util.go

package dnscrypt

import (
	"encoding/binary"
	"io"
	"net"
	"strings"

	"github.com/ameshkov/dnscrypt/v2/xsecretbox"
	"github.com/miekg/dns"
	"golang.org/x/crypto/nacl/box"
)

// Prior to encryption, queries are padded using the ISO/IEC 7816-4
// format. The padding starts with a byte valued 0x80 followed by a
// variable number of NUL bytes.
//
// ## Padding for client queries over UDP
//
// <client-query> <client-query-pad> must be at least <min-query-len>
// bytes. If the length of the client query is less than <min-query-len>,
// the padding length must be adjusted in order to satisfy this
// requirement.
//
// <min-query-len> is a variable length, initially set to 256 bytes, and
// must be a multiple of 64 bytes.
//
// ## Padding for client queries over TCP
//
// The length of <client-query-pad> is randomly chosen between 1 and 256
// bytes (including the leading 0x80), but the total length of <client-query>
// <client-query-pad> must be a multiple of 64 bytes.
//
// For example, an originally unpadded 56-bytes DNS query can be padded as:
//
// <56-bytes-query> 0x80 0x00 0x00 0x00 0x00 0x00 0x00 0x00
// or
// <56-bytes-query> 0x80 (0x00 * 71)
// or
// <56-bytes-query> 0x80 (0x00 * 135)
// or
// <56-bytes-query> 0x80 (0x00 * 199)
func pad(packet []byte) []byte {
	// get closest divisible by 64 to <packet-len> + 1 byte for 0x80
	minQuestionSize := len(packet) + 1 + (64 - (len(packet)+1)%64)

	// padded size can't be less than minUDPQuestionSize
	minQuestionSize = max(minUDPQuestionSize, minQuestionSize)

	packet = append(packet, 0x80)
	for len(packet) < minQuestionSize {
		packet = append(packet, 0)
	}

	return packet
}

// unpad - removes padding bytes
func unpad(packet []byte) ([]byte, error) {
	for i := len(packet); ; {
		if i == 0 {
			return nil, ErrInvalidPadding
		}
		i--
		if packet[i] == 0x80 {
			if i < minDNSPacketSize {
				return nil, ErrInvalidPadding
			}

			return packet[:i], nil
		} else if packet[i] != 0x00 {
			return nil, ErrInvalidPadding
		}
	}
}

// computeSharedKey - computes a shared key
func computeSharedKey(cryptoConstruction CryptoConstruction, secretKey *[keySize]byte, publicKey *[keySize]byte) ([keySize]byte, error) {
	if cryptoConstruction == XChacha20Poly1305 {
		sharedKey, err := xsecretbox.SharedKey(*secretKey, *publicKey)
		if err != nil {
			return sharedKey, err
		}
		return sharedKey, nil
	} else if cryptoConstruction == XSalsa20Poly1305 {
		sharedKey := [sharedKeySize]byte{}
		box.Precompute(&sharedKey, publicKey, secretKey)
		return sharedKey, nil
	}
	return [keySize]byte{}, ErrEsVersion
}

func max(a, b int) int {
	if a > b {
		return a
	}
	return b
}

func isDigit(b byte) bool { return b >= '0' && b <= '9' }

func dddToByte(s []byte) byte {
	return (s[0]-'0')*100 + (s[1]-'0')*10 + (s[2] - '0')
}

const (
	escapedByteSmall = "" +
		`\000\001\002\003\004\005\006\007\008\009` +
		`\010\011\012\013\014\015\016\017\018\019` +
		`\020\021\022\023\024\025\026\027\028\029` +
		`\030\031`
	escapedByteLarge = `\127\128\129` +
		`\130\131\132\133\134\135\136\137\138\139` +
		`\140\141\142\143\144\145\146\147\148\149` +
		`\150\151\152\153\154\155\156\157\158\159` +
		`\160\161\162\163\164\165\166\167\168\169` +
		`\170\171\172\173\174\175\176\177\178\179` +
		`\180\181\182\183\184\185\186\187\188\189` +
		`\190\191\192\193\194\195\196\197\198\199` +
		`\200\201\202\203\204\205\206\207\208\209` +
		`\210\211\212\213\214\215\216\217\218\219` +
		`\220\221\222\223\224\225\226\227\228\229` +
		`\230\231\232\233\234\235\236\237\238\239` +
		`\240\241\242\243\244\245\246\247\248\249` +
		`\250\251\252\253\254\255`
)

// escapeByte returns the \DDD escaping of b which must
// satisfy b < ' ' || b > '~'.
func escapeByte(b byte) string {
	if b < ' ' {
		return escapedByteSmall[b*4 : b*4+4]
	}

	b -= '~' + 1
	// The cast here is needed as b*4 may overflow byte.
	return escapedByteLarge[int(b)*4 : int(b)*4+4]
}

func packTxtString(buf []byte) string {
	var out strings.Builder
	out.Grow(3 + len(buf))
	for i := 0; i < len(buf); i++ {
		b := buf[i]
		switch {
		case b == '"' || b == '\\':
			out.WriteByte('\\')
			out.WriteByte(b)
		case b < ' ' || b > '~':
			out.WriteString(escapeByte(b))
		default:
			out.WriteByte(b)
		}
	}
	return out.String()
}

func unpackTxtString(s string) ([]byte, error) {
	bs := make([]byte, len(s))
	msg := make([]byte, 0)
	copy(bs, s)
	for i := 0; i < len(bs); i++ {
		if bs[i] == '\\' {
			i++
			if i == len(bs) {
				break
			}
			if i+2 < len(bs) && isDigit(bs[i]) && isDigit(bs[i+1]) && isDigit(bs[i+2]) {
				msg = append(msg, dddToByte(bs[i:]))
				i += 2
			} else if bs[i] == 't' {
				msg = append(msg, '\t')
			} else if bs[i] == 'r' {
				msg = append(msg, '\r')
			} else if bs[i] == 'n' {
				msg = append(msg, '\n')
			} else {
				msg = append(msg, bs[i])
			}
		} else {
			msg = append(msg, bs[i])
		}
	}
	return msg, nil
}

// normalize truncates the DNS response if needed depending on the protocol
func normalize(proto string, req *dns.Msg, res *dns.Msg) {
	size := dnsSize(proto, req)
	// DNSCrypt encryption adds a header to each message, we should
	// consider this when truncating a message.
	// 64 should cover all cases
	size = size - 64

	// Truncate response message
	res.Truncate(size)

	// In case of UDP it is safer to simply remove all response records
	// dns.Msg.Truncate method will not consider that we need a response
	// shorter than dns.MinMsgSize
	if res.Truncated && proto == "udp" {
		res.Answer = nil
	}
}

// dnsSize returns if buffer size *advertised* in the requests OPT record.
// Or when the request was over TCP, we return the maximum allowed size of 64K.
func dnsSize(proto string, r *dns.Msg) int {
	size := uint16(0)
	if o := r.IsEdns0(); o != nil {
		size = o.UDPSize()
	}

	if proto != "udp" {
		return dns.MaxMsgSize
	}

	if size < dns.MinMsgSize {
		return dns.MinMsgSize
	}

	// normalize size
	return int(size)
}

// readPrefixed -- reads a DNS message with a 2-byte prefix containing message length
func readPrefixed(conn net.Conn) ([]byte, error) {
	l := make([]byte, 2)
	_, err := conn.Read(l)
	if err != nil {
		return nil, err
	}
	packetLen := binary.BigEndian.Uint16(l)
	if packetLen > dns.MaxMsgSize {
		return nil, ErrQueryTooLarge
	}

	buf := make([]byte, packetLen)
	_, err = io.ReadFull(conn, buf)
	if err != nil {
		return nil, err
	}
	return buf, nil
}

// writePrefixed -- write a DNS message to a TCP connection
// it first writes a 2-byte prefix followed by the message itself
func writePrefixed(b []byte, conn net.Conn) error {
	l := make([]byte, 2)
	binary.BigEndian.PutUint16(l, uint16(len(b)))
	_, err := (&net.Buffers{l, b}).WriteTo(conn)
	return err
}

// isConnClosed - checks if the error signals of a closed server connecting
func isConnClosed(err error) bool {
	if err == nil {
		return false
	}
	nerr, ok := err.(*net.OpError)
	if !ok {
		return false
	}

	if strings.Contains(nerr.Err.Error(), "use of closed network connection") {
		return true
	}

	return false
}