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monitor.go
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monitor.go
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// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package monitor provides facilities for monitoring network
// interface and route changes. It primarily exists to know when
// portable devices move between different networks.
package monitor
import (
"encoding/json"
"errors"
"net/netip"
"runtime"
"sync"
"time"
"tailscale.com/net/interfaces"
"tailscale.com/types/logger"
)
// pollWallTimeInterval is how often we check the time to check
// for big jumps in wall (non-monotonic) time as a backup mechanism
// to get notified of a sleeping device waking back up.
// Usually there are also minor network change events on wake that let
// us check the wall time sooner than this.
const pollWallTimeInterval = 15 * time.Second
// message represents a message returned from an osMon.
type message interface {
// Ignore is whether we should ignore this message.
ignore() bool
}
// osMon is the interface that each operating system-specific
// implementation of the link monitor must implement.
type osMon interface {
Close() error
// Receive returns a new network interface change message. It
// should block until there's either something to return, or
// until the osMon is closed. After a Close, the returned
// error is ignored.
Receive() (message, error)
// IsInterestingInterface reports whether the provided interface should
// be considered for network change events.
IsInterestingInterface(iface string) bool
}
// ChangeFunc is a callback function that's called when the network
// changed. The changed parameter is whether the network changed
// enough for interfaces.State to have changed since the last
// callback.
type ChangeFunc func(changed bool, state *interfaces.State)
// An allocated callbackHandle's address is the Mon.cbs map key.
type callbackHandle byte
// Mon represents a monitoring instance.
type Mon struct {
logf logger.Logf
om osMon // nil means not supported on this platform
change chan struct{}
stop chan struct{} // closed on Stop
mu sync.Mutex // guards all following fields
cbs map[*callbackHandle]ChangeFunc
ruleDelCB map[*callbackHandle]RuleDeleteCallback
ifState *interfaces.State
gwValid bool // whether gw and gwSelfIP are valid
gw netip.Addr // our gateway's IP
gwSelfIP netip.Addr // our own IP address (that corresponds to gw)
started bool
closed bool
goroutines sync.WaitGroup
wallTimer *time.Timer // nil until Started; re-armed AfterFunc per tick
lastWall time.Time
timeJumped bool // whether we need to send a changed=true after a big time jump
}
// New instantiates and starts a monitoring instance.
// The returned monitor is inactive until it's started by the Start method.
// Use RegisterChangeCallback to get notified of network changes.
func New(logf logger.Logf) (*Mon, error) {
logf = logger.WithPrefix(logf, "monitor: ")
m := &Mon{
logf: logf,
cbs: map[*callbackHandle]ChangeFunc{},
change: make(chan struct{}, 1),
stop: make(chan struct{}),
lastWall: wallTime(),
}
st, err := m.interfaceStateUncached()
if err != nil {
return nil, err
}
m.ifState = st
m.om, err = newOSMon(logf, m)
if err != nil {
return nil, err
}
if m.om == nil {
return nil, errors.New("newOSMon returned nil, nil")
}
return m, nil
}
// InterfaceState returns the latest snapshot of the machine's network
// interfaces.
//
// The returned value is owned by Mon; it must not be modified.
func (m *Mon) InterfaceState() *interfaces.State {
m.mu.Lock()
defer m.mu.Unlock()
return m.ifState
}
func (m *Mon) interfaceStateUncached() (*interfaces.State, error) {
return interfaces.GetState()
}
// GatewayAndSelfIP returns the current network's default gateway, and
// the machine's default IP for that gateway.
//
// It's the same as interfaces.LikelyHomeRouterIP, but it caches the
// result until the monitor detects a network change.
func (m *Mon) GatewayAndSelfIP() (gw, myIP netip.Addr, ok bool) {
m.mu.Lock()
defer m.mu.Unlock()
if m.gwValid {
return m.gw, m.gwSelfIP, true
}
gw, myIP, ok = interfaces.LikelyHomeRouterIP()
if ok {
m.gw, m.gwSelfIP, m.gwValid = gw, myIP, true
}
return gw, myIP, ok
}
// RegisterChangeCallback adds callback to the set of parties to be
// notified (in their own goroutine) when the network state changes.
// To remove this callback, call unregister (or close the monitor).
func (m *Mon) RegisterChangeCallback(callback ChangeFunc) (unregister func()) {
handle := new(callbackHandle)
m.mu.Lock()
defer m.mu.Unlock()
m.cbs[handle] = callback
return func() {
m.mu.Lock()
defer m.mu.Unlock()
delete(m.cbs, handle)
}
}
// RuleDeleteCallback is a callback when a Linux IP policy routing
// rule is deleted. The table is the table number (52, 253, 354) and
// priority is the priority order number (for Tailscale rules
// currently: 5210, 5230, 5250, 5270)
type RuleDeleteCallback func(table uint8, priority uint32)
// RegisterRuleDeleteCallback adds callback to the set of parties to be
// notified (in their own goroutine) when a Linux ip rule is deleted.
// To remove this callback, call unregister (or close the monitor).
func (m *Mon) RegisterRuleDeleteCallback(callback RuleDeleteCallback) (unregister func()) {
handle := new(callbackHandle)
m.mu.Lock()
defer m.mu.Unlock()
if m.ruleDelCB == nil {
m.ruleDelCB = map[*callbackHandle]RuleDeleteCallback{}
}
m.ruleDelCB[handle] = callback
return func() {
m.mu.Lock()
defer m.mu.Unlock()
delete(m.ruleDelCB, handle)
}
}
// Start starts the monitor.
// A monitor can only be started & closed once.
func (m *Mon) Start() {
m.mu.Lock()
defer m.mu.Unlock()
if m.started || m.closed {
return
}
m.started = true
if shouldMonitorTimeJump {
m.wallTimer = time.AfterFunc(pollWallTimeInterval, m.pollWallTime)
}
if m.om == nil {
return
}
m.goroutines.Add(2)
go m.pump()
go m.debounce()
}
// Close closes the monitor.
func (m *Mon) Close() error {
m.mu.Lock()
if m.closed {
m.mu.Unlock()
return nil
}
m.closed = true
close(m.stop)
if m.wallTimer != nil {
m.wallTimer.Stop()
}
var err error
if m.om != nil {
err = m.om.Close()
}
started := m.started
m.mu.Unlock()
if started {
m.goroutines.Wait()
}
return err
}
// InjectEvent forces the monitor to pretend there was a network
// change and re-check the state of the network. Any registered
// ChangeFunc callbacks will be called within the event coalescing
// period (under a fraction of a second).
func (m *Mon) InjectEvent() {
select {
case m.change <- struct{}{}:
default:
// Another change signal is already
// buffered. Debounce will wake up soon
// enough.
}
}
func (m *Mon) stopped() bool {
select {
case <-m.stop:
return true
default:
return false
}
}
// pump continuously retrieves messages from the connection, notifying
// the change channel of changes, and stopping when a stop is issued.
func (m *Mon) pump() {
defer m.goroutines.Done()
for !m.stopped() {
msg, err := m.om.Receive()
if err != nil {
if m.stopped() {
return
}
// Keep retrying while we're not closed.
m.logf("error from link monitor: %v", err)
time.Sleep(time.Second)
continue
}
if rdm, ok := msg.(ipRuleDeletedMessage); ok {
m.notifyRuleDeleted(rdm)
continue
}
if msg.ignore() {
continue
}
m.InjectEvent()
}
}
func (m *Mon) notifyRuleDeleted(rdm ipRuleDeletedMessage) {
m.mu.Lock()
defer m.mu.Unlock()
for _, cb := range m.ruleDelCB {
go cb(rdm.table, rdm.priority)
}
}
// isInterestingInterface reports whether the provided interface should be
// considered when checking for network state changes.
// The ips parameter should be the IPs of the provided interface.
func (m *Mon) isInterestingInterface(i interfaces.Interface, ips []netip.Prefix) bool {
return m.om.IsInterestingInterface(i.Name) && interfaces.UseInterestingInterfaces(i, ips)
}
// debounce calls the callback function with a delay between events
// and exits when a stop is issued.
func (m *Mon) debounce() {
defer m.goroutines.Done()
for {
select {
case <-m.stop:
return
case <-m.change:
}
if curState, err := m.interfaceStateUncached(); err != nil {
m.logf("interfaces.State: %v", err)
} else {
m.mu.Lock()
oldState := m.ifState
changed := !curState.EqualFiltered(oldState, m.isInterestingInterface, interfaces.UseInterestingIPs)
if changed {
m.gwValid = false
m.ifState = curState
if s1, s2 := oldState.String(), curState.String(); s1 == s2 {
m.logf("[unexpected] network state changed, but stringification didn't: %v", s1)
m.logf("[unexpected] old: %s", jsonSummary(oldState))
m.logf("[unexpected] new: %s", jsonSummary(curState))
}
}
// See if we have a queued or new time jump signal.
if shouldMonitorTimeJump && m.checkWallTimeAdvanceLocked() {
m.resetTimeJumpedLocked()
if !changed {
// Only log if it wasn't an interesting change.
m.logf("time jumped (probably wake from sleep); synthesizing major change event")
changed = true
}
}
for _, cb := range m.cbs {
go cb(changed, m.ifState)
}
m.mu.Unlock()
}
select {
case <-m.stop:
return
case <-time.After(250 * time.Millisecond):
}
}
}
func jsonSummary(x any) any {
j, err := json.Marshal(x)
if err != nil {
return err
}
return j
}
func wallTime() time.Time {
// From time package's docs: "The canonical way to strip a
// monotonic clock reading is to use t = t.Round(0)."
return time.Now().Round(0)
}
func (m *Mon) pollWallTime() {
m.mu.Lock()
defer m.mu.Unlock()
if m.closed {
return
}
if m.checkWallTimeAdvanceLocked() {
m.InjectEvent()
}
m.wallTimer.Reset(pollWallTimeInterval)
}
// shouldMonitorTimeJump is whether we keep a regular periodic timer running in
// the background watching for jumps in wall time.
//
// We don't do this on mobile platforms for battery reasons, and because these
// platforms don't really sleep in the same way.
const shouldMonitorTimeJump = runtime.GOOS != "android" && runtime.GOOS != "ios"
// checkWallTimeAdvanceLocked reports whether wall time jumped more than 150% of
// pollWallTimeInterval, indicating we probably just came out of sleep. Once a
// time jump is detected it must be reset by calling resetTimeJumpedLocked.
func (m *Mon) checkWallTimeAdvanceLocked() bool {
if !shouldMonitorTimeJump {
panic("unreachable") // if callers are correct
}
now := wallTime()
if now.Sub(m.lastWall) > pollWallTimeInterval*3/2 {
m.timeJumped = true // it is reset by debounce.
}
m.lastWall = now
return m.timeJumped
}
// resetTimeJumpedLocked consumes the signal set by checkWallTimeAdvanceLocked.
func (m *Mon) resetTimeJumpedLocked() {
m.timeJumped = false
}
type ipRuleDeletedMessage struct {
table uint8
priority uint32
}
func (ipRuleDeletedMessage) ignore() bool { return true }