You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: dns.go
    path of file: ./repos/td/telegram/dcs
    
    the code of the file until where you have to start completion: package dcs

import (
	"bytes"
	"crypto/aes"
	"crypto/cipher"
	"crypto/rsa"
	"crypto/sha256"
	"encoding/base64"
	"math/big"
	"net"
	"sort"
	"sync"

	"github.com/go-faster/errors"

	"github.com/gotd/td/bin"
	"github.com/gotd/td/crypto"
	"github.com/gotd/td/tg"
)

var dnsKey struct {
	rsa.PublicKey
	once sync.Once
	eBig *big.Int
}

//nolint:gochecknoinits
func init() {
	dnsKey.once.Do(func() {
		k, err := crypto.ParseRSAPublicKeys([]byte(`-----BEGIN RSA PUBLIC KEY-----
MIIBCgKCAQEAyr+18Rex2ohtVy8sroGPBwXD3DOoKCSpjDqYoXgCqB7ioln4eDCF
fOBUlfXUEvM/fnKCpF46VkAftlb4VuPDeQSS/ZxZYEGqHaywlroV
    
func parseDNSList(input [256]byte) (tg.HelpConfigSimple, error) {
	// See https://github.com/tdlib/td/blob/master/td/telegram/ConfigManager.cpp#L148.
	x := new(big.Int).SetBytes(input[:])
	y := new(big.Int).Exp(x, dnsKey.eBig, dnsKey.N)

	dataRSA := make([]byte, 256)
	if !crypto.FillBytes(y, dataRSA) {
		return tg.HelpConfigSimple{}, errors.New("dataRSA has invalid size")
	}

	block, err := aes.NewCipher(dataRSA[:32])
	if err != nil {
		return tg.HelpConfigSimple{}, err
	}
	d := cipher.NewCBCDecrypter(block, dataRSA[16:32])
	dataCBC := dataRSA[32:]
	d.CryptBlocks(dataCBC, dataCBC)

	decrypted := dataCBC[:len(dataCBC)-16]
	decryptedHash := sha256.Sum256(decrypted)
	hash := dataCBC[len(dataCBC)-16:]

	if !bytes.Equal(decryptedHash[:16], hash) {
		return tg.HelpConfigSimple{}, errors.New("hash mismatch")
	}

	var cfg tg.HelpConfigSimple
	if err := cfg.Decode(&bin.Buffer{Buf: decrypted[4:]}); err != nil {
		return tg.HelpConfigSimple{}, err
	}
	return cfg, nil
}