Configurable backoff

clientapi/routing/rate_limiting.go

@@ -6,23 +6,28 @@ import (
 	"time"
 
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	"github.com/matrix-org/dendrite/internal/config"
 	"github.com/matrix-org/util"
 )
 
 type rateLimits struct {
-	limits       map[string]chan struct{}
-	limitsMutex  sync.RWMutex
-	maxRequests  int
-	timeInterval time.Duration
+	limits           map[string]chan struct{}
+	limitsMutex      sync.RWMutex
+	enabled          bool
+	requestThreshold int64
+	cooloffDuration  time.Duration
 }
 
-func newRateLimits() *rateLimits {
+func newRateLimits(cfg *config.RateLimiting) *rateLimits {
 	l := &rateLimits{
-		limits:       make(map[string]chan struct{}),
-		maxRequests:  10,
-		timeInterval: 250 * time.Millisecond,
+		limits:           make(map[string]chan struct{}),
+		enabled:          cfg.Enabled,
+		requestThreshold: cfg.Threshold,
+		cooloffDuration:  time.Duration(cfg.Cooloff) * time.Millisecond,
+	}
+	if l.enabled {
+		go l.clean()
 	}
-	go l.clean()
 	return l
 }
 
@@ -45,6 +50,15 @@ func (l *rateLimits) clean() {
 }
 
 func (l *rateLimits) rateLimit(req *http.Request) *util.JSONResponse {
+	// If rate limiting is disabled then do nothing.
+	if !l.enabled {
+		return nil
+	}
+
+	// Lock the map long enough to check for rate limiting. We hold it
+	// for longer here than we really need to but it makes sure that we
+	// also don't conflict with the cleaner goroutine which might clean
+	// up a channel after we have retrieved it otherwise.
 	l.limitsMutex.RLock()
 	defer l.limitsMutex.RUnlock()
 
@@ -59,7 +73,7 @@ func (l *rateLimits) rateLimit(req *http.Request) *util.JSONResponse {
 	// let's create one.
 	rateLimit, ok := l.limits[caller]
 	if !ok {
-		l.limits[caller] = make(chan struct{}, l.maxRequests)
+		l.limits[caller] = make(chan struct{}, l.requestThreshold)
 		rateLimit = l.limits[caller]
 	}
 
@@ -71,14 +85,14 @@ func (l *rateLimits) rateLimit(req *http.Request) *util.JSONResponse {
 		// We hit the rate limit. Tell the client to back off.
 		return &util.JSONResponse{
 			Code: http.StatusTooManyRequests,
-			JSON: jsonerror.LimitExceeded("You are sending too many requests too quickly!", l.timeInterval.Milliseconds()),
+			JSON: jsonerror.LimitExceeded("You are sending too many requests too quickly!", l.cooloffDuration.Milliseconds()),
 		}
 	}
 
 	// After the time interval, drain a resource from the rate limiting
 	// channel. This will free up space in the channel for new requests.
 	go func() {
-		<-time.After(l.timeInterval)
+		<-time.After(l.cooloffDuration)
 		<-rateLimit
 	}()
 	return nil

clientapi/routing/routing.go

@@ -60,7 +60,7 @@ func Setup(
 	keyAPI keyserverAPI.KeyInternalAPI,
 	extRoomsProvider api.ExtraPublicRoomsProvider,
 ) {
-	rateLimits := newRateLimits()
+	rateLimits := newRateLimits(&cfg.RateLimiting)
 	userInteractiveAuth := auth.NewUserInteractive(accountDB.GetAccountByPassword, cfg)
 
 	publicAPIMux.Handle("/versions",

dendrite-config.yaml

@@ -133,6 +133,14 @@ client_api:
     turn_username: ""
     turn_password: ""
 
+  # Settings for rate-limited endpoints. Rate limiting will kick in after the
+  # threshold number of "slots" have been taken by requests from a specific 
+  # host. Each "slot" will be released after the cooloff time in milliseconds.
+  rate_limiting:
+    enabled: true
+    threshold: 5
+    cooloff_ms: 500
+
 # Configuration for the Current State Server.
 current_state_server:
   internal_api:

internal/config/config_clientapi.go

@@ -34,6 +34,9 @@ type ClientAPI struct {
 
 	// TURN options
 	TURN TURN `yaml:"turn"`
+
+	// Rate-limiting options
+	RateLimiting RateLimiting `yaml:"rate_limiting"`
 }
 
 func (c *ClientAPI) Defaults() {
@@ -47,6 +50,7 @@ func (c *ClientAPI) Defaults() {
 	c.RecaptchaBypassSecret = ""
 	c.RecaptchaSiteVerifyAPI = ""
 	c.RegistrationDisabled = false
+	c.RateLimiting.Defaults()
 }
 
 func (c *ClientAPI) Verify(configErrs *ConfigErrors, isMonolith bool) {
@@ -61,6 +65,7 @@ func (c *ClientAPI) Verify(configErrs *ConfigErrors, isMonolith bool) {
 		checkNotEmpty(configErrs, "client_api.recaptcha_siteverify_api", string(c.RecaptchaSiteVerifyAPI))
 	}
 	c.TURN.Verify(configErrs)
+	c.RateLimiting.Verify(configErrs)
 }
 
 type TURN struct {
@@ -90,3 +95,29 @@ func (c *TURN) Verify(configErrs *ConfigErrors) {
 		}
 	}
 }
+
+type RateLimiting struct {
+	// Is rate limiting enabled or disabled?
+	Enabled bool `yaml:"enabled"`
+
+	// How many "slots" a user can occupy sending requests to a rate-limited
+	// endpoint before we apply rate-limiting
+	Threshold int64 `yaml:"threshold"`
+
+	// The cooloff period in milliseconds after a request before the "slot"
+	// is freed again
+	Cooloff int64 `yaml:"cooloff_ms"`
+}
+
+func (r *RateLimiting) Verify(configErrs *ConfigErrors) {
+	if r.Enabled {
+		checkPositive(configErrs, "client_api.rate_limiting.threshold", r.Threshold)
+		checkPositive(configErrs, "client_api.rate_limiting.cooloff_ms", r.Cooloff)
+	}
+}
+
+func (r *RateLimiting) Defaults() {
+	r.Enabled = true
+	r.Threshold = 5
+	r.Cooloff = 500
+}