Pull request: 3185 fix recursion vol.2

Merge in DNS/adguard-home from fix-recursion to master

Closes AdguardTeam#3185.

Squashed commit of the following:

commit c78650b
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon May 31 13:12:46 2021 +0300

    all: fix changelog

commit e43017a
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Sun May 30 22:39:05 2021 +0300

    dnsforward: reduce recursion ttl

commit 79208a8
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Sun May 30 22:29:27 2021 +0300

    dnsforward: only check recursion for private rdns

commit 1b8075b
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Sun May 30 22:18:11 2021 +0300

    dnsforward: rm recursion detecting from upstream

CHANGELOG.md

@@ -15,7 +15,8 @@ and this project adheres to
 
 ### Added
 
-- Detection and handling of recurrent requests ([#3185]).
+- Detection and handling of recurrent PTR requests for locally-served addresses
+  ([#3185]).
 - The ability to completely disable reverse DNS resolving of IPs from
   locally-served networks ([#3184]).
 - New flag `--local-frontend` to serve dinamically changeable frontend files

internal/dnsforward/dns.go

@@ -532,7 +532,6 @@ func (s *Server) processUpstream(ctx *dnsContext) (rc resultCode) {
 	}
 
 	// request was not filtered so let it be processed further
-	s.recDetector.add(*req)
 	if ctx.err = s.dnsProxy.Resolve(d); ctx.err != nil {
 		return resultCodeError
 	}

internal/dnsforward/dnsforward.go

@@ -124,7 +124,7 @@ func domainNameToSuffix(tld string) (suffix string) {
 
 const (
 	// recursionTTL is the time recursive request is cached for.
-	recursionTTL = 5 * time.Second
+	recursionTTL = 1 * time.Second
 	// cachedRecurrentReqNum is the maximum number of cached recurrent
 	// requests.
 	cachedRecurrentReqNum = 1000
@@ -265,13 +265,6 @@ func (s *Server) Exchange(ip net.IP) (host string, err error) {
 		return "", nil
 	}
 
-	var resolver *proxy.Proxy = s.localResolvers
-	if !s.subnetDetector.IsLocallyServedNetwork(ip) {
-		resolver = s.internalProxy
-	} else if !s.conf.UsePrivateRDNS {
-		return "", nil
-	}
-
 	arpa := dns.Fqdn(aghnet.ReverseAddr(ip))
 	req := &dns.Msg{
 		MsgHdr: dns.MsgHdr{
@@ -291,7 +284,16 @@ func (s *Server) Exchange(ip net.IP) (host string, err error) {
 		StartTime: time.Now(),
 	}
 
-	s.recDetector.add(*req)
+	var resolver *proxy.Proxy = s.internalProxy
+	if s.subnetDetector.IsLocallyServedNetwork(ip) {
+		if !s.conf.UsePrivateRDNS {
+			return "", nil
+		}
+
+		resolver = s.localResolvers
+		s.recDetector.add(*req)
+	}
+
 	if err = resolver.Resolve(ctx); err != nil {
 		return "", err
 	}

internal/dnsforward/recursiondetector.go

@@ -26,7 +26,7 @@ type recursionDetector struct {
 	ttl            time.Duration
 }
 
-// check checks if the passed req was already sent by s.
+// check checks if the passed req was already sent by the server.
 func (rd *recursionDetector) check(msg dns.Msg) (ok bool) {
 	if len(msg.Question) == 0 {
 		return false