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/**
 @page dhcpv6Hooks The Hooks API for the DHCPv6 Server

 @section dhcpv6HooksIntroduction 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.  Information for Kea maintainers can be
 found in the @ref hooksComponentDeveloperGuide.

 This manual is more specialised and is aimed at developers of hook
 code for the DHCPv6 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.
 - Skip flag action: the action taken by the server if a callout chooses to set
    the "skip" flag.

@section dhcpv6HooksHookPoints Hooks in the DHCPv6 Server

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

 @subsection dhcpv6HooksBuffer6Receive buffer6_receive

 - @b Arguments:
   - name: @b query6, type: isc::dhcp::Pkt6Ptr, direction: <b>in/out</b>

 - @b Description: This callout is executed when an incoming DHCPv6
   packet is received and the data stored in a buffer. The sole argument -
   query6 - contains a pointer to an isc::dhcp::Pkt6 object that contains
   the received information stored in the data_ field. Basic information
   like protocol, source/destination addresses and ports are set, but
   the contents of the buffer have not yet been parsed.  That means that
   the options_ field (that will eventually contain a list of objects
   representing the received options) is empty, so none of the methods
   that operate on it (e.g., getOption()) will work. The primary purpose
   of this early call is to offer the ability to modify incoming packets
   in their raw form. Unless you need to access to the raw data, it is
   usually better to install your callout on the pkt6_receive hook point.

 - <b>Skip flag action</b>: If any callout sets the skip flag, the
   server will assume that the callout parsed the buffer and added then
   necessary option objects to the options_ field; the server will not
   do any parsing. If the callout sets the skip flag but does not parse
   the buffer, the server will most probably drop the packet due to
   the absence of mandatory options. If you want to drop the packet,
   see the description of the skip flag in the pkt6_receive hook point.

 @subsection dhcpv6HooksPkt6Receive pkt6_receive

 - @b Arguments:
   - name: @b query6, type: isc::dhcp::Pkt6Ptr, direction: <b>in/out</b>

 - @b Description: This callout is executed when an incoming DHCPv6
   packet is received and its content is parsed. The sole argument -
   query6 - contains a pointer to an isc::dhcp::Pkt6 object that contains
   all information regarding incoming packet, including its source and
   destination addresses, the interface over which it was received, a list
   of all options present within and relay information.  All fields of
   the Pkt6 object can be modified at this time, except data_. (data_
   contains the incoming packet as raw buffer. By the time this hook is
   reached, that information has already been parsed and is available though
   other fields in the Pkt6 object.  For this reason, it doesn't make
   sense to modify it.)

 - <b>Skip flag action</b>: If any callout sets the skip flag, 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 dhcpv6HooksSubnet6Select subnet6_select

 - @b Arguments:
   - name: @b query6, type: isc::dhcp::Pkt6Ptr, direction: <b>in/out</b>
   - name: @b subnet6, type: isc::dhcp::Subnet6Ptr, direction: <b>in/out</b>
   - name: @b subnet6collection, type: const isc::dhcp::Subnet6Collection *, direction: <b>in</b>

 - @b Description: This callout is executed when a subnet is being
   selected for the incoming packet. All parameters, addresses and
   prefixes will be assigned from that subnet. A callout can select a
   different subnet if it wishes so, the list of all subnets currently
   configured being provided as 'subnet6collection'. The list itself must
   not be modified.

 - <b>Skip flag action</b>: If any callout installed on 'subnet6_select'
   sets the skip flag, the server will not select any subnet. Packet processing
   will continue, but will be severely limited (i.e. only global options
   will be assigned).

@subsection dhcpv6HooksLease6Select lease6_select

 - @b Arguments:
   - name: @b subnet6, type: isc::dhcp::Subnet6Ptr, direction: <b>in</b>
   - name: @b fake_allocation, type: bool, direction: <b>in</b>
   - name: @b lease6, type: isc::dhcp::Lease6Ptr, direction: <b>in/out</b>

 - @b Description: This callout is executed after the server engine
   has selected a lease for client's request but before the lease
   has been inserted into the database. Any modifications made to the
   isc::dhcp::Lease6 object will be stored in the lease's record in the
   database. The callout should make sure that any modifications are
   sanity checked as the server will use that data as is with no further
   checking.\n\n The server processes lease requests for SOLICIT and
   REQUEST in a very similar way. The only major difference is that
   for SOLICIT the lease is just selected; it is not inserted into
   the database.  It is possible to distinguish between SOLICIT and
   REQUEST by checking value of the fake_allocation flag: a value of true
   means that the lease won't be inserted into the database (SOLICIT),
   a value of false means that it will (REQUEST).

 - <b>Skip flag action</b>: If any callout installed on 'lease6_select'
   sets the skip flag, the server will not assign that particular lease.
   Packet processing will continue and the client may get other addresses
   or prefixes if it requested more than one address and/or prefix.

@subsection dhcpv6HooksLease6Renew lease6_renew

 - @b Arguments:
   - name: @b query6, type: isc::dhcp::PktPtr, direction: <b>in</b>
   - name: @b lease6, type: isc::dhcp::Lease6Ptr, direction: <b>in/out</b>
   - name: @b ia_na, type: boost::shared_ptr<Option6IA>, direction: <b>in/out</b>

 - @b Description: This callout is executed when the server engine is
   about to renew an existing lease. The client's request is provided as
   the query6 argument and the existing lease with the appropriate fields
   already modified is given in the lease6 argument. The final argument,
   ia_na, is the IA_NA option that will be sent back to the client.
   Callouts installed on the lease6_renew may modify the content of
   the lease6 object. Care should be taken however, as that modified
   information will be written to the database without any further
   checking. \n\n Although the envisaged usage assumes modification of T1,
   T2, preferred and valid lifetimes only, other parameters associated
   with the lease may be modified as well. The only exception is the addr_
   field, which must not be modified as it is used by the database to
   select the existing lease to be updated. Care should also be taken to
   modify the ia_na argument to match any changes in the lease6 argument.
   If a client sends more than one IA_NA option, callouts will be called
   separately for each IA_NA instance. The callout will be called only
   when the update is valid, i.e. conditions such as an invalid addresses
   or invalid iaid renewal attempts will not trigger this hook point.

 - <b>Skip flag action</b>: If any callout installed on 'lease6_renew'
   sets the skip flag, the server will not renew the lease. Under these
   circumstances, the callout should modify the ia_na argument to reflect
   this fact; otherwise the client will think the lease was renewed and continue
   to operate under this assumption.

@subsection dhcpv6HooksLease6Release lease6_release

 - @b Arguments:
   - name: @b query6, type: isc::dhcp::PktPtr, direction: <b>in</b>
   - name: @b lease6, type: isc::dhcp::Lease6Ptr, direction: <b>in/out</b>

 - @b Description: This callout is executed when the server engine is
   about to release an existing lease. The client's request is provided
   as the query6 argument and the existing lease is given in the lease6
   argument.  Although the lease6 structure may be modified, it doesn't
   make sense to do so as it will be destroyed immediately the callouts
   finish execution.

 - <b>Skip flag action</b>: If any callout installed on 'lease6_release'
   sets the skip flag, the server will not delete the lease, which will
   remain in the database until it expires. However, the server will send out
   the response back to the client as if it did.

@subsection dhcpv6HooksPkt6Send pkt6_send

 - @b Arguments:
   - name: @b response6, type: isc::dhcp::Pkt6Ptr, direction: <b>in/out</b>

 - @b Description: This callout is executed when server's response
   is about to be send back to the client. The sole argument - response6 -
   contains a pointer to an isc::dhcp::Pkt6 object that contains the
   packet, with set source and destination addresses, interface over which
   it will be send, list of all options and relay information.  All fields
   of the Pkt6 object can be modified at this time.  It should be noted that
   unless the callout sets the skip flag (see below), anything placed in the
   bufferOut_ field will be overwritten when the callout returns.
   (bufferOut_ is scratch space used for constructing the packet.)

 - <b>Skip flag action</b>: If any callout sets the skip flag, the server
   will assume that the callout did pack the transaction-id, message type and
   option objects into the bufferOut_ field and will skip packing part.
   Note that if the callout sets skip flag, but did not prepare the
   output buffer, the server will send a zero sized message that will be
   ignored by the client. If you want to drop the packet, please see
   skip flag in the buffer6_send hook point.

@subsection dhcpv6HooksBuffer6Send buffer6_send

 - @b Arguments:
   - name: @b response6, type: isc::dhcp::Pkt6Ptr, direction: <b>in/out</b>

 - @b Description: This callout is executed when server's response is
   assembled into binary form and is about to be send back to the
   client. The sole argument - response6 - contains a pointer to an
   isc::dhcp::Pkt6 object that contains the packet, with set source and
   destination addresses, interface over which it will be sent, list of
   all options and relay information. All options are already encoded
   in bufferOut_ field. It doesn't make sense to modify anything but the
   contents of bufferOut_ at this time (although if it is a requirement
   to modify that data, it will probably be found easier to modify the
   option objects in a callout attached to the pkt6_send hook).

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

*/
