AdGuardDNS/internal/dnsserver/msg.go

package dnsserver

import (
	"encoding/binary"
	"fmt"
	"slices"

	"github.com/miekg/dns"
)

// genErrorResponse creates a short DNS message with the specified rcode.
// it is supposed to be used for generating errors (server failure, bad format,
// etc.)
func genErrorResponse(req *dns.Msg, code int) (m *dns.Msg) {
	m = &dns.Msg{}
	m.SetRcode(req, code)

	return m
}

// packWithPrefix packs a DNS message with a 2-byte prefix with the message
// length by appending it into buf and returns it.
func packWithPrefix(m *dns.Msg, buf []byte) (packed []byte, err error) {
	buf, err = m.PackBuffer(buf)
	if err != nil {
		return nil, fmt.Errorf("packing buffer: %w", err)
	}

	l := len(buf)
	if l > dns.MaxMsgSize {
		// Generally shouldn't happen.
		return nil, fmt.Errorf("buffer too large: %d bytes", l)
	}

	// Try to reuse the slice if there is already space there.
	packed = slices.Grow(buf, 2)[:l+2]

	copy(packed[2:], buf)
	binary.BigEndian.PutUint16(packed[:2], uint16(l))

	return packed, nil
}