Add minimum cache TTL for successful DNS responses

libbeat/processors/dns/cache.go

@@ -35,8 +35,9 @@ func (r ptrRecord) IsExpired(now time.Time) bool {
 
 type ptrCache struct {
 	sync.RWMutex
-	data    map[string]ptrRecord
-	maxSize int
+	data          map[string]ptrRecord
+	maxSize       int
+	minSuccessTTL time.Duration
 }
 
 func (c *ptrCache) set(now time.Time, key string, ptr *PTR) {
@@ -49,7 +50,7 @@ func (c *ptrCache) set(now time.Time, key string, ptr *PTR) {
 
 	c.data[key] = ptrRecord{
 		host:    ptr.Host,
-		expires: now.Add(time.Duration(ptr.TTL) * time.Second),
+		expires: now.Add(maxDuration(time.Duration(ptr.TTL)*time.Second, c.minSuccessTTL)),
 	}
 }
 
@@ -135,11 +136,12 @@ func (ce *cachedError) Cause() error  { return ce.err }
 // reverse DNS queries. It caches the results of queries regardless of their
 // outcome (success or failure).
 type PTRLookupCache struct {
-	success    *ptrCache
-	failure    *failureCache
-	failureTTL time.Duration
-	resolver   PTRResolver
-	stats      cacheStats
+	success       *ptrCache
+	minSuccessTTL time.Duration
+	failure       *failureCache
+	failureTTL    time.Duration
+	resolver      PTRResolver
+	stats         cacheStats
 }
 
 type cacheStats struct {
@@ -155,8 +157,9 @@ func NewPTRLookupCache(reg *monitoring.Registry, conf CacheConfig, resolver PTRR
 
 	c := &PTRLookupCache{
 		success: &ptrCache{
-			data:    make(map[string]ptrRecord, conf.SuccessCache.InitialCapacity),
-			maxSize: conf.SuccessCache.MaxCapacity,
+			data:          make(map[string]ptrRecord, conf.SuccessCache.InitialCapacity),
+			maxSize:       conf.SuccessCache.MaxCapacity,
+			minSuccessTTL: conf.SuccessCache.MinTTL,
 		},
 		failure: &failureCache{
 			data:       make(map[string]failureRecord, conf.FailureCache.InitialCapacity),
@@ -208,3 +211,10 @@ func max(a, b int) int {
 	}
 	return b
 }
+
+func maxDuration(a time.Duration, b time.Duration) time.Duration {
+	if a >= b {
+		return a
+	}
+	return b
+}

libbeat/processors/dns/cache_test.go

@@ -19,8 +19,8 @@ package dns
 
 import (
 	"io"
-	"strings"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 
@@ -30,12 +30,14 @@ import (
 type stubResolver struct{}
 
 func (r *stubResolver) LookupPTR(ip string) (*PTR, error) {
-	if ip == gatewayIP {
+	switch ip {
+	case gatewayIP:
 		return &PTR{Host: gatewayName, TTL: gatewayTTL}, nil
-	} else if strings.HasSuffix(ip, "11") {
+	case gatewayIP + "1":
 		return nil, io.ErrUnexpectedEOF
+	case gatewayIP + "2":
+		return &PTR{Host: gatewayName, TTL: 0}, nil
 	}
-
 	return nil, &dnsError{"fake lookup returned NXDOMAIN"}
 }
 
@@ -98,4 +100,19 @@ func TestCache(t *testing.T) {
 		assert.EqualValues(t, 3, c.stats.Hit.Get())
 		assert.EqualValues(t, 3, c.stats.Miss.Get()) // Cache miss.
 	}
+
+	// Cache returned TTL=0 with MinTTL.
+	ptr, err = c.LookupPTR(gatewayIP + "2")
+	if assert.NoError(t, err) {
+		assert.EqualValues(t, gatewayName, ptr.Host)
+		// TTL counts down while in cache.
+		assert.EqualValues(t, 0, ptr.TTL)
+		assert.EqualValues(t, 3, c.stats.Hit.Get())
+		assert.EqualValues(t, 4, c.stats.Miss.Get())
+
+		minTTL := defaultConfig.CacheConfig.SuccessCache.MinTTL
+		expectedExpire := time.Now().Add(minTTL).Unix()
+		gotExpire := c.success.data[gatewayIP+"2"].expires.Unix()
+		assert.InDelta(t, expectedExpire, gotExpire, 1)
+	}
 }

libbeat/processors/dns/config.go

@@ -84,9 +84,12 @@ type CacheConfig struct {
 // CacheSettings define the caching behavior for an individual cache.
 type CacheSettings struct {
 	// TTL value for items in cache. Not used for success because we use TTL
-	// from the DNS record.
+	// from the DNS record or the minimum configured TTL.
 	TTL time.Duration `config:"ttl"`
 
+	// Minimum TTL value for successful DNS responses.
+	MinTTL time.Duration `config:"ttl.min" validate:"min=1"`
+
 	// Initial capacity. How much space is allocated at initialization.
 	InitialCapacity int `config:"capacity.initial" validate:"min=0"`
 
@@ -122,6 +125,9 @@ func (c *Config) Validate() error {
 
 // Validate validates the data contained in the CacheConfig.
 func (c *CacheConfig) Validate() error {
+	if c.SuccessCache.MinTTL <= 0 {
+		return errors.Errorf("success_cache.ttl.min must be > 0")
+	}
 	if c.FailureCache.TTL <= 0 {
 		return errors.Errorf("failure_cache.ttl must be > 0")
 	}
@@ -146,6 +152,7 @@ func (c *CacheConfig) Validate() error {
 var defaultConfig = Config{
 	CacheConfig: CacheConfig{
 		SuccessCache: CacheSettings{
+			MinTTL:          time.Minute,
 			InitialCapacity: 1000,
 			MaxCapacity:     10000,
 		},