Pull request 2096: 4923 gopacket dhcp vol.5

Updates #4923.

Squashed commit of the following:

commit 762a3f9b7d7d4dd8799e8bf4df632b8d50321bf8
Merge: 2af65b42a 34a34dc05
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Dec 12 11:13:21 2023 +0300

    Merge branch 'master' into 4923-gopacket-dhcp-vol.5

commit 2af65b42a62b92ec5bc28ef81eb6f08d0f9f443e
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Dec 8 16:22:51 2023 +0300

    dhcpsvc: imp code

commit 71233b9952b0d74e7e890d6755652877bc4c543b
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Dec 8 15:26:25 2023 +0300

    dhcpsvc: imp docs

commit 2949544f32b52d592f76e53062cf017e08073d29
Merge: 593e9edaa 214175eb4
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Dec 7 16:48:18 2023 +0300

    Merge branch 'master' into 4923-gopacket-dhcp-vol.5

commit 593e9edaa9e776db35b3bedfa942c015d7b1e4af
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Dec 7 16:45:10 2023 +0300

    dhcpsvc: imp docs

commit cdb1915c7db3bf36800b40bc6aedc0e20f55f899
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Dec 7 16:33:56 2023 +0300

    dhcpsvc: imp code

commit a0c423c2b39d674debf4c0fedf6208f656be861a
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Dec 5 20:56:43 2023 +0300

    dnspsvc: add opts

commit 050ab7f9410c206287c66376e21d36af8a3da384
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Dec 5 20:17:37 2023 +0300

    WIP

internal/dhcpsvc/v6.go

@@ -5,7 +5,9 @@ import (
 	"net/netip"
 	"time"
 
+	"github.com/AdguardTeam/golibs/log"
 	"github.com/google/gopacket/layers"
+	"golang.org/x/exp/slices"
 )
 
 // IPv6Config is the interface-specific configuration for DHCPv6.
@@ -13,8 +15,10 @@ type IPv6Config struct {
 	// RangeStart is the first address in the range to assign to DHCP clients.
 	RangeStart netip.Addr
 
-	// Options is the list of DHCP options to send to DHCP clients.
-	Options layers.DHCPOptions
+	// Options is the list of DHCP options to send to DHCP clients.  The options
+	// with zero length are treated as deletions of the corresponding options,
+	// either implicit or explicit.
+	Options layers.DHCPv6Options
 
 	// LeaseDuration is the TTL of a DHCP lease.
 	LeaseDuration time.Duration
@@ -58,6 +62,15 @@ type iface6 struct {
 	// name is the name of the interface.
 	name string
 
+	// implicitOpts are the DHCPv6 options listed in RFC 8415 (and others) and
+	// initialized with default values.  It must not have intersections with
+	// explicitOpts.
+	implicitOpts layers.DHCPv6Options
+
+	// explicitOpts are the user-configured options.  It must not have
+	// intersections with implicitOpts.
+	explicitOpts layers.DHCPv6Options
+
 	// leaseTTL is the time-to-live of dynamic leases on this interface.
 	leaseTTL time.Duration
 
@@ -78,11 +91,45 @@ func newIface6(name string, conf *IPv6Config) (i *iface6) {
 		return nil
 	}
 
-	return &iface6{
+	i = &iface6{
 		name:         name,
 		rangeStart:   conf.RangeStart,
 		leaseTTL:     conf.LeaseDuration,
 		raSLAACOnly:  conf.RASLAACOnly,
 		raAllowSLAAC: conf.RAAllowSLAAC,
 	}
+	i.implicitOpts, i.explicitOpts = conf.options()
+
+	return i
+}
+
+// options returns the implicit and explicit options for the interface.  The two
+// lists are disjoint and the implicit options are initialized with default
+// values.
+//
+// TODO(e.burkov):  Add implicit options according to RFC.
+func (conf *IPv6Config) options() (implicit, explicit layers.DHCPv6Options) {
+	// Set default values of host configuration parameters listed in RFC 8415.
+	implicit = layers.DHCPv6Options{}
+	slices.SortFunc(implicit, compareV6OptionCodes)
+
+	// Set values for explicitly configured options.
+	for _, exp := range conf.Options {
+		i, found := slices.BinarySearchFunc(implicit, exp, compareV6OptionCodes)
+		if found {
+			implicit = slices.Delete(implicit, i, i+1)
+		}
+
+		explicit = append(explicit, exp)
+	}
+
+	log.Debug("dhcpsvc: v6: implicit options: %s", implicit)
+	log.Debug("dhcpsvc: v6: explicit options: %s", explicit)
+
+	return implicit, explicit
+}
+
+// compareV6OptionCodes compares option codes of a and b.
+func compareV6OptionCodes(a, b layers.DHCPv6Option) (res int) {
+	return int(a.Code) - int(b.Code)
 }