syncthing/lib/nat/service.go

// Copyright (C) 2015 The Syncthing Authors.
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this file,
// You can obtain one at https://mozilla.org/MPL/2.0/.

package nat

import (
	"context"
	"fmt"
	"hash/fnv"
	"log/slog"
	"math/rand"
	"net"
	"slices"
	"sync"
	"time"

	"github.com/syncthing/syncthing/internal/slogutil"
	"github.com/syncthing/syncthing/lib/config"
	"github.com/syncthing/syncthing/lib/protocol"
)

// Service runs a loop for discovery of IGDs (Internet Gateway Devices) and
// setup/renewal of a port mapping.
type Service struct {
	id               protocol.DeviceID
	cfg              config.Wrapper
	processScheduled chan struct{}

	mappings []*Mapping
	enabled  bool
	mut      sync.RWMutex
}

func NewService(id protocol.DeviceID, cfg config.Wrapper) *Service {
	s := &Service{
		id:               id,
		cfg:              cfg,
		processScheduled: make(chan struct{}, 1),
	}
	cfgCopy := cfg.RawCopy()
	s.CommitConfiguration(cfgCopy, cfgCopy)
	return s
}

func (s *Service) CommitConfiguration(_, to config.Configuration) bool {
	s.mut.Lock()
	if !s.enabled && to.Options.NATEnabled {
		slog.Debug("Starting NAT service")
		s.enabled = true
		s.scheduleProcess()
	} else if s.enabled && !to.Options.NATEnabled {
		slog.Debug("Stopping NAT service")
		s.enabled = false
	}
	s.mut.Unlock()
	return true
}

func (s *Service) Serve(ctx context.Context) error {
	s.cfg.Subscribe(s)
	defer s.cfg.Unsubscribe(s)

	var announce sync.Once

	timer := time.NewTimer(0)

	for {
		select {
		case <-timer.C:
		case <-s.processScheduled:
			if !timer.Stop() {
				select {
				case <-timer.C:
				default:
				}
			}
		case <-ctx.Done():
			timer.Stop()
			s.mut.RLock()
			for _, mapping := range s.mappings {
				mapping.clearAddresses()
			}
			s.mut.RUnlock()
			return ctx.Err()
		}
		s.mut.RLock()
		enabled := s.enabled
		s.mut.RUnlock()
		if !enabled {
			continue
		}
		found, renewIn := s.process(ctx)
		timer.Reset(renewIn)
		if found != -1 {
			announce.Do(func() {
				slog.Info("Detected NAT services", "count", found)
			})
		}
	}
}

func (s *Service) process(ctx context.Context) (int, time.Duration) {
	// toRenew are mappings which are due for renewal
	// toUpdate are the remaining mappings, which will only be updated if one of
	// the old IGDs has gone away, or a new IGD has appeared, but only if we
	// actually need to perform a renewal.
	var toRenew, toUpdate []*Mapping

	renewIn := time.Duration(s.cfg.Options().NATRenewalM) * time.Minute
	if renewIn == 0 {
		// We always want to do renewal so lets just pick a nice sane number.
		renewIn = 30 * time.Minute
	}

	s.mut.RLock()
	for _, mapping := range s.mappings {
		mapping.mut.RLock()
		expires := mapping.expires
		mapping.mut.RUnlock()

		if expires.Before(time.Now()) {
			toRenew = append(toRenew, mapping)
		} else {
			toUpdate = append(toUpdate, mapping)
			mappingRenewIn := time.Until(expires)
			if mappingRenewIn < renewIn {
				renewIn = mappingRenewIn
			}
		}
	}
	s.mut.RUnlock()

	// Don't do anything, unless we really need to renew
	if len(toRenew) == 0 {
		return -1, renewIn
	}

	nats := discoverAll(ctx, time.Duration(s.cfg.Options().NATRenewalM)*time.Minute, time.Duration(s.cfg.Options().NATTimeoutS)*time.Second)

	for _, mapping := range toRenew {
		s.updateMapping(ctx, mapping, nats, true)
	}

	for _, mapping := range toUpdate {
		s.updateMapping(ctx, mapping, nats, false)
	}

	return len(nats), renewIn
}

func (s *Service) scheduleProcess() {
	select {
	case s.processScheduled <- struct{}{}: // 1-buffered
	default:
	}
}

func (s *Service) NewMapping(protocol Protocol, ipVersion IPVersion, ip net.IP, port int) *Mapping {
	mapping := &Mapping{
		protocol: protocol,
		address: Address{
			IP:   ip,
			Port: port,
		},
		extAddresses: make(map[string][]Address),
		ipVersion:    ipVersion,
	}

	s.mut.Lock()
	s.mappings = append(s.mappings, mapping)
	s.mut.Unlock()
	s.scheduleProcess()

	return mapping
}

// RemoveMapping does not actually remove the mapping from the IGD, it just
// internally removes it which stops renewing the mapping. Also, it clears any
// existing mapped addresses from the mapping, which as a result should cause
// discovery to reannounce the new addresses.
func (s *Service) RemoveMapping(mapping *Mapping) {
	s.mut.Lock()
	defer s.mut.Unlock()
	for i, existing := range s.mappings {
		if existing == mapping {
			mapping.clearAddresses()
			last := len(s.mappings) - 1
			s.mappings[i] = s.mappings[last]
			s.mappings[last] = nil
			s.mappings = s.mappings[:last]
			return
		}
	}
}

// updateMapping compares the addresses of the existing mapping versus the natds
// discovered, and removes any addresses of natds that do not exist, or tries to
// acquire mappings for natds which the mapping was unaware of before.
// Optionally takes renew flag which indicates whether or not we should renew
// mappings with existing natds
func (s *Service) updateMapping(ctx context.Context, mapping *Mapping, nats map[string]Device, renew bool) {
	renewalTime := time.Duration(s.cfg.Options().NATRenewalM) * time.Minute

	mapping.mut.Lock()

	mapping.expires = time.Now().Add(renewalTime)
	change := s.verifyExistingLocked(ctx, mapping, nats, renew)
	add := s.acquireNewLocked(ctx, mapping, nats)

	mapping.mut.Unlock()

	if change || add {
		mapping.notify()
	}
}

func (s *Service) verifyExistingLocked(ctx context.Context, mapping *Mapping, nats map[string]Device, renew bool) (change bool) {
	leaseTime := time.Duration(s.cfg.Options().NATLeaseM) * time.Minute

	for id, extAddrs := range mapping.extAddresses {
		select {
		case <-ctx.Done():
			return false
		default:
		}

		if nat, ok := nats[id]; !ok || len(extAddrs) == 0 {
			// Delete addresses for NATDevice's that do not exist anymore
			mapping.removeAddressLocked(id)
			change = true
			continue
		} else if renew {
			// Only perform renewals on the nat's that have the right local IP
			// address. For IPv6 the IP addresses are discovered by the service itself,
			// so this check is skipped.
			localIP := nat.GetLocalIPv4Address()
			if !mapping.validGateway(localIP) && nat.SupportsIPVersion(IPv4Only) {
				l.Debugf("Skipping %s for %s because of IP mismatch. %s != %s", id, mapping, mapping.address.IP, localIP)
				continue
			}

			if !nat.SupportsIPVersion(mapping.ipVersion) {
				l.Debugf("Skipping renew on gateway %s because it doesn't match the listener address family", nat.ID())
				continue
			}

			l.Debugf("Renewing %s -> %v open port on %s", mapping, extAddrs, id)
			// extAddrs either contains one IPv4 address, or possibly several
			// IPv6 addresses all using the same port.  Therefore the first
			// entry always has the external port.
			responseAddrs, err := s.tryNATDevice(ctx, nat, mapping.address, extAddrs[0].Port, mapping.protocol, leaseTime)
			if err != nil {
				slog.WarnContext(ctx, "Failed to renew open port", slog.String("mapping", mapping.String()), slog.Any("addresses", extAddrs), slog.String("id", id), slogutil.Error(err))
				mapping.removeAddressLocked(id)
				change = true
				continue
			}

			l.Debugf("Renewed %s -> %v open port on %s", mapping, extAddrs, id)

			// We shouldn't rely on the order in which the addresses are returned.
			// Therefore, we test for set equality and report change if there is any difference.
			if !addrSetsEqual(responseAddrs, extAddrs) {
				mapping.setAddressLocked(id, responseAddrs)
				change = true
			}
		}
	}

	return change
}

func (s *Service) acquireNewLocked(ctx context.Context, mapping *Mapping, nats map[string]Device) (change bool) {
	leaseTime := time.Duration(s.cfg.Options().NATLeaseM) * time.Minute
	addrMap := mapping.extAddresses

	for id, nat := range nats {
		select {
		case <-ctx.Done():
			return false
		default:
		}

		if _, ok := addrMap[id]; ok {
			continue
		}

		// Only perform mappings on the nat's that have the right local IP
		// address
		localIP := nat.GetLocalIPv4Address()
		if !mapping.validGateway(localIP) && nat.SupportsIPVersion(IPv4Only) {
			l.Debugf("Skipping %s for %s because of IP mismatch. %s != %s", id, mapping, mapping.address.IP, localIP)
			continue
		}

		l.Debugf("Trying to open port %s on %s", mapping, id)

		if !nat.SupportsIPVersion(mapping.ipVersion) {
			l.Debugf("Skipping firewall traversal on gateway %s because it doesn't match the listener address family", nat.ID())
			continue
		}

		addrs, err := s.tryNATDevice(ctx, nat, mapping.address, 0, mapping.protocol, leaseTime)
		if err != nil {
			slog.WarnContext(ctx, "Failed to acquire open port", slog.String("mapping", mapping.String()), slog.String("id", id), slogutil.Error(err))
			continue
		}

		l.Debugf("Opened port %s -> %v on %s", mapping, addrs, id)
		mapping.setAddressLocked(id, addrs)
		change = true
	}

	return change
}

// tryNATDevice tries to acquire a port mapping for the given internal address to
// the given external port. If external port is 0, picks a pseudo-random port.
func (s *Service) tryNATDevice(ctx context.Context, natd Device, intAddr Address, extPort int, protocol Protocol, leaseTime time.Duration) ([]Address, error) {
	var err error
	var port int
	// For IPv6, we just try to create the pinhole. If it fails, nothing can be done (probably no IGDv2 support).
	// If it already exists, the relevant UPnP standard requires that the gateway recognizes this and updates the lease time.
	// Since we usually have a global unicast IPv6 address so no conflicting mappings, we just request the port we're running on
	if natd.SupportsIPVersion(IPv6Only) {
		ipaddrs, err := natd.AddPinhole(ctx, protocol, intAddr, leaseTime)
		var addrs []Address
		for _, ipaddr := range ipaddrs {
			addrs = append(addrs, Address{
				ipaddr,
				intAddr.Port,
			})
		}

		if err != nil {
			l.Debugln("Error extending lease on", natd.ID(), err)
		}
		return addrs, err
	}
	// Generate a predictable random which is based on device ID + local port + hash of the device ID
	// number so that the ports we'd try to acquire for the mapping would always be the same for the
	// same device trying to get the same internal port.
	predictableRand := rand.New(rand.NewSource(int64(s.id.Short()) + int64(intAddr.Port) + hash(natd.ID())))

	if extPort != 0 {
		// First try renewing our existing mapping, if we have one.
		name := fmt.Sprintf("syncthing-%d", extPort)
		port, err = natd.AddPortMapping(ctx, protocol, intAddr.Port, extPort, name, leaseTime)
		if err == nil {
			extPort = port
			goto findIP
		}
		l.Debugf("Error extending lease on %v (external port %d -> internal port %d): %v", natd.ID(), extPort, intAddr.Port, err)
	}

	for i := 0; i < 10; i++ {
		select {
		case <-ctx.Done():
			return []Address{}, ctx.Err()
		default:
		}

		// Then try up to ten random ports.
		extPort = 1024 + predictableRand.Intn(65535-1024)
		name := fmt.Sprintf("syncthing-%d", extPort)
		port, err = natd.AddPortMapping(ctx, protocol, intAddr.Port, extPort, name, leaseTime)
		if err == nil {
			extPort = port
			goto findIP
		}
		err = fmt.Errorf("getting new lease on %s (external port %d -> internal port %d): %w", natd.ID(), extPort, intAddr.Port, err)
		l.Debugf("Error %s", err)
	}

	return nil, err

findIP:
	ip, err := natd.GetExternalIPv4Address(ctx)
	if err != nil {
		l.Debugf("Error getting external ip on %s: %s", natd.ID(), err)
		ip = nil
	}
	return []Address{
		{
			IP:   ip,
			Port: extPort,
		},
	}, nil
}

func (s *Service) String() string {
	return fmt.Sprintf("nat.Service@%p", s)
}

func hash(input string) int64 {
	h := fnv.New64a()
	h.Write([]byte(input))
	return int64(h.Sum64())
}

func addrSetsEqual(a []Address, b []Address) bool {
	if len(a) != len(b) {
		return false
	}

	for _, v := range a {
		if !slices.ContainsFunc(b, v.Equal) {
			return false
		}
	}

	return true
}