Pull request: all: do not use dhcp clients when server is off

Closes #2934.

Squashed commit of the following:

commit 856ea4ec0c3ffb1da447b93260da90d37cd5d45d
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Apr 15 17:39:46 2021 +0300

    dnsforward: imp spacing

commit fa748e5a26cb6a38b5f87c5498287cb734ce7a59
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Apr 15 17:33:44 2021 +0300

    dnsforward: imp code

commit 771ba0de449faffff1cea523e8bbcc1039c992db
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Apr 15 17:06:03 2021 +0300

    all: do not use dhcp clients when server is off

internal/dnsforward/dns.go

@@ -154,44 +154,60 @@ func isHostnameOK(hostname string) bool {
 	return true
 }
 
+func (s *Server) setTableHostToIP(t hostToIPTable) {
+	s.tableHostToIPLock.Lock()
+	defer s.tableHostToIPLock.Unlock()
+
+	s.tableHostToIP = t
+}
+
+func (s *Server) setTableIPToHost(t ipToHostTable) {
+	s.tableIPToHostLock.Lock()
+	defer s.tableIPToHostLock.Unlock()
+
+	s.tableIPToHost = t
+}
+
 func (s *Server) onDHCPLeaseChanged(flags int) {
+	add := true
 	switch flags {
 	case dhcpd.LeaseChangedAdded,
 		dhcpd.LeaseChangedAddedStatic,
 		dhcpd.LeaseChangedRemovedStatic:
-		//
+		// Go on.
+	case dhcpd.LeaseChangedRemovedAll:
+		add = false
 	default:
 		return
 	}
 
-	hostToIP := make(map[string]net.IP)
-	m := make(map[string]string)
+	var hostToIP hostToIPTable
+	var ipToHost ipToHostTable
+	if add {
+		hostToIP = make(hostToIPTable)
+		ipToHost = make(ipToHostTable)
 
-	ll := s.dhcpServer.Leases(dhcpd.LeasesAll)
+		ll := s.dhcpServer.Leases(dhcpd.LeasesAll)
 
-	for _, l := range ll {
-		if len(l.Hostname) == 0 || !isHostnameOK(l.Hostname) {
-			continue
+		for _, l := range ll {
+			if len(l.Hostname) == 0 || !isHostnameOK(l.Hostname) {
+				continue
+			}
+
+			lowhost := strings.ToLower(l.Hostname)
+
+			ipToHost[l.IP.String()] = lowhost
+
+			ip := make(net.IP, 4)
+			copy(ip, l.IP.To4())
+			hostToIP[lowhost] = ip
 		}
 
-		lowhost := strings.ToLower(l.Hostname)
-
-		m[l.IP.String()] = lowhost
-
-		ip := make(net.IP, 4)
-		copy(ip, l.IP.To4())
-		hostToIP[lowhost] = ip
+		log.Debug("dns: added %d A/PTR entries from DHCP", len(ipToHost))
 	}
 
-	log.Debug("dns: added %d A/PTR entries from DHCP", len(m))
-
-	s.tableHostToIPLock.Lock()
-	s.tableHostToIP = hostToIP
-	s.tableHostToIPLock.Unlock()
-
-	s.tablePTRLock.Lock()
-	s.tablePTR = m
-	s.tablePTRLock.Unlock()
+	s.setTableHostToIP(hostToIP)
+	s.setTableIPToHost(ipToHost)
 }
 
 // processDetermineLocal determines if the client's IP address is from
@@ -336,14 +352,14 @@ func (s *Server) processRestrictLocal(ctx *dnsContext) (rc resultCode) {
 // ipToHost tries to get a hostname leased by DHCP.  It's safe for concurrent
 // use.
 func (s *Server) ipToHost(ip net.IP) (host string, ok bool) {
-	s.tablePTRLock.Lock()
-	defer s.tablePTRLock.Unlock()
+	s.tableIPToHostLock.Lock()
+	defer s.tableIPToHostLock.Unlock()
 
-	if s.tablePTR == nil {
+	if s.tableIPToHost == nil {
 		return "", false
 	}
 
-	host, ok = s.tablePTR[ip.String()]
+	host, ok = s.tableIPToHost[ip.String()]
 
 	return host, ok
 }

internal/dnsforward/dns_test.go

@@ -91,7 +91,7 @@ func TestServer_ProcessInternalHosts_localRestriction(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			s := &Server{
 				autohostSuffix: defaultAutohostSuffix,
-				tableHostToIP: map[string]net.IP{
+				tableHostToIP: hostToIPTable{
 					"example": knownIP,
 				},
 			}
@@ -202,7 +202,7 @@ func TestServer_ProcessInternalHosts(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			s := &Server{
 				autohostSuffix: tc.suffix,
-				tableHostToIP: map[string]net.IP{
+				tableHostToIP: hostToIPTable{
 					"example": knownIP,
 				},
 			}

internal/dnsforward/dnsforward.go

@@ -43,6 +43,15 @@ var defaultBlockedHosts = []string{"version.bind", "id.server", "hostname.bind"}
 
 var webRegistered bool
 
+// hostToIPTable is an alias for the type of Server.tableHostToIP.
+type hostToIPTable = map[string]net.IP
+
+// ipToHostTable is an alias for the type of Server.tableIPToHost.
+//
+// TODO(a.garipov): Define an IPMap type in aghnet and use here and in other
+// places?
+type ipToHostTable = map[string]string
+
 // Server is the main way to start a DNS server.
 //
 // Example:
@@ -69,11 +78,11 @@ type Server struct {
 	subnetDetector *aghnet.SubnetDetector
 	localResolvers *proxy.Proxy
 
-	tableHostToIP     map[string]net.IP // "hostname -> IP" table for internal addresses (DHCP)
+	tableHostToIP     hostToIPTable
 	tableHostToIPLock sync.Mutex
 
-	tablePTR     map[string]string // "IP -> hostname" table for reverse lookup
-	tablePTRLock sync.Mutex
+	tableIPToHost     ipToHostTable
+	tableIPToHostLock sync.Mutex
 
 	// DNS proxy instance for internal usage
 	// We don't Start() it and so no listen port is required.