resolver.go

package libnetwork

import (
	"fmt"
	"math/rand"
	"net"
	"strings"
	"sync"
	"time"

	log "github.com/Sirupsen/logrus"
	"github.com/docker/libnetwork/iptables"
	"github.com/miekg/dns"
)

// Resolver represents the embedded DNS server in Docker. It operates
// by listening on container's loopback interface for DNS queries.
type Resolver interface {
	// Start starts the name server for the container
	Start() error
	// Stop stops the name server for the container. Stopped resolver
	// can be reused after running the SetupFunc again.
	Stop()
	// SetupFunc() provides the setup function that should be run
	// in the container's network namespace.
	SetupFunc() func()
	// NameServer() returns the IP of the DNS resolver for the
	// containers.
	NameServer() string
	// To configure external name servers the resolver should use
	SetExtServers([]string)
	// ResolverOptions returns resolv.conf options that should be set
	ResolverOptions() []string
}

const (
	resolverIP      = "127.0.0.11"
	dnsPort         = "53"
	ptrIPv4domain   = ".in-addr.arpa."
	ptrIPv6domain   = ".ip6.arpa."
	respTTL         = 600
	maxExtDNS       = 3 //max number of external servers to try
	extIOTimeout    = 3 * time.Second
	defaultRespSize = 512
)

type extDNSEntry struct {
	ipStr   string
	extConn net.Conn
	extOnce sync.Once
}

// resolver implements the Resolver interface
type resolver struct {
	sb         *sandbox
	extDNSList [maxExtDNS]extDNSEntry
	server     *dns.Server
	conn       *net.UDPConn
	tcpServer  *dns.Server
	tcpListen  *net.TCPListener
	err        error
}

func init() {
	rand.Seed(time.Now().Unix())
}

// NewResolver creates a new instance of the Resolver
func NewResolver(sb *sandbox) Resolver {
	return &resolver{
		sb:  sb,
		err: fmt.Errorf("setup not done yet"),
	}
}

func (r *resolver) SetupFunc() func() {
	return (func() {
		var err error

		// DNS operates primarily on UDP
		addr := &net.UDPAddr{
			IP: net.ParseIP(resolverIP),
		}

		r.conn, err = net.ListenUDP("udp", addr)
		if err != nil {
			r.err = fmt.Errorf("error in opening name server socket %v", err)
			return
		}
		laddr := r.conn.LocalAddr()
		_, ipPort, _ := net.SplitHostPort(laddr.String())

		// Listen on a TCP as well
		tcpaddr := &net.TCPAddr{
			IP: net.ParseIP(resolverIP),
		}

		r.tcpListen, err = net.ListenTCP("tcp", tcpaddr)
		if err != nil {
			r.err = fmt.Errorf("error in opening name TCP server socket %v", err)
			return
		}
		ltcpaddr := r.tcpListen.Addr()
		_, tcpPort, _ := net.SplitHostPort(ltcpaddr.String())
		rules := [][]string{
			{"-t", "nat", "-A", "OUTPUT", "-d", resolverIP, "-p", "udp", "--dport", dnsPort, "-j", "DNAT", "--to-destination", laddr.String()},
			{"-t", "nat", "-A", "POSTROUTING", "-s", resolverIP, "-p", "udp", "--sport", ipPort, "-j", "SNAT", "--to-source", ":" + dnsPort},
			{"-t", "nat", "-A", "OUTPUT", "-d", resolverIP, "-p", "tcp", "--dport", dnsPort, "-j", "DNAT", "--to-destination", ltcpaddr.String()},
			{"-t", "nat", "-A", "POSTROUTING", "-s", resolverIP, "-p", "tcp", "--sport", tcpPort, "-j", "SNAT", "--to-source", ":" + dnsPort},
		}

		for _, rule := range rules {
			r.err = iptables.RawCombinedOutputNative(rule...)
			if r.err != nil {
				return
			}
		}
		r.err = nil
	})
}

func (r *resolver) Start() error {
	// make sure the resolver has been setup before starting
	if r.err != nil {
		return r.err
	}
	s := &dns.Server{Handler: r, PacketConn: r.conn}
	r.server = s
	go func() {
		s.ActivateAndServe()
	}()

	tcpServer := &dns.Server{Handler: r, Listener: r.tcpListen}
	r.tcpServer = tcpServer
	go func() {
		tcpServer.ActivateAndServe()
	}()
	return nil
}

func (r *resolver) Stop() {
	if r.server != nil {
		r.server.Shutdown()
	}
	if r.tcpServer != nil {
		r.tcpServer.Shutdown()
	}
	r.conn = nil
	r.tcpServer = nil
	r.err = fmt.Errorf("setup not done yet")
}

func (r *resolver) SetExtServers(dns []string) {
	l := len(dns)
	if l > maxExtDNS {
		l = maxExtDNS
	}
	for i := 0; i < l; i++ {
		r.extDNSList[i].ipStr = dns[i]
	}
}

func (r *resolver) NameServer() string {
	return resolverIP
}

func (r *resolver) ResolverOptions() []string {
	return []string{"ndots:0"}
}

func setCommonFlags(msg *dns.Msg) {
	msg.RecursionAvailable = true
}

func shuffleAddr(addr []net.IP) []net.IP {
	for i := len(addr) - 1; i > 0; i-- {
		r := rand.Intn(i + 1)
		addr[i], addr[r] = addr[r], addr[i]
	}
	return addr
}

func (r *resolver) handleIPv4Query(name string, query *dns.Msg) (*dns.Msg, error) {
	addr := r.sb.ResolveName(name)
	if addr == nil {
		return nil, nil
	}

	log.Debugf("Lookup for %s: IP %v", name, addr)

	resp := new(dns.Msg)
	resp.SetReply(query)
	setCommonFlags(resp)

	if len(addr) > 1 {
		addr = shuffleAddr(addr)
	}

	for _, ip := range addr {
		rr := new(dns.A)
		rr.Hdr = dns.RR_Header{Name: name, Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: respTTL}
		rr.A = ip
		resp.Answer = append(resp.Answer, rr)
	}
	return resp, nil
}

func (r *resolver) handlePTRQuery(ptr string, query *dns.Msg) (*dns.Msg, error) {
	parts := []string{}

	if strings.HasSuffix(ptr, ptrIPv4domain) {
		parts = strings.Split(ptr, ptrIPv4domain)
	} else if strings.HasSuffix(ptr, ptrIPv6domain) {
		parts = strings.Split(ptr, ptrIPv6domain)
	} else {
		return nil, fmt.Errorf("invalid PTR query, %v", ptr)
	}

	host := r.sb.ResolveIP(parts[0])
	if len(host) == 0 {
		return nil, nil
	}

	log.Debugf("Lookup for IP %s: name %s", parts[0], host)
	fqdn := dns.Fqdn(host)

	resp := new(dns.Msg)
	resp.SetReply(query)
	setCommonFlags(resp)

	rr := new(dns.PTR)
	rr.Hdr = dns.RR_Header{Name: ptr, Rrtype: dns.TypePTR, Class: dns.ClassINET, Ttl: respTTL}
	rr.Ptr = fqdn
	resp.Answer = append(resp.Answer, rr)
	return resp, nil
}

func truncateResp(resp *dns.Msg, maxSize int, isTCP bool) {
	if !isTCP {
		resp.Truncated = true
	}

	// trim the Answer RRs one by one till the whole message fits
	// within the reply size
	for resp.Len() > maxSize {
		resp.Answer = resp.Answer[:len(resp.Answer)-1]
	}
}

func (r *resolver) ServeDNS(w dns.ResponseWriter, query *dns.Msg) {
	var (
		extConn net.Conn
		resp    *dns.Msg
		err     error
	)

	if query == nil || len(query.Question) == 0 {
		return
	}
	name := query.Question[0].Name
	if query.Question[0].Qtype == dns.TypeA {
		resp, err = r.handleIPv4Query(name, query)
	} else if query.Question[0].Qtype == dns.TypePTR {
		resp, err = r.handlePTRQuery(name, query)
	}

	if err != nil {
		log.Error(err)
		return
	}

	proto := w.LocalAddr().Network()
	maxSize := 0
	if proto == "tcp" {
		maxSize = dns.MaxMsgSize - 1
	} else if proto == "udp" {
		optRR := query.IsEdns0()
		if optRR != nil {
			maxSize = int(optRR.UDPSize())
		}
		if maxSize < defaultRespSize {
			maxSize = defaultRespSize
		}
	}

	if resp != nil {
		if resp.Len() > maxSize {
			truncateResp(resp, maxSize, proto == "tcp")
		}
	} else {
		for i := 0; i < maxExtDNS; i++ {
			extDNS := &r.extDNSList[i]
			if extDNS.ipStr == "" {
				break
			}
			log.Debugf("Querying ext dns %s:%s for %s[%d]", proto, extDNS.ipStr, name, query.Question[0].Qtype)

			extConnect := func() {
				addr := fmt.Sprintf("%s:%d", extDNS.ipStr, 53)
				extConn, err = net.DialTimeout(proto, addr, extIOTimeout)
			}

			// For udp clients connection is persisted to reuse for further queries.
			// Accessing extDNS.extConn be a race here between go rouines. Hence the
			// connection setup is done in a Once block and fetch the extConn again
			extConn = extDNS.extConn
			if extConn == nil || proto == "tcp" {
				if proto == "udp" {
					extDNS.extOnce.Do(func() {
						r.sb.execFunc(extConnect)
						extDNS.extConn = extConn
					})
					extConn = extDNS.extConn
				} else {
					r.sb.execFunc(extConnect)
				}
				if err != nil {
					log.Debugf("Connect failed, %s", err)
					continue
				}
			}

			// Timeout has to be set for every IO operation.
			extConn.SetDeadline(time.Now().Add(extIOTimeout))
			co := &dns.Conn{Conn: extConn}

			defer func() {
				if proto == "tcp" {
					co.Close()
				}
			}()
			err = co.WriteMsg(query)
			if err != nil {
				log.Debugf("Send to DNS server failed, %s", err)
				continue
			}

			resp, err = co.ReadMsg()
			if err != nil {
				log.Debugf("Read from DNS server failed, %s", err)
				continue
			}

			resp.Compress = true
			break
		}

		if resp == nil {
			return
		}
	}

	err = w.WriteMsg(resp)
	if err != nil {
		log.Errorf("error writing resolver resp, %s", err)
	}
}