Fix max-age in http server (#1890)

* Fix max-age in http server

Move the minMsgTTL to dnsutil and rename it MinimalTTL, move some
constants there as well.
Use these new function in server_https to correctly set the max-age
HTTP header.

Fixes: #1823

* Linter

plugin/pkg/dnsutil/ttl.go

@@ -0,0 +1,72 @@
+package dnsutil
+
+import (
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/response"
+
+	"github.com/miekg/dns"
+)
+
+// MinimalTTL scans the message returns the lowest TTL found taking into the response.Type of the message.
+func MinimalTTL(m *dns.Msg, mt response.Type) time.Duration {
+	if mt != response.NoError && mt != response.NameError && mt != response.NoData {
+		return MinimalDefaultTTL
+	}
+
+	// No data to examine, return a short ttl as a fail safe.
+	if len(m.Answer)+len(m.Ns)+len(m.Extra) == 0 {
+		return MinimalDefaultTTL
+	}
+
+	minTTL := MaximumDefaulTTL
+	for _, r := range m.Answer {
+		switch mt {
+		case response.NameError, response.NoData:
+			if r.Header().Rrtype == dns.TypeSOA {
+				minTTL = time.Duration(r.(*dns.SOA).Minttl) * time.Second
+			}
+		case response.NoError, response.Delegation:
+			if r.Header().Ttl < uint32(minTTL.Seconds()) {
+				minTTL = time.Duration(r.Header().Ttl) * time.Second
+			}
+		}
+	}
+	for _, r := range m.Ns {
+		switch mt {
+		case response.NameError, response.NoData:
+			if r.Header().Rrtype == dns.TypeSOA {
+				minTTL = time.Duration(r.(*dns.SOA).Minttl) * time.Second
+			}
+		case response.NoError, response.Delegation:
+			if r.Header().Ttl < uint32(minTTL.Seconds()) {
+				minTTL = time.Duration(r.Header().Ttl) * time.Second
+			}
+		}
+	}
+
+	for _, r := range m.Extra {
+		if r.Header().Rrtype == dns.TypeOPT {
+			// OPT records use TTL field for extended rcode and flags
+			continue
+		}
+		switch mt {
+		case response.NameError, response.NoData:
+			if r.Header().Rrtype == dns.TypeSOA {
+				minTTL = time.Duration(r.(*dns.SOA).Minttl) * time.Second
+			}
+		case response.NoError, response.Delegation:
+			if r.Header().Ttl < uint32(minTTL.Seconds()) {
+				minTTL = time.Duration(r.Header().Ttl) * time.Second
+			}
+		}
+	}
+	return minTTL
+}
+
+const (
+	// MinimalDefaultTTL is the absolute lowest TTL we use in CoreDNS.
+	MinimalDefaultTTL = 5 * time.Second
+	// MaximumDefaulTTL is the maximum TTL was use on RRsets in CoreDNS.
+	MaximumDefaulTTL = 1 * time.Hour
+)

plugin/pkg/dnsutil/ttl_test.go

@@ -0,0 +1,72 @@
+package dnsutil
+
+import (
+	"testing"
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/response"
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+// See https://github.com/kubernetes/dns/issues/121, add some specific tests for those use cases.
+
+func TestMinimalTTL(t *testing.T) {
+	m := new(dns.Msg)
+	m.SetQuestion("z.alm.im.", dns.TypeA)
+	m.Ns = []dns.RR{
+		test.SOA("alm.im.	1800	IN	SOA	ivan.ns.cloudflare.com. dns.cloudflare.com. 2025042470 10000 2400 604800 3600"),
+	}
+
+	utc := time.Now().UTC()
+
+	mt, _ := response.Typify(m, utc)
+	if mt != response.NoData {
+		t.Fatalf("Expected type to be response.NoData, got %s", mt)
+	}
+	dur := MinimalTTL(m, mt) // minTTL on msg is 3600 (neg. ttl on SOA)
+	if dur != time.Duration(3600*time.Second) {
+		t.Fatalf("Expected minttl duration to be %d, got %d", 3600, dur)
+	}
+
+	m.Rcode = dns.RcodeNameError
+	mt, _ = response.Typify(m, utc)
+	if mt != response.NameError {
+		t.Fatalf("Expected type to be response.NameError, got %s", mt)
+	}
+	dur = MinimalTTL(m, mt) // minTTL on msg is 3600 (neg. ttl on SOA)
+	if dur != time.Duration(3600*time.Second) {
+		t.Fatalf("Expected minttl duration to be %d, got %d", 3600, dur)
+	}
+}
+
+func BenchmarkMinimalTTL(b *testing.B) {
+	m := new(dns.Msg)
+	m.SetQuestion("example.org.", dns.TypeA)
+	m.Ns = []dns.RR{
+		test.A("a.example.org. 	1800	IN	A 127.0.0.53"),
+		test.A("b.example.org. 	1900	IN	A 127.0.0.53"),
+		test.A("c.example.org. 	1600	IN	A 127.0.0.53"),
+		test.A("d.example.org. 	1100	IN	A 127.0.0.53"),
+		test.A("e.example.org. 	1000	IN	A 127.0.0.53"),
+	}
+	m.Extra = []dns.RR{
+		test.A("a.example.org. 	1800	IN	A 127.0.0.53"),
+		test.A("b.example.org. 	1600	IN	A 127.0.0.53"),
+		test.A("c.example.org. 	1400	IN	A 127.0.0.53"),
+		test.A("d.example.org. 	1200	IN	A 127.0.0.53"),
+		test.A("e.example.org. 	1100	IN	A 127.0.0.53"),
+	}
+
+	utc := time.Now().UTC()
+	mt, _ := response.Typify(m, utc)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		dur := MinimalTTL(m, mt)
+		if dur != 1000*time.Second {
+			b.Fatalf("Wrong MinimalTTL %d, expected %d", dur, 1000*time.Second)
+		}
+	}
+}