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/v4.go

@@ -0,0 +1,113 @@
+package dhcpsvc
+
+import (
+	"fmt"
+	"net"
+	"net/netip"
+	"time"
+
+	"github.com/google/gopacket/layers"
+)
+
+// IPv4Config is the interface-specific configuration for DHCPv4.
+type IPv4Config struct {
+	// GatewayIP is the IPv4 address of the network's gateway.  It is used as
+	// the default gateway for DHCP clients and also used in calculating the
+	// network-specific broadcast address.
+	GatewayIP netip.Addr
+
+	// SubnetMask is the IPv4 subnet mask of the network.  It should be a valid
+	// IPv4 CIDR (i.e. all 1s followed by all 0s).
+	SubnetMask netip.Addr
+
+	// RangeStart is the first address in the range to assign to DHCP clients.
+	RangeStart netip.Addr
+
+	// RangeEnd is the last address in the range to assign to DHCP clients.
+	RangeEnd netip.Addr
+
+	// Options is the list of DHCP options to send to DHCP clients.
+	Options layers.DHCPOptions
+
+	// LeaseDuration is the TTL of a DHCP lease.
+	LeaseDuration time.Duration
+
+	// Enabled is the state of the DHCPv4 service, whether it is enabled or not
+	// on the specific interface.
+	Enabled bool
+}
+
+// validate returns an error in conf if any.
+func (conf *IPv4Config) validate() (err error) {
+	switch {
+	case conf == nil:
+		return errNilConfig
+	case !conf.Enabled:
+		return nil
+	case !conf.GatewayIP.Is4():
+		return mustBeErr("gateway ip", "be a valid ipv4", conf.GatewayIP)
+	case !conf.SubnetMask.Is4():
+		return mustBeErr("subnet mask", "be a valid ipv4 cidr mask", conf.SubnetMask)
+	case !conf.RangeStart.Is4():
+		return mustBeErr("range start", "be a valid ipv4", conf.RangeStart)
+	case !conf.RangeEnd.Is4():
+		return mustBeErr("range end", "be a valid ipv4", conf.RangeEnd)
+	case conf.LeaseDuration <= 0:
+		return mustBeErr("lease duration", "be less than %d", conf.LeaseDuration)
+	default:
+		return nil
+	}
+}
+
+// iface4 is a DHCP interface for IPv4 address family.
+type iface4 struct {
+	// gateway is the IP address of the network gateway.
+	gateway netip.Addr
+
+	// subnet is the network subnet.
+	subnet netip.Prefix
+
+	// addrSpace is the IPv4 address space allocated for leasing.
+	addrSpace ipRange
+
+	// name is the name of the interface.
+	name string
+
+	// TODO(e.burkov):  Add options.
+
+	// leaseTTL is the time-to-live of dynamic leases on this interface.
+	leaseTTL time.Duration
+}
+
+// newIface4 creates a new DHCP interface for IPv4 address family with the given
+// configuration.  It returns an error if the given configuration can't be used.
+func newIface4(name string, conf *IPv4Config) (i *iface4, err error) {
+	if !conf.Enabled {
+		return nil, nil
+	}
+
+	maskLen, _ := net.IPMask(conf.SubnetMask.AsSlice()).Size()
+	subnet := netip.PrefixFrom(conf.GatewayIP, maskLen)
+
+	switch {
+	case !subnet.Contains(conf.RangeStart):
+		return nil, fmt.Errorf("range start %s is not within %s", conf.RangeStart, subnet)
+	case !subnet.Contains(conf.RangeEnd):
+		return nil, fmt.Errorf("range end %s is not within %s", conf.RangeEnd, subnet)
+	}
+
+	addrSpace, err := newIPRange(conf.RangeStart, conf.RangeEnd)
+	if err != nil {
+		return nil, err
+	} else if addrSpace.contains(conf.GatewayIP) {
+		return nil, fmt.Errorf("gateway ip %s in the ip range %s", conf.GatewayIP, addrSpace)
+	}
+
+	return &iface4{
+		name:      name,
+		gateway:   conf.GatewayIP,
+		subnet:    subnet,
+		addrSpace: addrSpace,
+		leaseTTL:  conf.LeaseDuration,
+	}, nil
+}