New cache-harden-below-nxdomain option (#132)

* New cache-harden-below-nxdomain option

* Wire in the config

cache.go

@@ -5,6 +5,7 @@ import (
 	"expvar"
 	"math"
 	"math/rand"
+	"strings"
 	"sync"
 	"time"
 
@@ -48,6 +49,12 @@ type CacheOptions struct {
 	// Allows control over the order of answer RRs in cached responses. Default is to keep
 	// the order if nil.
 	ShuffleAnswerFunc AnswerShuffleFunc
+
+	// If enabled, will return NXDOMAIN for every name query under another name that is
+	// already cached as NXDOMAIN. For example, if example.com is in the cache with
+	// NXDOMAIN, a query for www.example.com will also immediately return NXDOMAIN.
+	// See RFC8020.
+	HardenBelowNXDOMAIN bool
 }
 
 // NewCache returns a new instance of a Cache resolver.
@@ -128,6 +135,26 @@ func (r *Cache) answerFromCache(q *dns.Msg) (*dns.Msg, bool) {
 	}
 	r.mu.Unlock()
 
+	// We couldn't find it in the cache, but a parent domain may already be with NXDOMAIN.
+	// Return that instead if enabled.
+	if answer == nil && r.HardenBelowNXDOMAIN {
+		name := q.Question[0].Name
+		newQ := q.Copy()
+		fragments := strings.Split(name, ".")
+		r.mu.Lock()
+		for i := 1; i < len(fragments)-1; i++ {
+			newQ.Question[0].Name = strings.Join(fragments[i:], ".")
+			if a := r.lru.get(newQ); a != nil {
+				if a.Rcode == dns.RcodeNameError {
+					r.mu.Unlock()
+					return nxdomain(q), true
+				}
+				break
+			}
+		}
+		r.mu.Unlock()
+	}
+
 	// Return a cache-miss if there's no answer record in the map
 	if answer == nil {
 		return nil, false

cache_test.go

@@ -99,6 +99,39 @@ func TestCacheNXDOMAIN(t *testing.T) {
 	require.Equal(t, 1, r.HitCount())
 }
 
+func TestCacheHardenBelowNXDOMAIN(t *testing.T) {
+	var ci ClientInfo
+	q := new(dns.Msg)
+	r := &TestResolver{
+		ResolveFunc: func(q *dns.Msg, ci ClientInfo) (*dns.Msg, error) {
+			a := new(dns.Msg)
+			a.SetReply(q)
+			a.SetRcode(q, dns.RcodeNameError)
+			return a, nil
+		},
+	}
+
+	opt := CacheOptions{
+		GCPeriod:            time.Minute,
+		HardenBelowNXDOMAIN: true,
+	}
+	c := NewCache("test-cache", r, opt)
+
+	// Cache an NXDOMAIN for the parent domain
+	q.SetQuestion("test.com.", dns.TypeA)
+	_, err := c.Resolve(q, ci)
+	require.NoError(t, err)
+	require.Equal(t, 1, r.HitCount())
+
+	// A sub-domain query should also return NXDOMAIN based on the cached
+	// record for the parent if HardenBelowNXDOMAIN is enabled.
+	q.SetQuestion("not.exist.test.com.", dns.TypeA)
+	a, err := c.Resolve(q, ci)
+	require.NoError(t, err)
+	require.Equal(t, 1, r.HitCount())
+	require.Equal(t, dns.RcodeNameError, a.Rcode)
+}
+
 func TestRoundRobinShuffle(t *testing.T) {
 	msg := &dns.Msg{
 		Answer: []dns.RR{

cmd/routedns/config.go

@@ -70,9 +70,10 @@ type group struct {
 	EDNS0Data  []byte                  `toml:"edns0-data"`  // EDNS0 modifier option data
 
 	// Cache options
-	CacheSize          int    `toml:"cache-size"`           // Max number of items to keep in the cache. Default 0 == unlimited
-	CacheNegativeTTL   uint32 `toml:"cache-negative-ttl"`   // TTL to apply to negative responses, default 60.
-	CacheAnswerShuffle string `toml:"cache-answer-shuffle"` // Algorithm to use for modifying the response order of cached items
+	CacheSize                int    `toml:"cache-size"`                  // Max number of items to keep in the cache. Default 0 == unlimited
+	CacheNegativeTTL         uint32 `toml:"cache-negative-ttl"`          // TTL to apply to negative responses, default 60.
+	CacheAnswerShuffle       string `toml:"cache-answer-shuffle"`        // Algorithm to use for modifying the response order of cached items
+	CacheHardenBelowNXDOMAIN bool   `toml:"cache-harden-below-nxdomain"` // Return NXDOMAIN if an NXDOMAIN is cached for a parent domain
 
 	// Blocklist options
 	Blocklist []string // Blocklist rules, only used by "blocklist" type

cmd/routedns/main.go

@@ -428,10 +428,11 @@ func instantiateGroup(id string, g group, resolvers map[string]rdns.Resolver) er
 			return fmt.Errorf("unsupported shuffle function %q", g.CacheAnswerShuffle)
 		}
 		opt := rdns.CacheOptions{
-			GCPeriod:          time.Duration(g.GCPeriod) * time.Second,
-			Capacity:          g.CacheSize,
-			NegativeTTL:       g.CacheNegativeTTL,
-			ShuffleAnswerFunc: shuffleFunc,
+			GCPeriod:            time.Duration(g.GCPeriod) * time.Second,
+			Capacity:            g.CacheSize,
+			NegativeTTL:         g.CacheNegativeTTL,
+			ShuffleAnswerFunc:   shuffleFunc,
+			HardenBelowNXDOMAIN: g.CacheHardenBelowNXDOMAIN,
 		}
 		resolvers[id] = rdns.NewCache(id, gr[0], opt)
 	case "response-blocklist-ip", "response-blocklist-cidr": // "response-blocklist-cidr" has been retired/renamed to "response-blocklist-ip"

doc/configuration.md

@@ -293,6 +293,7 @@ Options:
 - `cache-size` - Max number of responses to cache. Defaults to 0 which means no limit. Optional
 - `cache-negative-ttl` - TTL (in seconds) to apply to responses without a SOA. Default: 60. Optional
 - `cache-answer-shuffle` - Specifies a method for changing the order of cached A/AAAA answer records. Possible values `random` or `round-robin`. Defaults to static responses if not set.
+- `cache-harden-below-nxdomain` - Return NXDOMAIN for sudomain queries if the parent domain has a cached NXDOMAIN. See [RFC8020](https://tools.ietf.org/html/rfc8020).
 
 #### Examples