DNS: Use cache for NXDOMAIN (rcode 3 error) (#4560)

XTLS/Xray-core#4558 (comment)

Author: patterniha

app/dns/nameserver_doh.go

@@ -355,7 +355,7 @@ func (s *DoHNameServer) QueryIP(ctx context.Context, domain string, clientIP net
 		errors.LogDebug(ctx, "DNS cache is disabled. Querying IP for ", domain, " at ", s.name)
 	} else {
 		ips, ttl, err := s.findIPsForDomain(fqdn, option)
-		if err == nil || err == dns_feature.ErrEmptyResponse {
+		if err == nil || err == dns_feature.ErrEmptyResponse || dns_feature.RCodeFromError(err) == 3 {
 			errors.LogDebugInner(ctx, err, s.name, " cache HIT ", domain, " -> ", ips)
 			log.Record(&log.DNSLog{Server: s.name, Domain: domain, Result: ips, Status: log.DNSCacheHit, Elapsed: 0, Error: err})
 			return ips, ttl, err

app/dns/nameserver_quic.go

@@ -300,7 +300,7 @@ func (s *QUICNameServer) QueryIP(ctx context.Context, domain string, clientIP ne
 		errors.LogDebug(ctx, "DNS cache is disabled. Querying IP for ", domain, " at ", s.name)
 	} else {
 		ips, ttl, err := s.findIPsForDomain(fqdn, option)
-		if err == nil || err == dns_feature.ErrEmptyResponse {
+		if err == nil || err == dns_feature.ErrEmptyResponse || dns_feature.RCodeFromError(err) == 3 {
 			errors.LogDebugInner(ctx, err, s.name, " cache HIT ", domain, " -> ", ips)
 			log.Record(&log.DNSLog{Server: s.name, Domain: domain, Result: ips, Status: log.DNSCacheHit, Elapsed: 0, Error: err})
 			return ips, ttl, err

app/dns/nameserver_tcp.go

@@ -325,7 +325,7 @@ func (s *TCPNameServer) QueryIP(ctx context.Context, domain string, clientIP net
 		errors.LogDebug(ctx, "DNS cache is disabled. Querying IP for ", domain, " at ", s.name)
 	} else {
 		ips, ttl, err := s.findIPsForDomain(fqdn, option)
-		if err == nil || err == dns_feature.ErrEmptyResponse {
+		if err == nil || err == dns_feature.ErrEmptyResponse || dns_feature.RCodeFromError(err) == 3 {
 			errors.LogDebugInner(ctx, err, s.name, " cache HIT ", domain, " -> ", ips)
 			log.Record(&log.DNSLog{Server: s.name, Domain: domain, Result: ips, Status: log.DNSCacheHit, Elapsed: 0, Error: err})
 			return ips, ttl, err

app/dns/nameserver_udp.go

@@ -252,7 +252,7 @@ func (s *ClassicNameServer) QueryIP(ctx context.Context, domain string, clientIP
 		errors.LogDebug(ctx, "DNS cache is disabled. Querying IP for ", domain, " at ", s.name)
 	} else {
 		ips, ttl, err := s.findIPsForDomain(fqdn, option)
-		if err == nil || err == dns_feature.ErrEmptyResponse {
+		if err == nil || err == dns_feature.ErrEmptyResponse || dns_feature.RCodeFromError(err) == 3 {
 			errors.LogDebugInner(ctx, err, s.name, " cache HIT ", domain, " -> ", ips)
 			log.Record(&log.DNSLog{Server: s.name, Domain: domain, Result: ips, Status: log.DNSCacheHit, Elapsed: 0, Error: err})
 			return ips, ttl, err