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upstream.go
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upstream.go
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package dns
import (
"context"
"errors"
"fmt"
"net"
"runtime"
"sync"
"sync/atomic"
"time"
"github.com/cenkalti/backoff/v4"
"github.com/hashicorp/go-multierror"
"github.com/miekg/dns"
log "github.com/sirupsen/logrus"
"github.com/netbirdio/netbird/client/internal/peer"
)
const (
failsTillDeact = int32(5)
reactivatePeriod = 30 * time.Second
upstreamTimeout = 15 * time.Second
probeTimeout = 2 * time.Second
)
const testRecord = "."
type upstreamClient interface {
exchange(ctx context.Context, upstream string, r *dns.Msg) (*dns.Msg, time.Duration, error)
}
type UpstreamResolver interface {
serveDNS(r *dns.Msg) (*dns.Msg, time.Duration, error)
upstreamExchange(upstream string, r *dns.Msg) (*dns.Msg, time.Duration, error)
}
type upstreamResolverBase struct {
ctx context.Context
cancel context.CancelFunc
upstreamClient upstreamClient
upstreamServers []string
disabled bool
failsCount atomic.Int32
failsTillDeact int32
mutex sync.Mutex
reactivatePeriod time.Duration
upstreamTimeout time.Duration
deactivate func(error)
reactivate func()
statusRecorder *peer.Status
}
func newUpstreamResolverBase(ctx context.Context, statusRecorder *peer.Status) *upstreamResolverBase {
ctx, cancel := context.WithCancel(ctx)
return &upstreamResolverBase{
ctx: ctx,
cancel: cancel,
upstreamTimeout: upstreamTimeout,
reactivatePeriod: reactivatePeriod,
failsTillDeact: failsTillDeact,
statusRecorder: statusRecorder,
}
}
func (u *upstreamResolverBase) stop() {
log.Debugf("stopping serving DNS for upstreams %s", u.upstreamServers)
u.cancel()
}
// ServeDNS handles a DNS request
func (u *upstreamResolverBase) ServeDNS(w dns.ResponseWriter, r *dns.Msg) {
var err error
defer func() {
u.checkUpstreamFails(err)
}()
log.WithField("question", r.Question[0]).Trace("received an upstream question")
select {
case <-u.ctx.Done():
return
default:
}
for _, upstream := range u.upstreamServers {
var rm *dns.Msg
var t time.Duration
func() {
ctx, cancel := context.WithTimeout(u.ctx, u.upstreamTimeout)
defer cancel()
rm, t, err = u.upstreamClient.exchange(ctx, upstream, r)
}()
if err != nil {
if errors.Is(err, context.DeadlineExceeded) || isTimeout(err) {
log.WithError(err).WithField("upstream", upstream).
Warn("got an error while connecting to upstream")
continue
}
u.failsCount.Add(1)
log.WithError(err).WithField("upstream", upstream).
Error("got other error while querying the upstream")
return
}
if rm == nil {
log.WithError(err).WithField("upstream", upstream).
Warn("no response from upstream")
return
}
// those checks need to be independent of each other due to memory address issues
if !rm.Response {
log.WithError(err).WithField("upstream", upstream).
Warn("no response from upstream")
return
}
log.Tracef("took %s to query the upstream %s", t, upstream)
err = w.WriteMsg(rm)
if err != nil {
log.WithError(err).Error("got an error while writing the upstream resolver response")
}
// count the fails only if they happen sequentially
u.failsCount.Store(0)
return
}
u.failsCount.Add(1)
log.Error("all queries to the upstream nameservers failed with timeout")
}
// checkUpstreamFails counts fails and disables or enables upstream resolving
//
// If fails count is greater that failsTillDeact, upstream resolving
// will be disabled for reactivatePeriod, after that time period fails counter
// will be reset and upstream will be reactivated.
func (u *upstreamResolverBase) checkUpstreamFails(err error) {
u.mutex.Lock()
defer u.mutex.Unlock()
if u.failsCount.Load() < u.failsTillDeact || u.disabled {
return
}
select {
case <-u.ctx.Done():
return
default:
}
u.disable(err)
}
// probeAvailability tests all upstream servers simultaneously and
// disables the resolver if none work
func (u *upstreamResolverBase) probeAvailability() {
u.mutex.Lock()
defer u.mutex.Unlock()
select {
case <-u.ctx.Done():
return
default:
}
var success bool
var mu sync.Mutex
var wg sync.WaitGroup
var errors *multierror.Error
for _, upstream := range u.upstreamServers {
upstream := upstream
wg.Add(1)
go func() {
defer wg.Done()
err := u.testNameserver(upstream)
if err != nil {
errors = multierror.Append(errors, err)
log.Warnf("probing upstream nameserver %s: %s", upstream, err)
return
}
mu.Lock()
defer mu.Unlock()
success = true
}()
}
wg.Wait()
// didn't find a working upstream server, let's disable and try later
if !success {
u.disable(errors.ErrorOrNil())
}
}
// waitUntilResponse retries, in an exponential interval, querying the upstream servers until it gets a positive response
func (u *upstreamResolverBase) waitUntilResponse() {
exponentialBackOff := &backoff.ExponentialBackOff{
InitialInterval: 500 * time.Millisecond,
RandomizationFactor: 0.5,
Multiplier: 1.1,
MaxInterval: u.reactivatePeriod,
MaxElapsedTime: 0,
Stop: backoff.Stop,
Clock: backoff.SystemClock,
}
operation := func() error {
select {
case <-u.ctx.Done():
return backoff.Permanent(fmt.Errorf("exiting upstream retry loop for upstreams %s: parent context has been canceled", u.upstreamServers))
default:
}
for _, upstream := range u.upstreamServers {
if err := u.testNameserver(upstream); err != nil {
log.Tracef("upstream check for %s: %s", upstream, err)
} else {
// at least one upstream server is available, stop probing
return nil
}
}
log.Tracef("checking connectivity with upstreams %s failed. Retrying in %s", u.upstreamServers, exponentialBackOff.NextBackOff())
return fmt.Errorf("upstream check call error")
}
err := backoff.Retry(operation, exponentialBackOff)
if err != nil {
log.Warn(err)
return
}
log.Infof("upstreams %s are responsive again. Adding them back to system", u.upstreamServers)
u.failsCount.Store(0)
u.reactivate()
u.disabled = false
}
// isTimeout returns true if the given error is a network timeout error.
//
// Copied from k8s.io/apimachinery/pkg/util/net.IsTimeout
func isTimeout(err error) bool {
var neterr net.Error
if errors.As(err, &neterr) {
return neterr != nil && neterr.Timeout()
}
return false
}
func (u *upstreamResolverBase) disable(err error) {
if u.disabled {
return
}
// todo test the deactivation logic, it seems to affect the client
if runtime.GOOS != "ios" {
log.Warnf("Upstream resolving is Disabled for %v", reactivatePeriod)
u.deactivate(err)
u.disabled = true
go u.waitUntilResponse()
}
}
func (u *upstreamResolverBase) testNameserver(server string) error {
ctx, cancel := context.WithTimeout(u.ctx, probeTimeout)
defer cancel()
r := new(dns.Msg).SetQuestion(testRecord, dns.TypeSOA)
_, _, err := u.upstreamClient.exchange(ctx, server, r)
return err
}