		return false
	}

	if t.Header().Rrtype != TypeTSIG {
		return false
	}

	// t.OrigId -- need to check
	stripped, ok := stripTSIG(msg)
	if !ok {
		return false
	}
	buf, ok := tsigToBuf(t, stripped, reqmac, timers)
	if !ok {
		return false
	}

	h := hmac.NewMD5([]byte(rawsecret))
	io.WriteString(h, string(buf))
	return strings.ToUpper(hex.EncodeToString(h.Sum())) == strings.ToUpper(t.MAC)
}

// Create the buffer which we use for the MAC calculation.
func tsigToBuf(rr *RR_TSIG, msg []byte, reqmac string, timers bool) ([]byte, bool) {
	var (
		macbuf []byte
		buf    []byte
	)

	if reqmac != "" {
		m := new(macWireFmt)
		m.MACSize = uint16(len(reqmac) / 2)
		m.MAC = reqmac
		macbuf = make([]byte, len(reqmac)) // reqmac should be twice as long
		n, ok := packStruct(m, macbuf, 0)
		if !ok {
			return nil, false
		}
		macbuf = macbuf[:n]
	}

	tsigvar := make([]byte, DefaultMsgSize)
	if timers {
		tsig := new(tsigWireFmt)
		tsig.Name = strings.ToLower(rr.Header().Name)
		tsig.Class = rr.Header().Class
		tsig.Ttl = rr.Header().Ttl
		tsig.Algorithm = strings.ToLower(rr.Algorithm)
		tsig.TimeSigned = rr.TimeSigned
		tsig.Fudge = rr.Fudge
		tsig.Error = rr.Error
		tsig.OtherLen = rr.OtherLen
		tsig.OtherData = rr.OtherData
		n, ok1 := packStruct(tsig, tsigvar, 0)
		if !ok1 {
			return nil, false
		}
		tsigvar = tsigvar[:n]
	} else {
		tsig := new(timerWireFmt)
		tsig.TimeSigned = rr.TimeSigned
		tsig.Fudge = rr.Fudge
		n, ok1 := packStruct(tsig, tsigvar, 0)
		if !ok1 {
			return nil, false
		}
		tsigvar = tsigvar[:n]
	}
	if reqmac != "" {
		x := append(macbuf, msg...)
		buf = append(x, tsigvar...)
	} else {
		buf = append(msg, tsigvar...)
	}
	return buf, true
}

// Strip the TSIG from the pkt.
func stripTSIG(orig []byte) ([]byte, bool) {
	// Copied from msg.go's Unpack()
	// Header.
	var dh Header
	dns := new(Msg)
	msg := make([]byte, len(orig))
	copy(msg, orig) // fhhh.. another copy
	off := 0
	tsigoff := 0
	var ok bool
	if off, ok = unpackStruct(&dh, msg, off); !ok {
		return nil, false
	}
	if dh.Arcount == 0 {
		// No records at all in the additional.
		return nil, false
	}

	// 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++ {
		tsigoff = off
		dns.Extra[i], off, ok = unpackRR(msg, off)
		if dns.Extra[i].Header().Rrtype == TypeTSIG {
			// Adjust Arcount.
			arcount, _ := unpackUint16(msg, 10)
			msg[10], msg[11] = packUint16(arcount - 1)
			break
		}
	}
	if !ok {
		return nil, false
	}
	return msg[:tsigoff], true
}