Pull request: * all: move internal Go packages to internal/

Merge in DNS/adguard-home from 2234-move-to-internal to master

Squashed commit of the following:

commit d26a288cabeac86f9483fab307677b1027c78524
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Oct 30 12:44:18 2020 +0300

    * all: move internal Go packages to internal/

    Closes #2234.

internal/home/rdns.go

@@ -0,0 +1,129 @@
+package home
+
+import (
+	"encoding/binary"
+	"strings"
+	"time"
+
+	"github.com/AdguardTeam/AdGuardHome/internal/dnsforward"
+	"github.com/AdguardTeam/golibs/cache"
+	"github.com/AdguardTeam/golibs/log"
+	"github.com/miekg/dns"
+)
+
+// RDNS - module context
+type RDNS struct {
+	dnsServer *dnsforward.Server
+	clients   *clientsContainer
+	ipChannel chan string // pass data from DNS request handling thread to rDNS thread
+
+	// Contains IP addresses of clients to be resolved by rDNS
+	// If IP address is resolved, it stays here while it's inside Clients.
+	//  If it's removed from Clients, this IP address will be resolved once again.
+	// If IP address couldn't be resolved, it stays here for some time to prevent further attempts to resolve the same IP.
+	ipAddrs cache.Cache
+}
+
+// InitRDNS - create module context
+func InitRDNS(dnsServer *dnsforward.Server, clients *clientsContainer) *RDNS {
+	r := RDNS{}
+	r.dnsServer = dnsServer
+	r.clients = clients
+
+	cconf := cache.Config{}
+	cconf.EnableLRU = true
+	cconf.MaxCount = 10000
+	r.ipAddrs = cache.New(cconf)
+
+	r.ipChannel = make(chan string, 256)
+	go r.workerLoop()
+	return &r
+}
+
+// Begin - add IP address to rDNS queue
+func (r *RDNS) Begin(ip string) {
+	now := uint64(time.Now().Unix())
+	expire := r.ipAddrs.Get([]byte(ip))
+	if len(expire) != 0 {
+		exp := binary.BigEndian.Uint64(expire)
+		if exp > now {
+			return
+		}
+		// TTL expired
+	}
+	expire = make([]byte, 8)
+	const ttl = 1 * 60 * 60
+	binary.BigEndian.PutUint64(expire, now+ttl)
+	_ = r.ipAddrs.Set([]byte(ip), expire)
+
+	if r.clients.Exists(ip, ClientSourceRDNS) {
+		return
+	}
+
+	log.Tracef("rDNS: adding %s", ip)
+	select {
+	case r.ipChannel <- ip:
+		//
+	default:
+		log.Tracef("rDNS: queue is full")
+	}
+}
+
+// Use rDNS to get hostname by IP address
+func (r *RDNS) resolve(ip string) string {
+	log.Tracef("Resolving host for %s", ip)
+
+	req := dns.Msg{}
+	req.Id = dns.Id()
+	req.RecursionDesired = true
+	req.Question = []dns.Question{
+		{
+			Qtype:  dns.TypePTR,
+			Qclass: dns.ClassINET,
+		},
+	}
+	var err error
+	req.Question[0].Name, err = dns.ReverseAddr(ip)
+	if err != nil {
+		log.Debug("Error while calling dns.ReverseAddr(%s): %s", ip, err)
+		return ""
+	}
+
+	resp, err := r.dnsServer.Exchange(&req)
+	if err != nil {
+		log.Debug("Error while making an rDNS lookup for %s: %s", ip, err)
+		return ""
+	}
+	if len(resp.Answer) == 0 {
+		log.Debug("No answer for rDNS lookup of %s", ip)
+		return ""
+	}
+	ptr, ok := resp.Answer[0].(*dns.PTR)
+	if !ok {
+		log.Debug("not a PTR response for %s", ip)
+		return ""
+	}
+
+	log.Tracef("PTR response for %s: %s", ip, ptr.String())
+	if strings.HasSuffix(ptr.Ptr, ".") {
+		ptr.Ptr = ptr.Ptr[:len(ptr.Ptr)-1]
+	}
+
+	return ptr.Ptr
+}
+
+// Wait for a signal and then synchronously resolve hostname by IP address
+// Add the hostname:IP pair to "Clients" array
+func (r *RDNS) workerLoop() {
+	for {
+		var ip string
+		ip = <-r.ipChannel
+
+		host := r.resolve(ip)
+		if len(host) == 0 {
+			continue
+		}
+
+		_, _ = r.clients.AddHost(ip, host, ClientSourceRDNS)
+	}
+}