plugin/cache: add a new keepttl option (#5879)

adds a new option `keepttl` to the cache plugin

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

6 files changed, 87 insertions, 1 deletions

diff --git a/plugin/cache/README.md b/plugin/cache/README.md
index 6fc20ae2c..d516a91db 100644
--- a/plugin/cache/README.md
+++ b/plugin/cache/README.md
@@ -39,6 +39,7 @@ cache [TTL] [ZONES...] {
     serve_stale [DURATION] [REFRESH_MODE]
     servfail DURATION
     disable success|denial [ZONES...]
+    keepttl
 }
 ~~~
 
@@ -69,6 +70,11 @@ cache [TTL] [ZONES...] {
   greater than 5 minutes.
 * `disable`  disable the success or denial cache for the listed **ZONES**.  If no **ZONES** are given, the specified
   cache will be disabled for all zones.
+* `keepttl` do not age TTL when serving responses from cache. The entry will still be removed from cache
+  when the TTL expires as normal, but until it expires responses will include the original TTL instead
+  of the remaining TTL. This can be useful if CoreDNS is used as an authoritative server and you want
+  to serve a consistent TTL to downstream clients. This is **NOT** recommended when CoreDNS is caching
+  records it is not authoritative for because it could result in downstream clients using stale answers.
 
 ## Capacity and Eviction
 
@@ -135,4 +141,4 @@ example.org {
         disable denial sub.example.org
     }
 }
-~~~
\ No newline at end of file
+~~~
diff --git a/plugin/cache/cache.go b/plugin/cache/cache.go
index 54f2587fa..563b2abbe 100644
--- a/plugin/cache/cache.go
+++ b/plugin/cache/cache.go
@@ -48,6 +48,9 @@ type Cache struct {
 	pexcept []string
 	nexcept []string
 
+	// Keep ttl option
+	keepttl bool
+
 	// Testing.
 	now func() time.Time
 }
diff --git a/plugin/cache/cache_test.go b/plugin/cache/cache_test.go
index 861e9b8fd..5a023f400 100644
--- a/plugin/cache/cache_test.go
+++ b/plugin/cache/cache_test.go
@@ -690,6 +690,44 @@ func TestCacheWildcardMetadata(t *testing.T) {
 	}
 }
 
+func TestCacheKeepTTL(t *testing.T) {
+	defaultTtl := 60
+
+	c := New()
+	c.Next = ttlBackend(defaultTtl)
+
+	req := new(dns.Msg)
+	req.SetQuestion("cached.org.", dns.TypeA)
+	ctx := context.TODO()
+
+	// Cache cached.org. with 60s TTL
+	rec := dnstest.NewRecorder(&test.ResponseWriter{})
+	c.keepttl = true
+	c.ServeDNS(ctx, rec, req)
+
+	tests := []struct {
+		name          string
+		futureSeconds int
+	}{
+		{"cached.org.", 0},
+		{"cached.org.", 30},
+		{"uncached.org.", 60},
+	}
+
+	for i, tt := range tests {
+		rec := dnstest.NewRecorder(&test.ResponseWriter{})
+		c.now = func() time.Time { return time.Now().Add(time.Duration(tt.futureSeconds) * time.Second) }
+		r := req.Copy()
+		r.SetQuestion(tt.name, dns.TypeA)
+		c.ServeDNS(ctx, rec, r)
+
+		recTtl := rec.Msg.Answer[0].Header().Ttl
+		if defaultTtl != int(recTtl) {
+			t.Errorf("Test %d: expecting TTL=%d, got TTL=%d", i, defaultTtl, recTtl)
+		}
+	}
+}
+
 // wildcardMetadataBackend mocks a backend that reponds with a response for qname synthesized by wildcard
 // and sets the zone/wildcard metadata value
 func wildcardMetadataBackend(qname, wildcard string) plugin.Handler {
diff --git a/plugin/cache/handler.go b/plugin/cache/handler.go
index e60637af8..de7cae456 100644
--- a/plugin/cache/handler.go
+++ b/plugin/cache/handler.go
@@ -71,6 +71,11 @@ func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 		})
 	}
 
+	if c.keepttl {
+		// If keepttl is enabled we fake the current time to the stored
+		// one so that we always get the original TTL
+		now = i.stored
+	}
 	resp := i.toMsg(r, now, do, ad)
 	w.WriteMsg(resp)
 	return dns.RcodeSuccess, nil
diff --git a/plugin/cache/setup.go b/plugin/cache/setup.go
index 6a537d986..7e5dfa174 100644
--- a/plugin/cache/setup.go
+++ b/plugin/cache/setup.go
@@ -240,6 +240,11 @@ func cacheParse(c *caddy.Controller) (*Cache, error) {
 				default:
 					return nil, fmt.Errorf("cache type for disable must be %q or %q", Success, Denial)
 				}
+			case "keepttl":
+				if len(args) != 0 {
+					return nil, c.ArgErr()
+				}
+				ca.keepttl = true
 			default:
 				return nil, c.ArgErr()
 			}
diff --git a/plugin/cache/setup_test.go b/plugin/cache/setup_test.go
index 5d8b9653c..46ac5bd9e 100644
--- a/plugin/cache/setup_test.go
+++ b/plugin/cache/setup_test.go
@@ -231,3 +231,32 @@ func TestDisable(t *testing.T) {
 		}
 	}
 }
+
+func TestKeepttl(t *testing.T) {
+	tests := []struct {
+		input     string
+		shouldErr bool
+	}{
+		// positive
+		{"keepttl", false},
+		// negative
+		{"keepttl arg1", true},
+	}
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", fmt.Sprintf("cache {\n%s\n}", test.input))
+		ca, err := cacheParse(c)
+		if test.shouldErr && err == nil {
+			t.Errorf("Test %v: Expected error but found nil", i)
+			continue
+		} else if !test.shouldErr && err != nil {
+			t.Errorf("Test %v: Expected no error but found error: %v", i, err)
+			continue
+		}
+		if test.shouldErr {
+			continue
+		}
+		if !ca.keepttl {
+			t.Errorf("Test %v: Expected keepttl enabled but disabled", i)
+		}
+	}
+}