Pull request 2014: 4923 gopacket dhcp vol.3

Merge in DNS/adguard-home from 4923-gopacket-dhcp-vol.3 to master

Updates #4923.

Squashed commit of the following:

commit 1a09c436e5666a515084cd5e76cfccd67991ae5e
Merge: 95bcf0720 c3f141a0a
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Sep 28 19:38:57 2023 +0300

    Merge branch 'master' into 4923-gopacket-dhcp-vol.3

commit 95bcf07206434fd451632e819926871ba8c14f08
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Sep 28 13:19:42 2023 +0300

    dhcpsvc: fix interface to match legacy version

commit 5da513ce177319f19698c5a8e1d10affaaf5e85c
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Sep 28 12:32:21 2023 +0300

    dhcpsvc: make it build on 32bit

commit 37a935514b1cebdc817cdcd5ec3562baeafbc42d
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Sep 27 19:39:35 2023 +0300

    dhcpd: fix v6 as well

commit 03b5454b04c4fdb3fe928d661562883dc3e09d81
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Sep 27 19:34:17 2023 +0300

    dhcpsvc: imp code, docs

commit 91a0e451f78fba64578cc541f7ba66579c31d388
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Sep 22 15:25:58 2023 +0300

    dhcpsvc: imp filing

commit 57c91e1194caa00a69e62b6655b1b4e38b69b89f
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Sep 22 15:23:02 2023 +0300

    dhcpsvc: imp code

commit d86be56efbfc121c9fe2c5ecef992b4523e04d57
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Sep 14 12:24:39 2023 +0300

    dhcpsvc: imp code, docs

commit c9ef29057e9e378779d1a7938ad13b6eebda8f50
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Sep 13 17:53:55 2023 +0300

    dhcpsvc: add constructor, validations, tests

commit f2533ed64e4ef439603b9cdf9596f8b0c4a87cf1
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Sep 12 23:05:42 2023 +0500

    WIP

internal/dhcpsvc/server.go

@@ -0,0 +1,77 @@
+package dhcpsvc
+
+import (
+	"fmt"
+	"sync/atomic"
+	"time"
+
+	"golang.org/x/exp/maps"
+	"golang.org/x/exp/slices"
+)
+
+// DHCPServer is a DHCP server for both IPv4 and IPv6 address families.
+type DHCPServer struct {
+	// enabled indicates whether the DHCP server is enabled and can provide
+	// information about its clients.
+	enabled *atomic.Bool
+
+	// localTLD is the top-level domain name to use for resolving DHCP
+	// clients' hostnames.
+	localTLD string
+
+	// interfaces4 is the set of IPv4 interfaces sorted by interface name.
+	interfaces4 []*iface4
+
+	// interfaces6 is the set of IPv6 interfaces sorted by interface name.
+	interfaces6 []*iface6
+
+	// icmpTimeout is the timeout for checking another DHCP server's presence.
+	icmpTimeout time.Duration
+}
+
+// New creates a new DHCP server with the given configuration.  It returns an
+// error if the given configuration can't be used.
+//
+// TODO(e.burkov):  Use.
+func New(conf *Config) (srv *DHCPServer, err error) {
+	if !conf.Enabled {
+		// TODO(e.burkov):  Perhaps return [Empty]?
+		return nil, nil
+	}
+
+	ifaces4 := make([]*iface4, len(conf.Interfaces))
+	ifaces6 := make([]*iface6, len(conf.Interfaces))
+
+	ifaceNames := maps.Keys(conf.Interfaces)
+	slices.Sort(ifaceNames)
+
+	var i4 *iface4
+	var i6 *iface6
+
+	for _, ifaceName := range ifaceNames {
+		iface := conf.Interfaces[ifaceName]
+
+		i4, err = newIface4(ifaceName, iface.IPv4)
+		if err != nil {
+			return nil, fmt.Errorf("interface %q: ipv4: %w", ifaceName, err)
+		} else if i4 != nil {
+			ifaces4 = append(ifaces4, i4)
+		}
+
+		i6 = newIface6(ifaceName, iface.IPv6)
+		if i6 != nil {
+			ifaces6 = append(ifaces6, i6)
+		}
+	}
+
+	enabled := &atomic.Bool{}
+	enabled.Store(conf.Enabled)
+
+	return &DHCPServer{
+		enabled:     enabled,
+		interfaces4: ifaces4,
+		interfaces6: ifaces6,
+		localTLD:    conf.LocalDomainName,
+		icmpTimeout: conf.ICMPTimeout,
+	}, nil
+}