diff --git a/Gopkg.lock b/Gopkg.lock
index b8527d04..68e449d6 100644
--- a/Gopkg.lock
+++ b/Gopkg.lock
@@ -127,6 +127,14 @@
   revision = "8b15f938ed215522a37275106e847f6f0be85fe8"
   version = "v1.15.23"
 
+[[projects]]
+  digest = "1:cdee563173093e5ae7ab2a19c298e0904129719e1919a3c532b7bb0c3398b818"
+  name = "github.com/cenkalti/backoff"
+  packages = ["."]
+  pruneopts = "NUT"
+  revision = "1e4cf3da559842a91afcb6ea6141451e6c30c618"
+  version = "v2.1.1"
+
 [[projects]]
   digest = "1:03cfacdc6bfd46007c15786c1ece3fa074f89e5193a292f0f26d9e98c99c7cc2"
   name = "github.com/cloudflare/cloudflare-go"
@@ -667,6 +675,7 @@
     "github.com/aws/aws-sdk-go/aws/session",
     "github.com/aws/aws-sdk-go/service/lightsail",
     "github.com/aws/aws-sdk-go/service/route53",
+    "github.com/cenkalti/backoff",
     "github.com/cloudflare/cloudflare-go",
     "github.com/cpu/goacmedns",
     "github.com/decker502/dnspod-go",
diff --git a/acme/api/api.go b/acme/api/api.go
index 14b18f52..a62e49e4 100644
--- a/acme/api/api.go
+++ b/acme/api/api.go
@@ -2,12 +2,15 @@ package api
 
 import (
 	"bytes"
+	"context"
 	"crypto"
 	"encoding/json"
 	"errors"
 	"fmt"
 	"net/http"
+	"time"
 
+	"github.com/cenkalti/backoff"
 	"github.com/xenolf/lego/acme"
 	"github.com/xenolf/lego/acme/api/internal/nonces"
 	"github.com/xenolf/lego/acme/api/internal/secure"
@@ -64,34 +67,46 @@ func (a *Core) post(uri string, reqBody, response interface{}) (*http.Response,
 		return nil, errors.New("failed to marshal message")
 	}
 
-	return a.retrievablePost(uri, content, response, 0)
+	return a.retrievablePost(uri, content, response)
 }
 
 // postAsGet performs an HTTP POST ("POST-as-GET") request.
 // https://tools.ietf.org/html/draft-ietf-acme-acme-16#section-6.3
 func (a *Core) postAsGet(uri string, response interface{}) (*http.Response, error) {
-	return a.retrievablePost(uri, []byte{}, response, 0)
+	return a.retrievablePost(uri, []byte{}, response)
 }
 
-func (a *Core) retrievablePost(uri string, content []byte, response interface{}, retry int) (*http.Response, error) {
-	resp, err := a.signedPost(uri, content, response)
-	if err != nil {
-		// during tests, 5 retries allow to support ~50% of bad nonce.
-		if retry >= 5 {
-			log.Infof("too many retry on a nonce error, retry count: %d", retry)
-			return resp, err
-		}
-		switch err.(type) {
-		// Retry once if the nonce was invalidated
-		case *acme.NonceError:
-			log.Infof("nonce error retry: %s", err)
-			resp, err = a.retrievablePost(uri, content, response, retry+1)
-			if err != nil {
-				return resp, err
+func (a *Core) retrievablePost(uri string, content []byte, response interface{}) (*http.Response, error) {
+	// during tests, allow to support ~90% of bad nonce with a minimum of attempts.
+	bo := backoff.NewExponentialBackOff()
+	bo.InitialInterval = 200 * time.Millisecond
+	bo.MaxInterval = 5 * time.Second
+	bo.MaxElapsedTime = 20 * time.Second
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	var resp *http.Response
+	operation := func() error {
+		var err error
+		resp, err = a.signedPost(uri, content, response)
+		if err != nil {
+			switch err.(type) {
+			// Retry if the nonce was invalidated
+			case *acme.NonceError:
+				log.Infof("nonce error retry: %s", err)
+				return err
+			default:
+				cancel()
+				return err
 			}
-		default:
-			return resp, err
 		}
+
+		return nil
+	}
+
+	err := backoff.Retry(operation, backoff.WithContext(bo, ctx))
+	if err != nil {
+		return nil, err
 	}
 
 	return resp, nil
diff --git a/challenge/resolver/solver_manager.go b/challenge/resolver/solver_manager.go
index 55faf77a..840e19d8 100644
--- a/challenge/resolver/solver_manager.go
+++ b/challenge/resolver/solver_manager.go
@@ -1,12 +1,14 @@
 package resolver
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"sort"
 	"strconv"
 	"time"
 
+	"github.com/cenkalti/backoff"
 	"github.com/xenolf/lego/acme"
 	"github.com/xenolf/lego/acme/api"
 	"github.com/xenolf/lego/challenge"
@@ -90,16 +92,35 @@ func validate(core *api.Core, domain string, chlg acme.Challenge) error {
 		return nil
 	}
 
+	ra, err := strconv.Atoi(chlng.RetryAfter)
+	if err != nil {
+		// The ACME server MUST return a Retry-After.
+		// If it doesn't, we'll just poll hard.
+		// Boulder does not implement the ability to retry challenges or the Retry-After header.
+		// https://github.com/letsencrypt/boulder/blob/master/docs/acme-divergences.md#section-82
+		ra = 5
+	}
+	initialInterval := time.Duration(ra) * time.Second
+
+	bo := backoff.NewExponentialBackOff()
+	bo.InitialInterval = initialInterval
+	bo.MaxInterval = 10 * initialInterval
+	bo.MaxElapsedTime = 100 * initialInterval
+
+	ctx, cancel := context.WithCancel(context.Background())
+
 	// After the path is sent, the ACME server will access our server.
 	// Repeatedly check the server for an updated status on our request.
-	for {
+	operation := func() error {
 		authz, err := core.Authorizations.Get(chlng.AuthorizationURL)
 		if err != nil {
+			cancel()
 			return err
 		}
 
 		valid, err := checkAuthorizationStatus(authz)
 		if err != nil {
+			cancel()
 			return err
 		}
 
@@ -108,16 +129,10 @@ func validate(core *api.Core, domain string, chlg acme.Challenge) error {
 			return nil
 		}
 
-		ra, err := strconv.Atoi(chlng.RetryAfter)
-		if err != nil {
-			// The ACME server MUST return a Retry-After.
-			// If it doesn't, we'll just poll hard.
-			// Boulder does not implement the ability to retry challenges or the Retry-After header.
-			// https://github.com/letsencrypt/boulder/blob/master/docs/acme-divergences.md#section-82
-			ra = 5
-		}
-		time.Sleep(time.Duration(ra) * time.Second)
+		return errors.New("the server didn't respond to our request")
 	}
+
+	return backoff.Retry(operation, backoff.WithContext(bo, ctx))
 }
 
 func checkChallengeStatus(chlng acme.ExtendedChallenge) (bool, error) {
diff --git a/vendor/github.com/cenkalti/backoff/LICENSE b/vendor/github.com/cenkalti/backoff/LICENSE
new file mode 100644
index 00000000..89b81799
--- /dev/null
+++ b/vendor/github.com/cenkalti/backoff/LICENSE
@@ -0,0 +1,20 @@
+The MIT License (MIT)
+
+Copyright (c) 2014 Cenk Altı
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software is furnished to do so,
+subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
diff --git a/vendor/github.com/cenkalti/backoff/backoff.go b/vendor/github.com/cenkalti/backoff/backoff.go
new file mode 100644
index 00000000..3676ee40
--- /dev/null
+++ b/vendor/github.com/cenkalti/backoff/backoff.go
@@ -0,0 +1,66 @@
+// Package backoff implements backoff algorithms for retrying operations.
+//
+// Use Retry function for retrying operations that may fail.
+// If Retry does not meet your needs,
+// copy/paste the function into your project and modify as you wish.
+//
+// There is also Ticker type similar to time.Ticker.
+// You can use it if you need to work with channels.
+//
+// See Examples section below for usage examples.
+package backoff
+
+import "time"
+
+// BackOff is a backoff policy for retrying an operation.
+type BackOff interface {
+	// NextBackOff returns the duration to wait before retrying the operation,
+	// or backoff. Stop to indicate that no more retries should be made.
+	//
+	// Example usage:
+	//
+	// 	duration := backoff.NextBackOff();
+	// 	if (duration == backoff.Stop) {
+	// 		// Do not retry operation.
+	// 	} else {
+	// 		// Sleep for duration and retry operation.
+	// 	}
+	//
+	NextBackOff() time.Duration
+
+	// Reset to initial state.
+	Reset()
+}
+
+// Stop indicates that no more retries should be made for use in NextBackOff().
+const Stop time.Duration = -1
+
+// ZeroBackOff is a fixed backoff policy whose backoff time is always zero,
+// meaning that the operation is retried immediately without waiting, indefinitely.
+type ZeroBackOff struct{}
+
+func (b *ZeroBackOff) Reset() {}
+
+func (b *ZeroBackOff) NextBackOff() time.Duration { return 0 }
+
+// StopBackOff is a fixed backoff policy that always returns backoff.Stop for
+// NextBackOff(), meaning that the operation should never be retried.
+type StopBackOff struct{}
+
+func (b *StopBackOff) Reset() {}
+
+func (b *StopBackOff) NextBackOff() time.Duration { return Stop }
+
+// ConstantBackOff is a backoff policy that always returns the same backoff delay.
+// This is in contrast to an exponential backoff policy,
+// which returns a delay that grows longer as you call NextBackOff() over and over again.
+type ConstantBackOff struct {
+	Interval time.Duration
+}
+
+func (b *ConstantBackOff) Reset()                     {}
+func (b *ConstantBackOff) NextBackOff() time.Duration { return b.Interval }
+
+func NewConstantBackOff(d time.Duration) *ConstantBackOff {
+	return &ConstantBackOff{Interval: d}
+}
diff --git a/vendor/github.com/cenkalti/backoff/context.go b/vendor/github.com/cenkalti/backoff/context.go
new file mode 100644
index 00000000..7706faa2
--- /dev/null
+++ b/vendor/github.com/cenkalti/backoff/context.go
@@ -0,0 +1,63 @@
+package backoff
+
+import (
+	"context"
+	"time"
+)
+
+// BackOffContext is a backoff policy that stops retrying after the context
+// is canceled.
+type BackOffContext interface {
+	BackOff
+	Context() context.Context
+}
+
+type backOffContext struct {
+	BackOff
+	ctx context.Context
+}
+
+// WithContext returns a BackOffContext with context ctx
+//
+// ctx must not be nil
+func WithContext(b BackOff, ctx context.Context) BackOffContext {
+	if ctx == nil {
+		panic("nil context")
+	}
+
+	if b, ok := b.(*backOffContext); ok {
+		return &backOffContext{
+			BackOff: b.BackOff,
+			ctx:     ctx,
+		}
+	}
+
+	return &backOffContext{
+		BackOff: b,
+		ctx:     ctx,
+	}
+}
+
+func ensureContext(b BackOff) BackOffContext {
+	if cb, ok := b.(BackOffContext); ok {
+		return cb
+	}
+	return WithContext(b, context.Background())
+}
+
+func (b *backOffContext) Context() context.Context {
+	return b.ctx
+}
+
+func (b *backOffContext) NextBackOff() time.Duration {
+	select {
+	case <-b.ctx.Done():
+		return Stop
+	default:
+	}
+	next := b.BackOff.NextBackOff()
+	if deadline, ok := b.ctx.Deadline(); ok && deadline.Sub(time.Now()) < next {
+		return Stop
+	}
+	return next
+}
diff --git a/vendor/github.com/cenkalti/backoff/exponential.go b/vendor/github.com/cenkalti/backoff/exponential.go
new file mode 100644
index 00000000..a031a659
--- /dev/null
+++ b/vendor/github.com/cenkalti/backoff/exponential.go
@@ -0,0 +1,153 @@
+package backoff
+
+import (
+	"math/rand"
+	"time"
+)
+
+/*
+ExponentialBackOff is a backoff implementation that increases the backoff
+period for each retry attempt using a randomization function that grows exponentially.
+
+NextBackOff() is calculated using the following formula:
+
+ randomized interval =
+     RetryInterval * (random value in range [1 - RandomizationFactor, 1 + RandomizationFactor])
+
+In other words NextBackOff() will range between the randomization factor
+percentage below and above the retry interval.
+
+For example, given the following parameters:
+
+ RetryInterval = 2
+ RandomizationFactor = 0.5
+ Multiplier = 2
+
+the actual backoff period used in the next retry attempt will range between 1 and 3 seconds,
+multiplied by the exponential, that is, between 2 and 6 seconds.
+
+Note: MaxInterval caps the RetryInterval and not the randomized interval.
+
+If the time elapsed since an ExponentialBackOff instance is created goes past the
+MaxElapsedTime, then the method NextBackOff() starts returning backoff.Stop.
+
+The elapsed time can be reset by calling Reset().
+
+Example: Given the following default arguments, for 10 tries the sequence will be,
+and assuming we go over the MaxElapsedTime on the 10th try:
+
+ Request #  RetryInterval (seconds)  Randomized Interval (seconds)
+
+  1          0.5                     [0.25,   0.75]
+  2          0.75                    [0.375,  1.125]
+  3          1.125                   [0.562,  1.687]
+  4          1.687                   [0.8435, 2.53]
+  5          2.53                    [1.265,  3.795]
+  6          3.795                   [1.897,  5.692]
+  7          5.692                   [2.846,  8.538]
+  8          8.538                   [4.269, 12.807]
+  9         12.807                   [6.403, 19.210]
+ 10         19.210                   backoff.Stop
+
+Note: Implementation is not thread-safe.
+*/
+type ExponentialBackOff struct {
+	InitialInterval     time.Duration
+	RandomizationFactor float64
+	Multiplier          float64
+	MaxInterval         time.Duration
+	// After MaxElapsedTime the ExponentialBackOff stops.
+	// It never stops if MaxElapsedTime == 0.
+	MaxElapsedTime time.Duration
+	Clock          Clock
+
+	currentInterval time.Duration
+	startTime       time.Time
+}
+
+// Clock is an interface that returns current time for BackOff.
+type Clock interface {
+	Now() time.Time
+}
+
+// Default values for ExponentialBackOff.
+const (
+	DefaultInitialInterval     = 500 * time.Millisecond
+	DefaultRandomizationFactor = 0.5
+	DefaultMultiplier          = 1.5
+	DefaultMaxInterval         = 60 * time.Second
+	DefaultMaxElapsedTime      = 15 * time.Minute
+)
+
+// NewExponentialBackOff creates an instance of ExponentialBackOff using default values.
+func NewExponentialBackOff() *ExponentialBackOff {
+	b := &ExponentialBackOff{
+		InitialInterval:     DefaultInitialInterval,
+		RandomizationFactor: DefaultRandomizationFactor,
+		Multiplier:          DefaultMultiplier,
+		MaxInterval:         DefaultMaxInterval,
+		MaxElapsedTime:      DefaultMaxElapsedTime,
+		Clock:               SystemClock,
+	}
+	b.Reset()
+	return b
+}
+
+type systemClock struct{}
+
+func (t systemClock) Now() time.Time {
+	return time.Now()
+}
+
+// SystemClock implements Clock interface that uses time.Now().
+var SystemClock = systemClock{}
+
+// Reset the interval back to the initial retry interval and restarts the timer.
+func (b *ExponentialBackOff) Reset() {
+	b.currentInterval = b.InitialInterval
+	b.startTime = b.Clock.Now()
+}
+
+// NextBackOff calculates the next backoff interval using the formula:
+// 	Randomized interval = RetryInterval +/- (RandomizationFactor * RetryInterval)
+func (b *ExponentialBackOff) NextBackOff() time.Duration {
+	// Make sure we have not gone over the maximum elapsed time.
+	if b.MaxElapsedTime != 0 && b.GetElapsedTime() > b.MaxElapsedTime {
+		return Stop
+	}
+	defer b.incrementCurrentInterval()
+	return getRandomValueFromInterval(b.RandomizationFactor, rand.Float64(), b.currentInterval)
+}
+
+// GetElapsedTime returns the elapsed time since an ExponentialBackOff instance
+// is created and is reset when Reset() is called.
+//
+// The elapsed time is computed using time.Now().UnixNano(). It is
+// safe to call even while the backoff policy is used by a running
+// ticker.
+func (b *ExponentialBackOff) GetElapsedTime() time.Duration {
+	return b.Clock.Now().Sub(b.startTime)
+}
+
+// Increments the current interval by multiplying it with the multiplier.
+func (b *ExponentialBackOff) incrementCurrentInterval() {
+	// Check for overflow, if overflow is detected set the current interval to the max interval.
+	if float64(b.currentInterval) >= float64(b.MaxInterval)/b.Multiplier {
+		b.currentInterval = b.MaxInterval
+	} else {
+		b.currentInterval = time.Duration(float64(b.currentInterval) * b.Multiplier)
+	}
+}
+
+// Returns a random value from the following interval:
+// 	[randomizationFactor * currentInterval, randomizationFactor * currentInterval].
+func getRandomValueFromInterval(randomizationFactor, random float64, currentInterval time.Duration) time.Duration {
+	var delta = randomizationFactor * float64(currentInterval)
+	var minInterval = float64(currentInterval) - delta
+	var maxInterval = float64(currentInterval) + delta
+
+	// Get a random value from the range [minInterval, maxInterval].
+	// The formula used below has a +1 because if the minInterval is 1 and the maxInterval is 3 then
+	// we want a 33% chance for selecting either 1, 2 or 3.
+	return time.Duration(minInterval + (random * (maxInterval - minInterval + 1)))
+}
diff --git a/vendor/github.com/cenkalti/backoff/retry.go b/vendor/github.com/cenkalti/backoff/retry.go
new file mode 100644
index 00000000..e936a506
--- /dev/null
+++ b/vendor/github.com/cenkalti/backoff/retry.go
@@ -0,0 +1,82 @@
+package backoff
+
+import "time"
+
+// An Operation is executing by Retry() or RetryNotify().
+// The operation will be retried using a backoff policy if it returns an error.
+type Operation func() error
+
+// Notify is a notify-on-error function. It receives an operation error and
+// backoff delay if the operation failed (with an error).
+//
+// NOTE that if the backoff policy stated to stop retrying,
+// the notify function isn't called.
+type Notify func(error, time.Duration)
+
+// Retry the operation o until it does not return error or BackOff stops.
+// o is guaranteed to be run at least once.
+//
+// If o returns a *PermanentError, the operation is not retried, and the
+// wrapped error is returned.
+//
+// Retry sleeps the goroutine for the duration returned by BackOff after a
+// failed operation returns.
+func Retry(o Operation, b BackOff) error { return RetryNotify(o, b, nil) }
+
+// RetryNotify calls notify function with the error and wait duration
+// for each failed attempt before sleep.
+func RetryNotify(operation Operation, b BackOff, notify Notify) error {
+	var err error
+	var next time.Duration
+	var t *time.Timer
+
+	cb := ensureContext(b)
+
+	b.Reset()
+	for {
+		if err = operation(); err == nil {
+			return nil
+		}
+
+		if permanent, ok := err.(*PermanentError); ok {
+			return permanent.Err
+		}
+
+		if next = cb.NextBackOff(); next == Stop {
+			return err
+		}
+
+		if notify != nil {
+			notify(err, next)
+		}
+
+		if t == nil {
+			t = time.NewTimer(next)
+			defer t.Stop()
+		} else {
+			t.Reset(next)
+		}
+
+		select {
+		case <-cb.Context().Done():
+			return err
+		case <-t.C:
+		}
+	}
+}
+
+// PermanentError signals that the operation should not be retried.
+type PermanentError struct {
+	Err error
+}
+
+func (e *PermanentError) Error() string {
+	return e.Err.Error()
+}
+
+// Permanent wraps the given err in a *PermanentError.
+func Permanent(err error) *PermanentError {
+	return &PermanentError{
+		Err: err,
+	}
+}
diff --git a/vendor/github.com/cenkalti/backoff/ticker.go b/vendor/github.com/cenkalti/backoff/ticker.go
new file mode 100644
index 00000000..e41084b0
--- /dev/null
+++ b/vendor/github.com/cenkalti/backoff/ticker.go
@@ -0,0 +1,82 @@
+package backoff
+
+import (
+	"sync"
+	"time"
+)
+
+// Ticker holds a channel that delivers `ticks' of a clock at times reported by a BackOff.
+//
+// Ticks will continue to arrive when the previous operation is still running,
+// so operations that take a while to fail could run in quick succession.
+type Ticker struct {
+	C        <-chan time.Time
+	c        chan time.Time
+	b        BackOffContext
+	stop     chan struct{}
+	stopOnce sync.Once
+}
+
+// NewTicker returns a new Ticker containing a channel that will send
+// the time at times specified by the BackOff argument. Ticker is
+// guaranteed to tick at least once.  The channel is closed when Stop
+// method is called or BackOff stops. It is not safe to manipulate the
+// provided backoff policy (notably calling NextBackOff or Reset)
+// while the ticker is running.
+func NewTicker(b BackOff) *Ticker {
+	c := make(chan time.Time)
+	t := &Ticker{
+		C:    c,
+		c:    c,
+		b:    ensureContext(b),
+		stop: make(chan struct{}),
+	}
+	t.b.Reset()
+	go t.run()
+	return t
+}
+
+// Stop turns off a ticker. After Stop, no more ticks will be sent.
+func (t *Ticker) Stop() {
+	t.stopOnce.Do(func() { close(t.stop) })
+}
+
+func (t *Ticker) run() {
+	c := t.c
+	defer close(c)
+
+	// Ticker is guaranteed to tick at least once.
+	afterC := t.send(time.Now())
+
+	for {
+		if afterC == nil {
+			return
+		}
+
+		select {
+		case tick := <-afterC:
+			afterC = t.send(tick)
+		case <-t.stop:
+			t.c = nil // Prevent future ticks from being sent to the channel.
+			return
+		case <-t.b.Context().Done():
+			return
+		}
+	}
+}
+
+func (t *Ticker) send(tick time.Time) <-chan time.Time {
+	select {
+	case t.c <- tick:
+	case <-t.stop:
+		return nil
+	}
+
+	next := t.b.NextBackOff()
+	if next == Stop {
+		t.Stop()
+		return nil
+	}
+
+	return time.After(next)
+}
diff --git a/vendor/github.com/cenkalti/backoff/tries.go b/vendor/github.com/cenkalti/backoff/tries.go
new file mode 100644
index 00000000..cfeefd9b
--- /dev/null
+++ b/vendor/github.com/cenkalti/backoff/tries.go
@@ -0,0 +1,35 @@
+package backoff
+
+import "time"
+
+/*
+WithMaxRetries creates a wrapper around another BackOff, which will
+return Stop if NextBackOff() has been called too many times since
+the last time Reset() was called
+
+Note: Implementation is not thread-safe.
+*/
+func WithMaxRetries(b BackOff, max uint64) BackOff {
+	return &backOffTries{delegate: b, maxTries: max}
+}
+
+type backOffTries struct {
+	delegate BackOff
+	maxTries uint64
+	numTries uint64
+}
+
+func (b *backOffTries) NextBackOff() time.Duration {
+	if b.maxTries > 0 {
+		if b.maxTries <= b.numTries {
+			return Stop
+		}
+		b.numTries++
+	}
+	return b.delegate.NextBackOff()
+}
+
+func (b *backOffTries) Reset() {
+	b.numTries = 0
+	b.delegate.Reset()
+}