Add TLS resolver connection reusing and pooling

Also, fix caching issues and add more tests

resolver/block-detection.go

@@ -0,0 +1,61 @@
+package resolver
+
+import (
+	"net"
+
+	"github.com/miekg/dns"
+)
+
+// Supported upstream block detections
+const (
+	BlockDetectionRefused     = "refused"
+	BlockDetectionZeroIP      = "zeroip"
+	BlockDetectionEmptyAnswer = "empty"
+	BlockDetectionDisabled    = "disabled"
+)
+
+func isBlockedUpstream(resolver *Resolver, answer *dns.Msg) bool {
+	if resolver.UpstreamBlockDetection == BlockDetectionDisabled {
+		return false
+	}
+
+	switch resolver.UpstreamBlockDetection {
+	case BlockDetectionRefused:
+		return answer.Rcode == dns.RcodeRefused
+	case BlockDetectionZeroIP:
+		if answer.Rcode != dns.RcodeSuccess {
+			return false
+		}
+		var ips []net.IP
+		for _, rr := range answer.Answer {
+			switch v := rr.(type) {
+			case *dns.A:
+				ips = append(ips, v.A)
+			case *dns.AAAA:
+				ips = append(ips, v.AAAA)
+			}
+		}
+
+		if len(ips) == 0 {
+			return false // we expected an empty IP
+		}
+
+		for _, ip := range ips {
+			if ip.To4() != nil {
+				if !ip.Equal(net.IPv4zero) {
+					return false
+				}
+			} else {
+				if !ip.To16().Equal(net.IPv6zero) {
+					return false
+				}
+			}
+		}
+
+		return true
+	case BlockDetectionEmptyAnswer:
+		return answer.Rcode == dns.RcodeNameError && len(answer.Ns) == 0 && len(answer.Answer) == 0 && len(answer.Extra) == 0
+	}
+
+	return false
+}