syncthing/internal/discover/client_udp.go

// Copyright (C) 2014 The Syncthing Authors.
//
// This program is free software: you can redistribute it and/or modify it
// under the terms of the GNU General Public License as published by the Free
// Software Foundation, either version 3 of the License, or (at your option)
// any later version.
//
// This program is distributed in the hope that it will be useful, but WITHOUT
// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
// more details.
//
// You should have received a copy of the GNU General Public License along
// with this program. If not, see <http://www.gnu.org/licenses/>.

package discover

import (
	"encoding/hex"
	"io"
	"net"
	"net/url"
	"strconv"
	"sync"
	"time"

	"github.com/syncthing/syncthing/internal/protocol"
)

func init() {
	for _, proto := range []string{"udp", "udp4", "udp6"} {
		Register(proto, func(uri *url.URL, pkt *Announce) (Client, error) {
			c := &UDPClient{}
			err := c.Start(uri, pkt)
			if err != nil {
				return nil, err
			}
			return c, nil
		})
	}
}

type UDPClient struct {
	url *url.URL

	id protocol.DeviceID

	stop          chan struct{}
	wg            sync.WaitGroup
	listenAddress *net.UDPAddr

	globalBroadcastInterval time.Duration
	errorRetryInterval      time.Duration

	status bool
	mut    sync.RWMutex
}

func (d *UDPClient) Start(uri *url.URL, pkt *Announce) error {
	d.url = uri
	d.id = protocol.DeviceIDFromBytes(pkt.This.ID)
	d.stop = make(chan struct{})

	params := uri.Query()
	// The address must not have a port, as otherwise both announce and lookup
	// sockets would try to bind to the same port.
	addr, err := net.ResolveUDPAddr(d.url.Scheme, params.Get("listenaddress")+":0")
	if err != nil {
		return err
	}
	d.listenAddress = addr

	broadcastSeconds, err := strconv.ParseUint(params.Get("broadcast"), 0, 0)
	if err != nil {
		d.globalBroadcastInterval = DefaultGlobalBroadcastInterval
	} else {
		d.globalBroadcastInterval = time.Duration(broadcastSeconds) * time.Second
	}

	retrySeconds, err := strconv.ParseUint(params.Get("retry"), 0, 0)
	if err != nil {
		d.errorRetryInterval = DefaultErrorRetryInternval
	} else {
		d.errorRetryInterval = time.Duration(retrySeconds) * time.Second
	}

	d.wg.Add(1)
	go d.broadcast(pkt.MustMarshalXDR())
	return nil
}

func (d *UDPClient) broadcast(pkt []byte) {
	defer d.wg.Done()
	timer := time.NewTimer(0)

	conn, err := net.ListenUDP(d.url.Scheme, d.listenAddress)
	for err != nil {
		timer.Reset(d.errorRetryInterval)
		l.Warnf("Global UDP discovery (%s): %v; trying again in %v", d.url, err, d.errorRetryInterval)
		select {
		case <-d.stop:
			return
		case <-timer.C:
		}
		conn, err = net.ListenUDP(d.url.Scheme, d.listenAddress)
	}
	defer conn.Close()

	remote, err := net.ResolveUDPAddr(d.url.Scheme, d.url.Host)
	for err != nil {
		timer.Reset(d.errorRetryInterval)
		l.Warnf("Global UDP discovery (%s): %v; trying again in %v", d.url, err, d.errorRetryInterval)
		select {
		case <-d.stop:
			return
		case <-timer.C:
		}
		remote, err = net.ResolveUDPAddr(d.url.Scheme, d.url.Host)
	}

	timer.Reset(0)

	for {
		select {
		case <-d.stop:
			return

		case <-timer.C:
			var ok bool

			if debug {
				l.Debugf("Global UDP discovery (%s): send announcement -> %v\n%s", d.url, remote, hex.Dump(pkt))
			}

			_, err := conn.WriteTo(pkt, remote)
			if err != nil {
				if debug {
					l.Debugf("discover %s: warning: %s", d.url, err)
				}
				ok = false
			} else {
				// Verify that the announce server responds positively for our device ID

				time.Sleep(1 * time.Second)

				res := d.Lookup(d.id)
				if debug {
					l.Debugf("discover %s: external lookup check: %v", d.url, res)
				}
				ok = len(res) > 0
			}

			d.mut.Lock()
			d.status = ok
			d.mut.Unlock()

			if ok {
				timer.Reset(d.globalBroadcastInterval)
			} else {
				timer.Reset(d.errorRetryInterval)
			}
		}
	}
}

func (d *UDPClient) Lookup(device protocol.DeviceID) []string {
	extIP, err := net.ResolveUDPAddr(d.url.Scheme, d.url.Host)
	if err != nil {
		if debug {
			l.Debugf("discover %s: %v; no external lookup", d.url, err)
		}
		return nil
	}

	conn, err := net.DialUDP(d.url.Scheme, d.listenAddress, extIP)
	if err != nil {
		if debug {
			l.Debugf("discover %s: %v; no external lookup", d.url, err)
		}
		return nil
	}
	defer conn.Close()

	err = conn.SetDeadline(time.Now().Add(5 * time.Second))
	if err != nil {
		if debug {
			l.Debugf("discover %s: %v; no external lookup", d.url, err)
		}
		return nil
	}

	buf := Query{QueryMagic, device[:]}.MustMarshalXDR()
	_, err = conn.Write(buf)
	if err != nil {
		if debug {
			l.Debugf("discover %s: %v; no external lookup", d.url, err)
		}
		return nil
	}

	buf = make([]byte, 2048)
	n, err := conn.Read(buf)
	if err != nil {
		if err, ok := err.(net.Error); ok && err.Timeout() {
			// Expected if the server doesn't know about requested device ID
			return nil
		}
		if debug {
			l.Debugf("discover %s: %v; no external lookup", d.url, err)
		}
		return nil
	}

	if debug {
		l.Debugf("discover %s: read external:\n%s", d.url, hex.Dump(buf[:n]))
	}

	var pkt Announce
	err = pkt.UnmarshalXDR(buf[:n])
	if err != nil && err != io.EOF {
		if debug {
			l.Debugln("discover %s:", d.url, err)
		}
		return nil
	}

	var addrs []string
	for _, a := range pkt.This.Addresses {
		deviceAddr := net.JoinHostPort(net.IP(a.IP).String(), strconv.Itoa(int(a.Port)))
		addrs = append(addrs, deviceAddr)
	}
	return addrs
}

func (d *UDPClient) Stop() {
	if d.stop != nil {
		close(d.stop)
		d.wg.Wait()
	}
}

func (d *UDPClient) StatusOK() bool {
	d.mut.RLock()
	defer d.mut.RUnlock()
	return d.status
}

func (d *UDPClient) Address() string {
	return d.url.String()
}