path: root/edgemesh/pkg/server/dns.go

package server

import (
	"bufio"
	"encoding/binary"
	"errors"
	"fmt"
	"net"
	"os"
	"strings"
	"time"
	"unsafe"

	"k8s.io/klog"

	"github.com/kubeedge/beehive/pkg/core/context"
	"github.com/kubeedge/kubeedge/edge/pkg/metamanager/client"
	"github.com/kubeedge/kubeedge/edgemesh/pkg/common"
)

var (
	dnsQr       = uint16(0x8000)
	oneByteSize = uint16(1)
	twoByteSize = uint16(2)
	ttl         = uint32(64)
	fakeIp      = []byte{5, 5, 5, 5}
)

const (
	aRecord   = 1
	bufSize   = 1024
	notImplem = uint16(0x0004)
)

type dnsHeader struct {
	transactionID uint16
	flags         uint16
	queNum        uint16
	ansNum        uint16
	authNum       uint16
	additNum      uint16
}

type dnsQuestion struct {
	from    *net.UDPAddr
	head    *dnsHeader
	name    []byte
	queByte []byte
	qType   uint16
	qClasss uint16
	queNum  uint16
}

type dnsAnswer struct {
	name    []byte
	qType   uint16
	qClass  uint16
	ttl     uint32
	dataLen uint16
	addr    []byte
}

//define the dns Question list type
type dnsQs []dnsQuestion

//metaClient is a query client
var metaClient client.CoreInterface

//dnsConn save DNS server
var dnsConn *net.UDPConn

//DnsStart is a External interface
func DnsStart() {
	startDnsServer()
}

// startDnsServer start the DNS Server
func startDnsServer() {
	// init meta client
	c := context.GetContext(context.MsgCtxTypeChannel)
	metaClient = client.New(c)
	//get DNS server name
	lip, err := getIP()
	if err != nil {
		klog.Errorf("Dns server Start error : %s", err)
		return
	}

	laddr := &net.UDPAddr{
		IP:   lip,
		Port: 53,
	}
	udpConn, err := net.ListenUDP("udp", laddr)
	defer udpConn.Close()
	if err != nil {
		klog.Errorf("Dns server Start error : %s", err)
	}
	dnsConn = udpConn
	for {
		req := make([]byte, bufSize)
		n, from, err := dnsConn.ReadFromUDP(req)
		if err != nil || n <= 0 {
			klog.Infof("DNS server get an IO error : %s", err)
			continue
		}

		que, err := parseDnsQuery(req[:n])
		if err != nil {
			continue
		}
		for _, q := range que {
			q.from = from
		}
		rsp := make([]byte, 0)
		rsp, err = recordHandler(que, req[0:n])
		if err != nil {
			klog.Infof("DNS server get an resolve abnormal : %s", err)
			continue
		}
		dnsConn.WriteTo(rsp, from)
	}
}

//recordHandler returns the Answer for the dns question
func recordHandler(que []dnsQuestion, req []byte) (rsp []byte, err error) {
	var exist bool
	for _, q := range que {
		domainName := string(q.name)
		exist, err = lookupFromMetaManager(domainName)
		if err != nil {
			rsp = nil
			return
		}
		if !exist {
			//if this service don't belongs to this cluster
			go getfromRealDNS(req, q.from)
			return rsp, fmt.Errorf("get from real DNS")
		}
	}

	//gen
	pre, err := modifyRspPrefix(que)
	rsp = append(rsp, pre...)
	for _, q := range que {
		//create a deceptive rep
		dnsAns := &dnsAnswer{
			name:    q.name,
			qType:   q.qType,
			qClass:  q.qClasss,
			ttl:     ttl,
			dataLen: uint16(len(fakeIp)),
			addr:    fakeIp,
		}
		ans := dnsAns.getAnswer()
		rsp = append(rsp, ans...)
	}

	return rsp, nil
}

//parseDnsQuery returns question of the dns request
func parseDnsQuery(req []byte) (que []dnsQuestion, err error) {
	head := &dnsHeader{}
	head.getHeader(req)
	if !head.isAQurey() {
		return nil, errors.New("Ignore")
	}

	question := make(dnsQs, head.queNum)

	offset := uint16(unsafe.Sizeof(dnsHeader{}))
	question.getQuestion(req, offset, head)

	que = question
	err = nil
	return
}

//isAQuery judge if the dns pkg is a Qurey process
func (h *dnsHeader) isAQurey() bool {
	if h.flags&dnsQr != dnsQr {
		return true
	}
	return false
}

//getHeader get dns pkg head
func (h *dnsHeader) getHeader(req []byte) {
	h.transactionID = binary.BigEndian.Uint16(req[0:2])
	h.flags = binary.BigEndian.Uint16(req[2:4])
	h.queNum = binary.BigEndian.Uint16(req[4:6])
	h.ansNum = binary.BigEndian.Uint16(req[6:8])
	h.authNum = binary.BigEndian.Uint16(req[8:10])
	h.additNum = binary.BigEndian.Uint16(req[10:12])
}

//getQuestion get dns questions
func (q dnsQs) getQuestion(req []byte, offset uint16, head *dnsHeader) {
	ost := offset
	qNum := uint16(len(q))

	for i := uint16(0); i < qNum; i++ {
		tmp := ost
		ost = q[i].getQName(req, ost)
		q[i].qType = binary.BigEndian.Uint16(req[ost : ost+twoByteSize])
		ost += twoByteSize
		q[i].qClasss = binary.BigEndian.Uint16(req[ost : ost+twoByteSize])
		ost += twoByteSize
		q[i].head = head
		q[i].queByte = req[tmp:ost]
	}
}

//getAnswer Generate Answer for the dns question
func (d *dnsAnswer) getAnswer() (ans []byte) {
	ans = make([]byte, 0)

	if d.qType == aRecord {
		ans = append(ans, 0xc0)
		ans = append(ans, 0x0c)

		tmp16 := make([]byte, 2)
		tmp32 := make([]byte, 4)

		binary.BigEndian.PutUint16(tmp16, d.qType)
		ans = append(ans, tmp16...)
		binary.BigEndian.PutUint16(tmp16, d.qClass)
		ans = append(ans, tmp16...)
		binary.BigEndian.PutUint32(tmp32, d.ttl)
		ans = append(ans, tmp32...)
		binary.BigEndian.PutUint16(tmp16, d.dataLen)
		ans = append(ans, tmp16...)
		ans = append(ans, d.addr...)
	}

	return ans
}

// getQName get dns question domain name
func (q *dnsQuestion) getQName(req []byte, offset uint16) uint16 {
	ost := offset

	for {
		qbyte := uint16(req[ost])

		if qbyte == 0x00 {
			q.name = q.name[:uint16(len(q.name))-oneByteSize]
			return ost + oneByteSize
		}
		ost += oneByteSize
		q.name = append(q.name, req[ost:ost+qbyte]...)
		q.name = append(q.name, 0x2e)
		ost += qbyte
	}
}

// lookupFromMetaManager implement confirm the service exists
func lookupFromMetaManager(serviceUrl string) (exist bool, err error) {
	name, namespace := common.SplitServiceKey(serviceUrl)
	s, _ := metaClient.Services(namespace).Get(name)
	if s != nil {
		klog.Infof("Service %s is found in this cluster. namespace : %s, name: %s", serviceUrl, namespace, name)
		return true, nil
	}
	klog.Infof("Service %s is not found in this cluster", serviceUrl)
	return false, nil
}

// getfromRealDNS returns the dns response from the real DNS server
func getfromRealDNS(req []byte, from *net.UDPAddr) {
	rsp := make([]byte, 0)
	ips, err := parseNameServer()
	if err != nil {
		return
	}

	laddr := &net.UDPAddr{
		IP:   net.IPv4zero,
		Port: 0,
	}

	for _, ip := range ips { // get from real
		raddr := &net.UDPAddr{
			IP:   ip,
			Port: 53,
		}
		conn, err := net.DialUDP("udp", laddr, raddr)
		defer conn.Close()
		if err != nil {
			continue
		}
		_, err = conn.Write(req)
		if err != nil {
			continue
		}
		if err = conn.SetReadDeadline(time.Now().Add(time.Minute)); err != nil {
			continue
		}
		var n int
		buf := make([]byte, bufSize)
		n, err = conn.Read(buf)
		if err != nil {
			continue
		}

		if n > 0 {
			rsp = append(rsp, buf[:n]...)
			dnsConn.WriteToUDP(rsp, from)
			break
		}
	}
}

// parseNameServer gets the nameserver from the resolv.conf
func parseNameServer() ([]net.IP, error) {
	file, err := os.Open("/etc/resolv.conf")
	if err != nil {
		return nil, fmt.Errorf("error opening /etc/resolv.conf : %s", err)
	}
	defer file.Close()

	scan := bufio.NewScanner(file)
	scan.Split(bufio.ScanLines)

	ip := make([]net.IP, 0)

	for scan.Scan() { //get name server
		serverString := scan.Text()
		fmt.Println(serverString)
		if strings.Contains(serverString, "nameserver") {
			tmpString := strings.Replace(serverString, "nameserver", "", 1)
			nameserver := strings.TrimSpace(tmpString)
			sip := net.ParseIP(nameserver)
			if sip != nil {
				ip = append(ip, sip)
			}
		}
	}
	if len(ip) == 0 {
		return nil, fmt.Errorf("there is no nameserver in /etc/resolv.conf")
	}
	return ip, nil
}

// modifyRspPrefix use req' head generate a rsp head
func modifyRspPrefix(que []dnsQuestion) (pre []byte, err error) {
	ansNum := len(que)
	if ansNum == 0 {
		return
	}
	//use head in que. All the same
	rspHead := que[0].head
	rspHead.converQueryRsp(true)
	if que[0].qType == aRecord {
		rspHead.setAnswerNum(uint16(ansNum))
	} else {
		rspHead.setAnswerNum(0)
	}

	rspHead.setRspRcode(que)
	pre = rspHead.getByteFromDnsHeader()

	for _, q := range que {
		pre = append(pre, q.queByte...)
	}

	err = nil
	return
}

// converQueryRsp conversion the dns head to a response for one query
func (h *dnsHeader) converQueryRsp(isRsp bool) {
	if isRsp {
		h.flags |= dnsQr
	} else {
		h.flags |= dnsQr
	}
}

// set the Answer num for dns head
func (h *dnsHeader) setAnswerNum(num uint16) {
	h.ansNum = num
}

// set the dns response return code
func (h *dnsHeader) setRspRcode(que dnsQs) {
	for _, q := range que {
		if q.qType != aRecord {
			h.flags &= (^notImplem)
			h.flags |= notImplem
		}
	}
}

//getByteFromDnsHeader implement from dnsHeader struct to []byte
func (h *dnsHeader) getByteFromDnsHeader() (rspHead []byte) {
	rspHead = make([]byte, unsafe.Sizeof(*h))

	idxTran := unsafe.Sizeof(h.transactionID)
	idxflag := unsafe.Sizeof(h.flags) + idxTran
	idxque := unsafe.Sizeof(h.ansNum) + idxflag
	idxans := unsafe.Sizeof(h.ansNum) + idxque
	idxaut := unsafe.Sizeof(h.authNum) + idxans
	idxadd := unsafe.Sizeof(h.additNum) + idxaut

	binary.BigEndian.PutUint16(rspHead[:idxTran], h.transactionID)
	binary.BigEndian.PutUint16(rspHead[idxTran:idxflag], h.flags)
	binary.BigEndian.PutUint16(rspHead[idxflag:idxque], h.queNum)
	binary.BigEndian.PutUint16(rspHead[idxque:idxans], h.ansNum)
	binary.BigEndian.PutUint16(rspHead[idxans:idxaut], h.authNum)
	binary.BigEndian.PutUint16(rspHead[idxaut:idxadd], h.additNum)
	return
}