Pull request 1978: 4923 gopacket dhcp vol.2

Merge in DNS/adguard-home from 4923-gopacket-dhcp-vol.2 to master

Updates #4923.

Squashed commit of the following:

commit d0ef7d44af9790ed55401f6f65c7149f4c3658f7
Merge: f92b4c72d a4fdc3e8e
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Aug 30 13:43:41 2023 +0300

    Merge branch 'master' into 4923-gopacket-dhcp-vol.2

commit f92b4c72de03ceacb9b8890b7cf4307688795ce5
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Aug 28 12:33:29 2023 +0300

    dhcpd: imp code

commit 63f0fce99a0343af2670943770cfef4694ae93ed
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Aug 24 15:01:34 2023 +0300

    all: imp dhcpd code

commit 563b43b4b5ab6c9c9046c7f09008ea3ef344f4e9
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Jul 28 17:03:55 2023 +0300

    dhcpd: imp indexing

commit 340d3efa90ac4d34ba3d18702692de0fbc0247be
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Jul 28 16:22:43 2023 +0300

    all: adapt current code

internal/dhcpd/v6_unix.go

@@ -26,15 +26,14 @@ const valueIAID = "ADGH" // value for IANA.ID
 //
 // TODO(a.garipov): Think about unifying this and v4Server.
 type v6Server struct {
-	srv        *server6.Server
-	leasesLock sync.Mutex
-	leases     []*Lease
-	ipAddrs    [256]byte
-	sid        dhcpv6.DUID
-
-	ra raCtx // RA module
-
+	ra   raCtx
 	conf V6ServerConf
+	sid  dhcpv6.DUID
+	srv  *server6.Server
+
+	leases     []*Lease
+	leasesLock sync.Mutex
+	ipAddrs    [256]byte
 }
 
 // WriteDiskConfig4 - write configuration
@@ -59,6 +58,34 @@ func ip6InRange(start, ip net.IP) bool {
 	return start[15] <= ip[15]
 }
 
+// HostByIP implements the [Interface] interface for *v6Server.
+func (s *v6Server) HostByIP(ip netip.Addr) (host string) {
+	s.leasesLock.Lock()
+	defer s.leasesLock.Unlock()
+
+	for _, l := range s.leases {
+		if l.IP == ip {
+			return l.Hostname
+		}
+	}
+
+	return ""
+}
+
+// IPByHost implements the [Interface] interface for *v6Server.
+func (s *v6Server) IPByHost(host string) (ip netip.Addr) {
+	s.leasesLock.Lock()
+	defer s.leasesLock.Unlock()
+
+	for _, l := range s.leases {
+		if l.Hostname == host {
+			return l.IP
+		}
+	}
+
+	return netip.Addr{}
+}
+
 // ResetLeases resets leases.
 func (s *v6Server) ResetLeases(leases []*Lease) (err error) {
 	defer func() { err = errors.Annotate(err, "dhcpv6: %w") }()