// Copyright (C) 2013-2025 Internet Systems Consortium, Inc. ("ISC")
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

/**
 @page dhcpv4Hooks The Hooks API for the DHCPv4 Server

 @section dhcpv4HooksIntroduction Introduction
 Kea features an API (the "Hooks" API) that allows user-written code to
 be integrated into Kea and called at specific points in its processing.
 An overview of the API and a tutorial for writing such code can be found in
 the @ref hooksdgDevelopersGuide. It also includes information how hooks
 framework can be used to implement additional control commands for the
 DHCPv4 server. Information for Kea maintainers can be found in the
 @ref hooksComponentDeveloperGuide.

 This manual is more specialized and is aimed at developers of hook
 code for the DHCPv4 server. It describes each hook point, what the callouts
 attached to the hook are able to do, and the arguments passed to the
 callouts.  Each entry in this manual has the following information:

 - Name of the hook point.
 - Arguments for the callout.  As well as the argument name and data type, the
   information includes the direction, which can be one of:
   - @b in - the server passes values to the callout but ignored any data
     returned.
   - @b out - the callout is expected to set this value.
   - <b>in/out</b> - the server passes a value to the callout and uses whatever
     value the callout sends back.  Note that the callout may choose not to
     do any modification, in which case the server will use whatever value
     it sent to the callout.
 - Description of the hook. This explains where in the processing the hook
   is located, the possible actions a callout attached to this hook could take,
   and a description of the data passed to the callouts.
 - Next step status: the action taken by the server when a callout chooses to set
   status to specified value. Actions not listed explicitly are not supported.
   If a callout sets status to unsupported value, this specific value will be
   ignored and treated as if the status was CONTINUE.

@section dhcpv4HooksHookPoints Hooks in the DHCPv4 Server

The following list is roughly ordered by appearance of specific hook points during
packet processing, but the exact order depends on the actual processing. Hook points
that are not specific to packet processing (e.g. lease expiration) will be added
to the end of this list.

 @subsection dhcp4HooksDhcpv4SrvConfigured dhcp4_srv_configured
 - @b Arguments:
   - name: @b io_context, type: isc::asiolink::IOServicePtr, direction: <b>in</b>
   - name: @b network_state, type: isc::dhcp::NetworkStatePtr, direction: <b>in</b>
   - name: @b json_config, type: isc::data::ConstElementPtr, direction: <b>in</b>
   - name: @b server_config, type: isc::dhcp::SrvConfigPtr, direction: <b>in</b>

 - @b Description: this callout is executed when the server has completed
   its (re)configuration. The server provides received and parsed configuration
   structures to the hook library.
   If the library uses any IO operations, it should create a local IOService
   object and register it with the IOServiceMgr. This way the local IOService is
   used by the server to run asynchronous operations. The hooks library can use
   the local IOService object to schedule asynchronous tasks which are triggered
   by the DHCP server's main loop. The hook library can use the local IOService
   until the library is unloaded at which stage it must unregister it.
   The "io_context" parameter gives access to the main IOService, but it's use
   has been deprecated in favor of a local IOService to avoid issues when
   unloading the library. The parameter has been deprecated and will be removed
   in future versions.
   The NetworkState object provides access to the DHCP service state of the
   server and allows for enabling and disabling the DHCP service from the hooks
   libraries.

 - <b>Next step status</b>: If any callout sets the status to DROP, the server
   will interrupt the reconfiguration process. The hook callout is expected to
   have completed the cause of the interruption under the "error" handle argument
   with a std::string which is then logged. Finally, this leads to the
   termination of kea-dhcp4.

 @subsection dhcpv4HooksCb4Update cb4_updated
 - @b Arguments:
   - name: audit_entries, type isc::db::AuditEntryCollectionPtr, direction: <b>in</b>

 - @b Description: this callout is executed when the server has completed
   a configuration update using the Config Backend. The server provides
   the audit entries as a never null pointer to a not empty collection
   copied from the update apply method argument.

 - <b>Next step status</b>: Status codes returned by the callouts installed on
   this hook point are ignored.

 @subsection dhcpv4HooksBuffer4Receive buffer4_receive

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in/out</b>

 - @b Description: this callout is executed when the server has received a
   buffer containing a DHCPv4 message, but the message hasn't yet been parsed.
   The sole argument "query4" contains a pointer to the isc::dhcp::Pkt4
   object, which contains the source and destination address of the
   received packet, the interface over which the packet has been received, and
   a raw buffer, stored in the data_ field, containing the DHCPv4 message
   in the wire format. None of the packet fields (op_, hlen_, chaddr_, etc.)
   are set yet. Callouts installed on this hook point can modify the data
    in the received buffer. The server will parse the buffer afterwards.

 - <b>Next step status</b>: If any callout sets the status to DROP, the server
   will drop the packet and start processing the next one.
   If any callout sets the status to SKIP, the server will
   skip the buffer parsing. In this case there is an expectation that
   the callout will parse the options carried in the buffer, create
   @c isc::dhcp::Option objects (or derived) and add them to the "query4"
   object using the @c isc::dhcp::Pkt4::addOption.
   Otherwise the server will find out that some mandatory options are
   missing (e.g. DHCP Message Type) and will drop the message.

 @subsection dhcpv4HooksPkt4Receive pkt4_receive

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in/out</b>

 - @b Description: this callout is executed when an incoming DHCPv4
   packet is received and its content has been parsed. The sole
   argument "query4" contains a pointer to an isc::dhcp::Pkt4 object
   that contains all information regarding incoming packet, including
   its source and destination addresses, interface over which it was
   received, a list of all options present within and the relay
   information.  All fields of the Pkt4 object can be modified at this
   time. By the time this hook is reached, the contents of the data_
   field has been already parsed and stored in other fields. Therefore,
   the modification in the data_ field has no effect.

 - <b>Next step status</b>: If any callout sets the status to SKIP or DROP, the server will
   drop the packet and start processing the next one.  The reason for the drop
   will be logged if logging is set to the appropriate debug level.

@subsection dhcpv4HooksSubnet4Select subnet4_select

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in/out</b>
   - name: @b subnet4, type: isc::dhcp::ConstSubnet4Ptr, direction: <b>in/out</b>
   - name: @b subnet4collection, type: const isc::dhcp::Subnet4Collection *,
     direction: <b>in</b>

 - @b Description: this callout is executed when a subnet is being
   selected for the incoming packet. All parameters and addresses
   will be assigned from that subnet. A callout can select a
   different subnet if it wishes so. The list of all subnets currently
   configured are provided as "subnet4collection". The list itself must
   not be modified.

 - <b>Next step status</b>: If any callout installed on the "subnet4_select" hook
   sets the next step status to DROP, the server cancels current processing,
   drop the packet and start processing the next one.
   If any callout sets the status to SKIP, the server will not select any subnet.
   Packet processing will continue, but will be severely limited.

@subsection dhcpv4HooksHost4Identifier host4_identifier

 - @b Arguments:
   - name: @b query4, type isc::dhcp::Pkt4Ptr, direction: <b>in</b>
   - name: @b id_type, type isc::dhcp::Host::IdentifierType, direction: <b>in/out</b>
   - name: @b id_value, type std::vector<uint8_t>, direction: <b>out</b>

 - @b Description: this callout is executed only if flexible identifiers are
   enabled, i.e. host-reservation-identifiers contain 'flex-id' value. This
   callout enables external library to provide values for flexible identifiers.
   To be able to use this feature, flex_id hook library is required.

 - <b>Next step status</b>: If a callout installed on the "host4_identifier" hook
   point sets the next step status to value other than NEXT_STEP_CONTINUE, the
   identifier will not be used.

When the "early-global-reservations-lookup" flag is true this callout is
called before "subnet4_select".

@subsection dhcpv4HooksLeaseSelect lease4_select

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in</b>
   - name: @b subnet4, type: isc::dhcp::ConstSubnet4Ptr, direction: <b>in</b>
   - name: @b fake_allocation, type: bool, direction: <b>in</b>
   - name: @b lease4, type: isc::dhcp::Lease4Ptr, direction: <b>in/out</b>

 - @b Description: this callout is executed after the server engine
   has selected a lease for the client's request, but before the lease has
   been inserted into the database. Any modifications made to the
   "lease4" object will affect the lease's record in the database.
   The callout should sanity check all modifications as the server will
   use that data as is, with no further checking.\n\n
   The server processes lease requests for DHCPDISCOVER and DHCPREQUEST in a
   very similar way. The only major difference is that for DHCPDISCOVER
   the lease is only selected, but not inserted into the database. The callouts
   may distinguish between DHCPDISCOVER and DHCPREQUEST by checking the
   value of the "fake_allocation" flag: a value of true indicates that the
   lease won't be inserted into the database (DHCPDISCOVER case), a value of
   false indicates that it will (DHCPREQUEST case).

 - <b>Next step status</b>: If any callout installed on the "lease4_select" hook
   sets the next step action to SKIP, the server will not assign any lease and
   the callouts become responsible for the lease assignment. If the callouts
   fail to provide a lease, the packet processing will continue, but client
   will not get an address.

@subsection dhcpv4HooksLeaseRenew lease4_renew

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in</b>
   - name: @b subnet4, type: isc::dhcp::ConstSubnet4Ptr, direction: <b>in</b>
   - name: @b clientid, type: isc::dhcp::ClientId, direction: <b>in</b>
   - name: @b hwaddr, type: isc::dhcp::HWAddr, direction: <b>in</b>
   - name: @b lease4, type: isc::dhcp::Lease4Ptr, direction: <b>in/out</b>

 - @b Description: this callout is executed when the server engine
   is about to renew a lease, as a result of receiving DHCPREQUEST/Renewing
   packet. The "lease4" argument points to @c isc::dhcp::Lease4 object that
   contains the updated values. Callout can modify those values. Care should
   be taken as the server will attempt to update the lease in the database
   without any additional checks.

 - <b>Next step status</b>: If any callout installed on the "lease4_renew" hook
   sets the next step action to SKIP, the server will not update the lease in the
   database and will continue using the old values instead.

@subsection dhcpv4HooksLeaseRelease lease4_release

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in</b>
   - name: @b lease4, type: isc::dhcp::Lease4Ptr, direction: <b>in</b>

 - @b Description: this callout is executed when the server engine
   is about to release a lease, as a result of receiving DHCPRELEASE packet.
   The "lease4" argument points to @c Lease4 object that contains the lease to
   be released. It doesn't make sense to modify it at this time.

 - <b>Next step status</b>: If any callout installed on the "lease4_release" hook
   sets the next step action to SKIP or DROP, the server will not delete the lease.
   It will be kept in the database and will go through the regular expiration/reuse
   process.

@subsection dhcpv4HooksLeaseDecline lease4_decline

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in</b>
   - name: @b lease4, type: isc::dhcp::Lease4Ptr, direction: <b>in</b>

 - @b Description: this callout is executed when the server engine
   is about to decline a lease, as a result of receiving DHCPDECLINE packet.
   The server already sanity checked it (the packet is sane, attempts to decline
   a lease that is valid and belongs to the client that requests its decline).
   The "lease4" argument points to @c Lease4 object that contains the lease to
   be released. Note this lease still contains client identifying information.
   That data is provided for informational purposes and it doesn't make sense to
   modify it at this time. All the information will be removed from the lease
   before it is updated in the database.

 - <b>Next step status</b>: If any callout installed on the "lease4_decline" hook
   sets the next step action to SKIP or DROP, the server will not decline the lease.
   Care should be taken when setting this status.  The lease will be kept in
   the database as it is and the client will incorrectly assume that the server
   marked this lease as unavailable. If the client restarts its configuration,
   it will get the same (not declined) lease as a result.

@subsection dhcpv4HooksDdns4Update ddns4_update

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in</b>
   - name: @b response4, type: isc::dhcp::Pkt4Ptr, direction: <b>in</b>
   - name: @b subnet4, type: isc::dhcp::ConstSubnet4Ptr, direction: <b>in</b>
   - name: @b hostname, type: std::string, direction: <b>in/out</b>
   - name: @b fwd-update, type: bool, direction: <b>in/out</b>
   - name: @b rev-update, type: bool, direction: <b>in/out</b>
   - name: @b ddns-params, type: isc::dhcp::DdnsParamsPtr, direction: <b>in</b>

 - @b Description: this callout is executed after the server has selected
   a lease and has formed a host name to associate with the lease and/or use
   as the basis for the FQDN for DNS updates (if enabled), but before the
   lease has been committed or any NameChangeRequests have been generated to
   send to <b>kea-dhcp-ddns</b>.  Thus it provides an opportunity to alter the
   host name as well as whether or not forward and/or reverse updates are
   enabled.

   Upon entry into the callout, the arguments <b>hostname</b>,<b>fwd-update</b>,
   and <b>rev-update</b> have been set by the server based on the client packet,
   and various configuration values (e.g host reservations, DDNS behavioral
   parameters, etc).  Upon return from the callout, any changes to these
   values will be applied as follows:
   - If <b>hostname</b> has changed it will be used to update the outbound
   host name (option 12) if it exists, the output FQDN option (option 81)
   if it exists, and used as the FQDN sent in DNS updates
   - Forward DNS update(s) will be done if <b>fwd-update</b> is true (and
   <b>kea-dhcp-ddns</b> connectivity is enabled)
   - Reverse DNS update(s) will be done if <b>rev-update</b> is true (and
   <b>kea-dhcp-ddns</b> connectivity is enabled)

 - <b>Next step status</b>: Not applicable, its value will be ignored.

@subsection dhcpv4Leases4Committed leases4_committed

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in</b>
   - name: @b response4, type: isc::dhcp::Pkt4Ptr, direction: <b>in</b>
   - name: @b leases4, type: isc::dhcp::Lease4CollectionPtr, direction: <b>in</b>
   - name: @b deleted_leases4, type: isc::dhcp::Lease4CollectionPtr, direction: <b>in</b>

 - @b Description: this callout is executed when the server has applied all
   lease changes as a result of DHCP message processing. This includes
   writing new lease into the database, releasing an old lease for this
   client or declining a lease. This callout is executed only for the
   DHCP client messages which may cause lease changes, i.e. DHCPREQUEST,
   DHCPRELEASE and DHCPDECLINE. This callout is not executed for DHCPDISCOVER
   and DHCPINFORM. If the callouts are executed as a result of DHCPREQUEST
   message, it is possible that both leases collections hold leases to be
   handled. This is the case when the new lease allocation replaces an existing
   lease for the client. The "deleted_leases4" object will hold a previous
   lease instance and the "leases4" object will hold the new lease for this
   client. The callouts should be prepared to handle such situation. When
   the callout is executed as a result DHCPRELEASE, the callout will typically
   receive only one lease (being released) in the "deleted_leases4" object.
   Both leases collections are always provided to the callouts,
   even though they may sometimes be empty.

 - <b>Next step status</b>: If any callout installed on the "leases4_committed"
   sets the next step action to DROP the server will drop the processed query.
   If it sets the next step action to PARK, the server will park the processed
   packet (hold packet processing) until the hook libraries explicitly unpark
   the packet after they are done performing asynchronous operations.

@subsection dhcpv4Lease4Offer lease4_offer

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in</b>
   - name: @b response4, type: isc::dhcp::Pkt4Ptr, direction: <b>in</b>
   - name: @b leases4, type: isc::dhcp::Lease4CollectionPtr, direction: <b>in</b>
   - name: @b offer_lifetime, type: uint32_t, direction: <b>in</b>
   - name: @b old_lease, type: isc::dhcp::Lease4Ptr, direction: <b>in</b>
   - name: @b host, type: isc::dhcp::HostPtr, direction: <b>in</b>

 - @b Description: this callout is executed when the server has received
   DHCPDISCOVER from the client and the DHCPOFFER has been constructed but not
   yet sent back. The argument "query4" contains a pointer to an isc::dhcp::Pkt4
   object that contains all information regarding incoming DHCPDISCOVER packet.
   The "response4" argument contains a pointer to an isc::dhcp::Pkt4 object that
   contains all information regarding the outgoing::DHCPOFFER packet.
   The "leases4" object will hold the pointer to the new candidate lease for this
   client. The "offer_lifetime" argument comes from the context information for the
   DHCPv4 lease allocation. If "offer_lifetime" is not zero,  it indicates that temporary
   allocation on DHCPDISCOVER is enabled and the lease to be offered already exists in
   the lease store. The "old_lease" argument also comes from the context information
   for the DHCPv4 lease allocation. If not Null, it is a pointer to an old lease that
   the client had before.  The "host" argument is a pointer to the host reserveration
   associated with the lease. If there is no such reservation the argument is Null.

 - <b>Next step status</b>: If any callout installed on the "lease4_offer" hook point
   sets the next step action to DROP, the server will drop the processed query
   and discard the prepared DHCPOFFER rather than sending it to the client.
   If it sets the next step action to PARK, the server will park the processed
   packet (hold packet processing) until the hook libraries explicitly unpark
   the packet after they are done performing asynchronous operations.

   Prior to unparking the packet, callouts are expected to set the argument,
   "offer_address_in_use" to false if the address is available and the offer should
   be sent to the client. If the address should be declined, callouts should set
   the argument to true. This will cause the server to discard the DHCPOFFER, mark
   the lease DECLINED in the lease store, update the relevant statistics, and then
   invoke callouts installed for lease4_server_decline.

@subsection dhcpv4Lease4ServerDecline lease4_server_decline

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in</b>
   - name: @b lease4, type: isc::dhcp::Lease4Ptr, direction: <b>in</b>

 - @b Description: this callout is executed after a lease4_offer callout has
   determined that the lease to be offered is already in use by an unknown
   client.  At this point in processing, the offer has been discarded, the lease
   has been marked DECLINED in the lease store, and the relevant statistics have
   been updated.  The argument "query4" contains a pointer to an isc::dhcp::Pkt4
   object that contains all information regarding incoming DHCPDISCOVER packet.
   The "leases4" object will hold the pointer to the declined lease.

 - <b>Next step status</b>: Not applicable, its value will be ignored.

@subsection dhcpv4HooksPkt4Send pkt4_send

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in</b>
   - name: @b response4, type: isc::dhcp::Pkt4Ptr, direction: <b>in/out</b>
   - name: @b subnet4, type: isc::dhcp::ConstSubnet4Ptr, direction: <b>in</b>

 - @b Description: this callout is executed when server's response
   is about to be sent back to the client. The argument "response4"
   contains a pointer to an isc::dhcp::Pkt4 object carrying the
   packet, with source and destination addresses set, interface over which
   it will be sent, and a list of all options and relay information.  All fields
   of the @c Pkt4 object can be modified at this time, except @c buffer_out_.
   (This is scratch space used for constructing the packet after all
   pkt4_send callouts are complete, so any changes to that field will
   be overwritten.)\n\n
   The argument query4 contains a pointer to the corresponding query packet
   (allowing to perform correlation between response and query). This object
   cannot be modified.
   The argument subnet4 contains a pointer to the selected subnet (if one).
   This object cannot be modified.

 - <b>Next step action</b>: if any callout installed on the "pkt4_send" hook
   sets the next step action to SKIP, the server will not construct the raw
   buffer. The expectation is that if the callout set skip flag, it is
   responsible for constructing raw form on its own. Otherwise the output
   packet will be sent with zero length. If any callout set the next step action
   to DROP, the server will drop the packet.

@subsection dhcpv4HooksBuffer4Send buffer4_send

 - @b Arguments:
   - name: @b response4, type: isc::dhcp::Pkt4Ptr, direction: <b>in/out</b>

 - @b Description: this callout is executed when server's response
   is about to be sent back to the client. The sole argument "response4"
   contains a pointer to an @c isc::dhcp::Pkt4 object that contains the
   packet, with source and destination addresses set, interface over which
   it will be sent, and a list of all options and relay information. The raw
   on-wire form is already prepared in @c buffer_out_ (see
   @c isc::dhcp::Pkt4::getBuffer())
   Callouts should not modify the packet fields or options contents at this
   time, because they were already used to construct on-wire buffer. Their
   modification would have no effect.

 - <b>Next step status</b>: if any callout sets the next step action to SKIP or DROP,
   the server will drop this response packet. However, the original request
   packet from a client was processed, so server's state most likely has changed
   (e.g. lease was allocated). Setting this flag merely stops the change
   being communicated to the client.

@subsection dhcpv4HooksLease4Expire lease4_expire

- @b Arguments:
  - name: @b lease4, type: isc::dhcp::Lease4Ptr, direction: <b>in/out</b>
  - name: @b remove_lease, type: bool, direction: <b>in</b>

- @b Description: this callout is executed for each expired lease when
  the server performs reclamation of the expired leases. During this
  process the server executes "lease4_expire" callout, removes the DNS
  records associated with this lease and finally removes the lease from
  the database or updates its status to "expired-reclaimed". The "lease4"
  argument contains the pointer to the lease being reclaimed. The second
  argument "remove_lease" indicates if the reclaimed leases should be
  removed from the lease database (if true), or their state should be
  set to "expired-reclaimed" in the lease database. This argument
  is only used by the callout if it takes responsibility for the lease
  reclamation, i.e. it sets the "skip" flag to "true". The "remove_lease"
  argument is set to "true" if the "flush-reclaimed-timer-wait-time" is
  set to 0 in the server configuration file.

- <b>Next step status</b>: if the callout sets the next step action to SKIP,
  the server will assume that the callout has fully reclaimed the lease, i.e.
  performed the DNS update and updated the lease in the database. The
  server will not perform any further actions on the lease for which the
  skip flag has been set. It is important to note that if the callout
  sets this flag but fails to reclaim the lease in the database, the
  reclamation routine will repeatedly process this lease in subsequent
  runs. Therefore, the implementors of this callout must make sure that
  skip flag is only set when the lease has been actually reclaimed in the
  database by the callout.

@subsection dhcpv4HooksLease4Recover lease4_recover

- @b Arguments:
  - name: @b lease4, type: isc::dhcp::Lease4Ptr, direction: <b>in</b>

- @b Description: this callout is executed for each declined lease that
  has expired (was put aside for the duration of decline-probation-period)
  and is being recovered. The lease has already been stripped
  from any client identifying information when it was put into declined
  state. In principle the callouts can modify the lease in this hook,
  but it makes little sense. There's no useful data in the lease, except
  the IPv4 address (which must not be modified).

- <b>Next step status</b>: if the callout sets the next step action to SKIP,
  the server will skip the lease recovery. In other words, it will keep
  the lease as is. This is not recommended in general, as the declined
  expired leases will remain in the database and their recovery will
  be attempted during the next reclaim cycle.

@subsection dhcpv4HooksCommandProcessed command_processed

 - @b Arguments:
   - name: @b name, type: std::string, direction: <b>in</b>
   - name: @b arguments type: isc::data::ConstElementPtr, direction: <b>in</b>
   - name: @b response, type: isc::data::ConstElementPtr, direction: <b>in/out</b>

 - @b Description: this callout is executed after the DHCPv4 server receives
   and processes a control command over the command channel (typically unix domain socket).
   The "name" argument is the name of the command processed.
   The "arguments" argument is a pointer to the parsed JSON structure
   containing the command's input arguments.  The "response" argument
   is the parsed JSON structure containing the response generated by
   the command processing.

 - <b>Next step status</b>: Not applicable, its value will be ignored.

@section dhcpv4HooksOptionsAccess Accessing DHCPv4 Options within a Packet
When the server constructs a response message to a client it includes
DHCP options configured for this client in a response message. Apart
from the dynamically created options, such as Client FQDN option, it
typically includes many options specified in the server configuration
and held within the configuration structures by @c CfgMgr. Option
instances are created once, during server configuration, and the
@c CfgMgr holds pointers to those instances until the next server
reconfiguration.

When the server includes an option in a response message it copies
a pointer to the instance of this option, rather than entire option.
This ensures the good performance of response message creation. However,
it also implies that any modification to the option carried in the
DHCP response will affect an instance of this option in the server
configuration structures. This is obviously not desired as it would
affect all subsequent DHCP transactions involving this option. The
DHCP server code avoids modifying the options included in the messages
so it is possible to ensure good performance without a risk of
accidentally modifying server configuration. The situation is
different with hooks libraries which purpose is, in many cases,
to modify values of options inserted by the server.

Thus, @c Pkt class provides a mechanism to return a copy of an
option to a caller (e.g. a callout), rather than an instance
shared with the @c CfgMgr. This mechanism is enabled for all instances
of @c Pkt4 passed to the callouts, i.e. "query4" and "response4"
arguments. It is also automatically disabled when the callout
returns the control back to the server.

At every hook point, where the server passes an instance of a packet
to the callouts, the server calls
@c isc::dhcp::Pkt4::setCopyRetrievedOptions (true)
to force copying options retrieved by @c isc::dhcp::Pkt4::getOption
within callouts. The copied option replaces an original option within a
packet and any modification to the option content by the callout
would only affect the option instance associated with the packet.

On the other hand, copying each retrieved option may be expensive.
If performance of a hook library is a concern, it is possible for the
hook library to disable copying retrieved options by calling
@c isc::dhcp::Pkt4::setCopyRetrievedOptions (false) within a callout.
In this case however, the hook library implementer must be aware that any
modification of the option instance would affect the server configuration
and may disrupt server's operation. Thus, disabling copying of retrieved
options is not recommended unless the hook library is not intended
to modify configured options carried within a packet.

*/
