+ dns: respond to PTR requests for internal IP addresses from DHCP

Close #1682

Squashed commit of the following:

commit 2fad3544bf8853b1f8f19ad8b7bc8a490c96e533
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Jun 22 17:32:45 2020 +0300

    minor

commit 7c17992424702d95e6de91f30e8ae2dfcd8de257
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Jun 22 16:09:34 2020 +0300

    build

commit 16a52e11a015a97d3cbf30362482a4abd052192b
Merge: 7b6a73c8 2c47053c
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Jun 22 16:08:32 2020 +0300

    Merge remote-tracking branch 'origin/master' into 1682-dhcp-resolve

commit 7b6a73c84b5cb9a073a9dfb7d7bdecd22e1e1318
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Jun 22 16:01:34 2020 +0300

    tests

commit c2654abb2e5e7b7e3a04e4ddb8e1064b37613929
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Jun 1 15:15:13 2020 +0300

    + dnsforward: respond to PTR requests for internal IP addresses

    {[IP] => "host"} <- DNSforward <-(leases)-- DHCP

dnsforward/handle_dns.go

@@ -1,9 +1,12 @@
 package dnsforward
 
 import (
+	"strings"
 	"time"
 
+	"github.com/AdguardTeam/AdGuardHome/dhcpd"
 	"github.com/AdguardTeam/AdGuardHome/dnsfilter"
+	"github.com/AdguardTeam/AdGuardHome/util"
 	"github.com/AdguardTeam/dnsproxy/proxy"
 	"github.com/AdguardTeam/golibs/log"
 	"github.com/miekg/dns"
@@ -39,6 +42,7 @@ func (s *Server) handleDNSRequest(_ *proxy.Proxy, d *proxy.DNSContext) error {
 	type modProcessFunc func(ctx *dnsContext) int
 	mods := []modProcessFunc{
 		processInitial,
+		processInternalIPAddrs,
 		processFilteringBeforeRequest,
 		processUpstream,
 		processDNSSECAfterResponse,
@@ -88,11 +92,82 @@ func processInitial(ctx *dnsContext) int {
 	return resultDone
 }
 
+func (s *Server) onDHCPLeaseChanged(flags int) {
+	switch flags {
+	case dhcpd.LeaseChangedAdded,
+		dhcpd.LeaseChangedAddedStatic,
+		dhcpd.LeaseChangedRemovedStatic:
+		//
+	default:
+		return
+	}
+
+	m := make(map[string]string)
+	ll := s.dhcpServer.Leases(dhcpd.LeasesAll)
+	for _, l := range ll {
+		if len(l.Hostname) == 0 {
+			continue
+		}
+		m[l.IP.String()] = l.Hostname
+	}
+	log.Debug("DNS: added %d PTR entries from DHCP", len(m))
+	s.tablePTRLock.Lock()
+	s.tablePTR = m
+	s.tablePTRLock.Unlock()
+}
+
+// Respond to PTR requests if the target IP address is leased by our DHCP server
+func processInternalIPAddrs(ctx *dnsContext) int {
+	s := ctx.srv
+	req := ctx.proxyCtx.Req
+	if req.Question[0].Qtype != dns.TypePTR {
+		return resultDone
+	}
+
+	arpa := req.Question[0].Name
+	arpa = strings.TrimSuffix(arpa, ".")
+	arpa = strings.ToLower(arpa)
+	ip := util.DNSUnreverseAddr(arpa)
+	if ip == nil {
+		return resultDone
+	}
+
+	s.tablePTRLock.Lock()
+	if s.tablePTR == nil {
+		s.tablePTRLock.Unlock()
+		return resultDone
+	}
+	host, ok := s.tablePTR[ip.String()]
+	s.tablePTRLock.Unlock()
+	if !ok {
+		return resultDone
+	}
+
+	log.Debug("DNS: reverse-lookup: %s -> %s", arpa, host)
+
+	resp := s.makeResponse(req)
+	ptr := &dns.PTR{}
+	ptr.Hdr = dns.RR_Header{
+		Name:   req.Question[0].Name,
+		Rrtype: dns.TypePTR,
+		Ttl:    s.conf.BlockedResponseTTL,
+		Class:  dns.ClassINET,
+	}
+	ptr.Ptr = host + "."
+	resp.Answer = append(resp.Answer, ptr)
+	ctx.proxyCtx.Res = resp
+	return resultDone
+}
+
 // Apply filtering logic
 func processFilteringBeforeRequest(ctx *dnsContext) int {
 	s := ctx.srv
 	d := ctx.proxyCtx
 
+	if d.Res != nil {
+		return resultDone // response is already set - nothing to do
+	}
+
 	s.RLock()
 	// Synchronize access to s.dnsFilter so it won't be suddenly uninitialized while in use.
 	// This could happen after proxy server has been stopped, but its workers are not yet exited.