xref: /dports/net/kea/kea-2.0.1/src/bin/dhcp4/dhcp4_srv.h

// Copyright (C) 2011-2021 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/.

#ifndef DHCPV4_SRV_H
#define DHCPV4_SRV_H

#include <asiolink/io_service.h>
#include <dhcp/dhcp4.h>
#include <dhcp/option.h>
#include <dhcp/option_string.h>
#include <dhcp/option4_client_fqdn.h>
#include <dhcp/option_custom.h>
#include <dhcp/pkt4.h>
#include <dhcp_ddns/ncr_msg.h>
#include <dhcpsrv/alloc_engine.h>
#include <dhcpsrv/callout_handle_store.h>
#include <dhcpsrv/cb_ctl_dhcp4.h>
#include <dhcpsrv/cfg_option.h>
#include <dhcpsrv/d2_client_mgr.h>
#include <dhcpsrv/network_state.h>
#include <dhcpsrv/subnet.h>
#include <hooks/callout_handle.h>
#include <process/daemon.h>

#include <functional>
#include <iostream>
#include <queue>

// Undefine the macro OPTIONAL which is defined in some operating
// systems but conflicts with a member of the RequirementLevel enum in
// the server class.

#ifdef OPTIONAL
#undef OPTIONAL
#endif

namespace isc {
namespace dhcp {

/// @brief DHCPv4 message exchange.
///
/// This class represents the DHCPv4 message exchange. The message exchange
/// consists of the single client message, server response to this message
/// and the mechanisms to generate the server's response. The server creates
/// the instance of the @c Dhcpv4Exchange for each inbound message that it
/// accepts for processing.
///
/// The use of the @c Dhcpv4Exchange object as a central repository of
/// information about the message exchange simplifies the API of the
/// @c Dhcpv4Srv class.
///
/// Another benefit of using this class is that different methods of the
/// @c Dhcpv4Srv may share information. For example, the constructor of this
/// class selects the subnet and multiple methods of @c Dhcpv4Srv use this
/// subnet, without the need to select it again.
///
/// @todo This is the initial version of this class. In the future a lot of
/// code from the @c Dhcpv4Srv class will be migrated here.
class Dhcpv4Exchange {
public:
    /// @brief Constructor.
    ///
    /// The constructor selects the subnet for the query and checks for the
    /// static host reservations for the client which has sent the message.
    /// The information about the reservations is stored in the
    /// @c AllocEngine::ClientContext4 object, which can be obtained by
    /// calling the @c getContext.
    ///
    /// @param alloc_engine Pointer to the instance of the Allocation Engine
    /// used by the server.
    /// @param query Pointer to the client message.
    /// @param subnet Pointer to the subnet to which the client belongs.
    /// @param drop if it is true the packet will be dropped.
    Dhcpv4Exchange(const AllocEnginePtr& alloc_engine, const Pkt4Ptr& query,
                   const Subnet4Ptr& subnet, bool& drop);

    /// @brief Initializes the instance of the response message.
    ///
    /// The type of the response depends on the type of the query message.
    /// For the DHCPDISCOVER the DHCPOFFER is created. For the DHCPREQUEST
    /// and DHCPINFORM the DHCPACK is created. For the DHCPRELEASE the
    /// response is not initialized.
    void initResponse();

    /// @brief Initializes the DHCPv6 part of the response message
    ///
    /// Called by initResponse() when the query is a DHCP4o6 message
    void initResponse4o6();

    /// @brief Returns the pointer to the query from the client.
    Pkt4Ptr getQuery() const {
        return (query_);
    }

    /// @brief Returns the pointer to the server's response.
    ///
    /// The returned pointer is NULL if the query type is DHCPRELEASE or DHCPDECLINE.
    Pkt4Ptr getResponse() const {
        return (resp_);
    }

    /// @brief Removes the response message by resetting the pointer to NULL.
    void deleteResponse() {
        resp_.reset();
    }

    /// @brief Returns the copy of the context for the Allocation engine.
    AllocEngine::ClientContext4Ptr getContext() const {
        return (context_);
    }

    /// @brief Returns the configured option list (non-const version)
    CfgOptionList& getCfgOptionList() {
        return (cfg_option_list_);
    }

    /// @brief Returns the configured option list (const version)
    const CfgOptionList& getCfgOptionList() const {
        return (cfg_option_list_);
    }

    /// @brief Sets reserved values of siaddr, sname and file in the
    /// server's response.
    void setReservedMessageFields();

    /// @brief Assigns classes retrieved from host reservation database.
    ///
    /// @param context pointer to the context.
    static void setReservedClientClasses(AllocEngine::ClientContext4Ptr context);

    /// @brief Assigns classes retrieved from host reservation database
    /// if they haven't been yet set.
    ///
    /// This function sets reserved client classes in case they haven't
    /// been set after fetching host reservations from the database.
    /// This is the case when the client has non-global host reservation
    /// and the selected subnet belongs to a shared network.
    void conditionallySetReservedClientClasses();

    /// @brief Assigns incoming packet to zero or more classes.
    ///
    /// @note This is done in two phases: first the content of the
    /// vendor-class-identifier option is used as a class, by
    /// calling @ref classifyByVendor(). Second, the classification match
    /// expressions are evaluated. The resulting classes will be stored
    /// in the packet (see @ref isc::dhcp::Pkt4::classes_ and
    /// @ref isc::dhcp::Pkt4::inClass).
    ///
    /// @param pkt packet to be classified
    static void classifyPacket(const Pkt4Ptr& pkt);

private:

    /// @public
    /// @brief Assign class using vendor-class-identifier option
    ///
    /// @note This is the first part of @ref classifyPacket
    ///
    /// @param pkt packet to be classified
    static void classifyByVendor(const Pkt4Ptr& pkt);

    /// @brief Evaluate classes.
    ///
    /// @note Second part of the classification.
    ///
    /// Evaluate expressions of client classes: if it returns true the class
    /// is added to the incoming packet.
    ///
    /// @param pkt packet to be classified.
    /// @param depend_on_known if false classes depending on the KNOWN or
    /// UNKNOWN classes are skipped, if true only these classes are evaluated.
    static void evaluateClasses(const Pkt4Ptr& pkt, bool depend_on_known);

    /// @brief Copies default parameters from client's to server's message
    ///
    /// Some fields are copied from client's message into server's response,
    /// e.g. client HW address, number of hops, transaction-id etc.
    ///
    /// @warning This message is called internally by @c initResponse and
    /// thus it doesn't check if the resp_ value has been initialized. The
    /// calling method is responsible for making sure that @c resp_ is
    /// not NULL.
    void copyDefaultFields();

    /// @brief Copies default options from client's to server's message
    ///
    /// Some options are copied from client's message into server's response,
    /// e.g. Relay Agent Info option, Subnet Selection option etc.
    ///
    /// @warning This message is called internally by @c initResponse and
    /// thus it doesn't check if the resp_ value has been initialized. The
    /// calling method is responsible for making sure that @c resp_ is
    /// not NULL.
    void copyDefaultOptions();

    /// @brief Set host identifiers within a context.
    ///
    /// This method sets an ordered list of host identifier types and
    /// values which the server should use to find host reservations.
    /// The order of the set is determined by the configuration parameter,
    /// host-reservation-identifiers
    void setHostIdentifiers();

    /// @brief Pointer to the allocation engine used by the server.
    AllocEnginePtr alloc_engine_;

    /// @brief Pointer to the DHCPv4 message sent by the client.
    Pkt4Ptr query_;

    /// @brief Pointer to the DHCPv4 message to be sent to the client.
    Pkt4Ptr resp_;

    /// @brief Context for use with allocation engine.
    AllocEngine::ClientContext4Ptr context_;

    /// @brief Configured option list.
    /// @note The configured option list is an *ordered* list of
    /// @c CfgOption objects used to append options to the response.
    CfgOptionList cfg_option_list_;
};

/// @brief Type representing the pointer to the @c Dhcpv4Exchange.
typedef boost::shared_ptr<Dhcpv4Exchange> Dhcpv4ExchangePtr;


/// @brief DHCPv4 server service.
///
/// This singleton class represents DHCPv4 server. It contains all
/// top-level methods and routines necessary for server operation.
/// In particular, it instantiates IfaceMgr, loads or generates DUID
/// that is going to be used as server-identifier, receives incoming
/// packets, processes them, manages leases assignment and generates
/// appropriate responses.
///
/// This class does not support any controlling mechanisms directly.
/// See the derived \ref ControlledDhcpv4Srv class for support for
/// command and configuration updates over msgq.
class Dhcpv4Srv : public process::Daemon {
private:

    /// @brief Pointer to IO service used by the server.
    asiolink::IOServicePtr io_service_;

public:

    /// @brief defines if certain option may, must or must not appear
    typedef enum {
        FORBIDDEN,
        MANDATORY,
        OPTIONAL
    } RequirementLevel;

    /// @brief Default constructor.
    ///
    /// Instantiates necessary services, required to run DHCPv4 server.
    /// In particular, creates IfaceMgr that will be responsible for
    /// network interaction. Will instantiate lease manager, and load
    /// old or create new DUID. It is possible to specify alternate
    /// port on which DHCPv4 server will listen on and alternate port
    /// where DHCPv4 server sends all responses to. Those are mostly useful
    /// for testing purposes. The Last two arguments of the constructor
    /// should be left at default values for normal server operation.
    /// They should be set to 'false' when creating an instance of this
    /// class for unit testing because features they enable require
    /// root privileges.
    ///
    /// @param server_port specifies port number to listen on
    /// @param client_port specifies port number to send to
    /// @param use_bcast configure sockets to support broadcast messages.
    /// @param direct_response_desired specifies if it is desired to
    /// use direct V4 traffic.
    Dhcpv4Srv(uint16_t server_port = DHCP4_SERVER_PORT,
              uint16_t client_port = 0,
              const bool use_bcast = true,
              const bool direct_response_desired = true);

    /// @brief Destructor. Used during DHCPv4 service shutdown.
    virtual ~Dhcpv4Srv();

    /// @brief Checks if the server is running in unit test mode.
    ///
    /// @return true if the server is running in unit test mode,
    /// false otherwise.
    bool inTestMode() const {
        return (server_port_ == 0);
    }

    /// @brief Returns pointer to the IO service used by the server.
    asiolink::IOServicePtr& getIOService() {
        return (io_service_);
    }

    /// @brief Returns pointer to the network state used by the server.
    NetworkStatePtr& getNetworkState() {
        return (network_state_);
    }

    /// @brief Returns an object which controls access to the configuration
    /// backends.
    ///
    /// @return Pointer to the instance of the object which controls
    /// access to the configuration backends.
    CBControlDHCPv4Ptr getCBControl() const {
        return (cb_control_);
    }

    /// @brief returns Kea version on stdout and exit.
    /// redeclaration/redefinition. @ref isc::process::Daemon::getVersion()
    static std::string getVersion(bool extended);

    /// @brief Main server processing loop.
    ///
    /// Main server processing loop. Call the processing step routine
    /// until shut down.
    ///
    /// @return The value returned by @c Daemon::getExitValue().
    int run();

    /// @brief Main server processing step.
    ///
    /// Main server processing step. Receives one incoming packet, calls
    /// the processing packet routing and (if necessary) transmits
    /// a response.
    void run_one();

    /// @brief Process a single incoming DHCPv4 packet and sends the response.
    ///
    /// It verifies correctness of the passed packet, calls per-type processXXX
    /// methods, generates appropriate answer, sends the answer to the client.
    ///
    /// @param query A pointer to the packet to be processed.
    void processPacketAndSendResponse(Pkt4Ptr& query);

    /// @brief Process a single incoming DHCPv4 packet and sends the response.
    ///
    /// It verifies correctness of the passed packet, calls per-type processXXX
    /// methods, generates appropriate answer, sends the answer to the client.
    ///
    /// @param query A pointer to the packet to be processed.
    void processPacketAndSendResponseNoThrow(Pkt4Ptr& query);

    /// @brief Process an unparked DHCPv4 packet and sends the response.
    ///
    /// @param callout_handle pointer to the callout handle.
    /// @param query A pointer to the packet to be processed.
    /// @param rsp A pointer to the response.
    void sendResponseNoThrow(hooks::CalloutHandlePtr& callout_handle,
                             Pkt4Ptr& query, Pkt4Ptr& rsp);

    /// @brief Process a single incoming DHCPv4 packet.
    ///
    /// It verifies correctness of the passed packet, calls per-type processXXX
    /// methods, generates appropriate answer.
    ///
    /// @param query A pointer to the packet to be processed.
    /// @param rsp A pointer to the response.
    /// @param allow_packet_park Indicates if parking a packet is allowed.
    void processPacket(Pkt4Ptr& query, Pkt4Ptr& rsp,
                       bool allow_packet_park = true);

    /// @brief Process a single incoming DHCPv4 query.
    ///
    /// It calls per-type processXXX methods, generates appropriate answer.
    ///
    /// @param query A pointer to the packet to be processed.
    /// @param rsp A pointer to the response.
    /// @param allow_packet_park Indicates if parking a packet is allowed.
    void processDhcp4Query(Pkt4Ptr& query, Pkt4Ptr& rsp,
                           bool allow_packet_park);

    /// @brief Process a single incoming DHCPv4 query.
    ///
    /// It calls per-type processXXX methods, generates appropriate answer,
    /// sends the answer to the client.
    ///
    /// @param query A pointer to the packet to be processed.
    /// @param rsp A pointer to the response.
    /// @param allow_packet_park Indicates if parking a packet is allowed.
    void processDhcp4QueryAndSendResponse(Pkt4Ptr& query, Pkt4Ptr& rsp,
                                          bool allow_packet_park);

    /// @brief Instructs the server to shut down.
    void shutdown() override;

    ///
    /// @name Public accessors returning values required to (re)open sockets.
    ///
    //@{
    ///
    /// @brief Get UDP port on which server should listen.
    ///
    /// Typically, server listens on UDP port number 67. Other ports are used
    /// for testing purposes only.
    ///
    /// @return UDP port on which server should listen.
    uint16_t getServerPort() const {
        return (server_port_);
    }

    /// @brief Return bool value indicating that broadcast flags should be set
    /// on sockets.
    ///
    /// @return A bool value indicating that broadcast should be used (if true).
    bool useBroadcast() const {
        return (use_bcast_);
    }
    //@}

    /// @brief Starts DHCP_DDNS client IO if DDNS updates are enabled.
    ///
    /// If updates are enabled, it instructs the D2ClientMgr singleton to
    /// enter send mode.  If D2ClientMgr encounters errors it may throw
    /// D2ClientError. This method does not catch exceptions.
    void startD2();

    /// @brief Stops DHCP_DDNS client IO if DDNS updates are enabled.
    ///
    /// If updates are enabled, it instructs the D2ClientMgr singleton to
    /// leave send mode.  If D2ClientMgr encounters errors it may throw
    /// D2ClientError. This method does not catch exceptions.
    void stopD2();

    /// @brief Implements the error handler for DHCP_DDNS IO errors
    ///
    /// Invoked when a NameChangeRequest send to kea-dhcp-ddns completes with
    /// a failed status.  These are communications errors, not data related
    /// failures.
    ///
    /// This method logs the failure and then suspends all further updates.
    /// Updating can only be restored by reconfiguration or restarting the
    /// server.  There is currently no retry logic so the first IO error that
    /// occurs will suspend updates.
    /// @todo We may wish to make this more robust or sophisticated.
    ///
    /// @param result Result code of the send operation.
    /// @param ncr NameChangeRequest which failed to send.
    virtual void d2ClientErrorHandler(const dhcp_ddns::
                                      NameChangeSender::Result result,
                                      dhcp_ddns::NameChangeRequestPtr& ncr);

    /// @brief Discards parked packets
    /// Clears the packet parking lots of all packets.
    /// Called during reconfigure and shutdown.
    void discardPackets();

    /// @brief Returns value of the test_send_responses_to_source_ flag.
    ///
    /// @return value of the test_send_responses_to_source_ flag.
    bool getSendResponsesToSource() const {
        return (test_send_responses_to_source_);
    }

protected:

    /// @name Functions filtering and sanity-checking received messages.
    ///
    /// @todo These functions are supposed to be moved to a new class which
    /// will manage different rules for accepting and rejecting messages.
    /// Perhaps ticket #3116 is a good opportunity to do it.
    ///
    //@{
    /// @brief Checks whether received message should be processed or discarded.
    ///
    /// This function checks whether received message should be processed or
    /// discarded. It should be called on the beginning of message processing
    /// (just after the message has been decoded). This message calls a number
    /// of other functions which check whether message should be processed,
    /// using different criteria.
    ///
    /// This function should be extended when new criteria for accepting
    /// received message have to be implemented. This function is meant to
    /// aggregate all early filtering checks on the received message. By having
    /// a single function like this, we are avoiding bloat of the server's main
    /// loop.
    ///
    /// @warning This function should remain exception safe.
    ///
    /// @param query Received message.
    ///
    /// @return true if the message should be further processed, or false if
    /// the message should be discarded.
    bool accept(const Pkt4Ptr& query) const;

    /// @brief Check if a message sent by directly connected client should be
    /// accepted or discarded.
    ///
    /// This function checks if the received message is from directly connected
    /// client. If it is, it checks that it should be processed or discarded.
    ///
    /// Note that this function doesn't validate all addresses being carried in
    /// the message. The primary purpose of this function is to filter out
    /// direct messages in the local network for which there is no suitable
    /// subnet configured. For example, this function accepts unicast messages
    /// because unicasts may be used by clients located in remote networks to
    /// to renew existing leases. If their notion of address is wrong, the
    /// server will have to sent a NAK, instead of dropping the message.
    /// Detailed validation of such messages is performed at later stage of
    /// processing.
    ///
    /// This function accepts the following messages:
    /// - all valid relayed messages,
    /// - all unicast messages,
    /// - all broadcast messages except DHCPINFORM received on the interface
    /// for which the suitable subnet exists (is configured).
    /// - all DHCPINFORM messages with source address or ciaddr set.
    ///
    /// @param query Message sent by a client.
    ///
    /// @return true if message is accepted for further processing, false
    /// otherwise.
    bool acceptDirectRequest(const Pkt4Ptr& query) const;

    /// @brief Check if received message type is valid for the server to
    /// process.
    ///
    /// This function checks that the received message type belongs to
    /// the range of types recognized by the server and that the
    /// message of this type should be processed by the server.
    ///
    /// The messages types accepted for processing are:
    /// - Discover
    /// - Request
    /// - Release
    /// - Decline
    /// - Inform
    ///
    /// @param query Message sent by a client.
    ///
    /// @return true if message is accepted for further processing, false
    /// otherwise.
    bool acceptMessageType(const Pkt4Ptr& query) const;

    /// @brief Verifies if the server id belongs to our server.
    ///
    /// This function checks if the server identifier carried in the specified
    /// DHCPv4 message belongs to this server. If the server identifier option
    /// is absent or the value carried by this option is equal to one of the
    /// server identifiers used by the server, the true is returned. If the
    /// server identifier option is present, but it doesn't match any server
    /// identifier used by this server, the false value is returned.
    ///
    /// @param pkt DHCPv4 message which server identifier is to be checked.
    ///
    /// @return true, if the server identifier is absent or matches one of the
    /// server identifiers that the server is using; false otherwise.
    bool acceptServerId(const Pkt4Ptr& pkt) const;
    //@}

    /// @brief Verifies if specified packet meets RFC requirements
    ///
    /// Checks if mandatory option is really there, that forbidden option
    /// is not there, and that client-id or server-id appears only once.
    ///
    /// @param query Pointer to the client's message.
    /// @param serverid expectation regarding server-id option
    /// @throw RFCViolation if any issues are detected
    static void sanityCheck(const Pkt4Ptr& query, RequirementLevel serverid);

    /// @brief Processes incoming DISCOVER and returns response.
    ///
    /// Processes received DISCOVER message and verifies that its sender
    /// should be served. In particular, a lease is selected and sent
    /// as an offer to a client if it should be served.
    ///
    /// @param discover DISCOVER message received from client
    ///
    /// @return OFFER message or NULL
    Pkt4Ptr processDiscover(Pkt4Ptr& discover);

    /// @brief Processes incoming REQUEST and returns REPLY response.
    ///
    /// Processes incoming REQUEST message and verifies that its sender
    /// should be served. In particular, verifies that requested lease
    /// is valid, not expired, not reserved, not used by other client and
    /// that requesting client is allowed to use it.
    ///
    /// Returns ACK message, NAK message, or NULL
    ///
    /// @param request a message received from client
    /// @param [out] context pointer to the client context where allocated
    /// and deleted leases are stored.
    ///
    /// @return ACK or NAK message
    Pkt4Ptr processRequest(Pkt4Ptr& request, AllocEngine::ClientContext4Ptr& context);

    /// @brief Processes incoming DHCPRELEASE messages.
    ///
    /// In DHCPv4, server does not respond to RELEASE messages, therefore
    /// this function does not return anything.
    ///
    /// @param release message received from client
    /// @param [out] context pointer to the client context where released
    /// lease is stored.
    void processRelease(Pkt4Ptr& release, AllocEngine::ClientContext4Ptr& context);

    /// @brief Process incoming DHCPDECLINE messages.
    ///
    /// This method processes incoming DHCPDECLINE. In particular, it extracts
    /// Requested IP Address option, checks that the address really belongs to
    /// the client and if it does, calls @ref declineLease.
    ///
    /// @param decline message received from client
    /// @param [out] context pointer to the client context where declined
    /// lease is stored.
    void processDecline(Pkt4Ptr& decline, AllocEngine::ClientContext4Ptr& context);

    /// @brief Processes incoming DHCPINFORM messages.
    ///
    /// @param inform message received from client
    ///
    /// @return DHCPACK to be sent to the client.
    Pkt4Ptr processInform(Pkt4Ptr& inform);

    /// @brief Build the configured option list
    ///
    /// @note The configured option list is an *ordered* list of
    /// @c CfgOption objects used to append options to the response.
    ///
    /// @param ex The exchange where the configured option list is cached
    void buildCfgOptionList(Dhcpv4Exchange& ex);

    /// @brief Appends options requested by client.
    ///
    /// This method assigns options that were requested by client
    /// (sent in PRL) or are enforced by server.
    ///
    /// @param ex The exchange holding both the client's message and the
    /// server's response.
    void appendRequestedOptions(Dhcpv4Exchange& ex);

    /// @brief Appends requested vendor options as requested by client.
    ///
    /// This method is similar to \ref appendRequestedOptions(), but uses
    /// vendor options. The major difference is that vendor-options use
    /// its own option spaces (there may be more than one distinct set of vendor
    /// options, each with unique vendor-id). Vendor options are requested
    /// using separate options within their respective vendor-option spaces.
    ///
    /// @param ex The exchange holding both the client's message and the
    /// server's response.
    void appendRequestedVendorOptions(Dhcpv4Exchange& ex);

    /// @brief Assigns a lease and appends corresponding options
    ///
    /// This method chooses the most appropriate lease for requesting
    /// client and assigning it. Options corresponding to the lease
    /// are added to specific message.
    ///
    /// This method may reset the pointer to the response in the @c ex object
    /// to indicate that the response should not be sent to the client.
    /// The caller must check if the response is is null after calling
    /// this method.
    ///
    /// The response type in the @c ex object may be set to DHCPACK or DHCPNAK.
    ///
    /// @param ex DHCPv4 exchange holding the client's message to be checked.
    void assignLease(Dhcpv4Exchange& ex);

    /// @brief Update client name and DNS flags in the lease and response
    ///
    /// There are two cases when the client name (FQDN or hostname) and DNS
    /// flags need to updated after the lease has been allocated:
    /// 1. If the name is being generated from the lease address
    /// 2. If the allocation changed the chosen subnet
    ///
    /// In the first case this function will generate the name from the
    /// lease address.  In either case, the name and DNS flags are updated
    /// in the lease and in the response packet.
    ///
    /// @param ctx reference to the client context
    /// @param lease reference to the client lease
    /// @param query reference to the client query
    /// @param resp reference to the client response
    /// @param client_name_changed - true if the new values are already in
    /// the lease
    void postAllocateNameUpdate(const AllocEngine::ClientContext4Ptr& ctx,
                                const Lease4Ptr& lease, const Pkt4Ptr& query,
                                const Pkt4Ptr& resp, bool client_name_changed);

    /// @brief Adds the T1 and T2 timers to the outbound response as appropriate
    ///
    /// This method determines if either of the timers T1 (option 58) and T2
    /// (option 59) should be sent to the client.  It is influenced by the
    /// lease's subnet's values for renew-timer, rebind-timer,
    /// calculate-tee-times, t1-percent, and t2-percent as follows:
    ///
    /// By default neither T1 nor T2 will be sent.
    ///
    /// T2:
    ///
    /// If rebind-timer is set use its value, otherwise if calculate-tee-times
    /// is true use the value given by valid lease time * t2-percent.  Either
    /// way the value will only be sent if it is less than the valid lease time.
    ///
    /// T1:
    ///
    /// If renew-timer is set use its value, otherwise if calculate-tee-times
    /// is true use the value given by valid lease time * t1-percent.  Either
    /// way the value will only be sent if it is less than T2 when T2 is being
    /// sent, or less than the valid lease time if T2 is not being sent.
    ///
    /// @param lease lease being assigned to the client
    /// @param subnet the subnet to which the lease belongs
    /// @param resp outbound response for the client to which timers are added.
    void setTeeTimes(const Lease4Ptr& lease, const Subnet4Ptr& subnet, Pkt4Ptr resp);

    /// @brief Append basic options if they are not present.
    ///
    /// This function adds the following basic options if they
    /// are not yet added to the response message:
    /// - Subnet Mask,
    /// - Router,
    /// - Name Server,
    /// - Domain Name,
    /// - Server Identifier.
    ///
    /// @param ex DHCPv4 exchange holding the client's message to be checked.
    void appendBasicOptions(Dhcpv4Exchange& ex);

    /// @brief Sets fixed fields of the outgoing packet.
    ///
    /// If the incoming packets belongs to a class and that class defines
    /// next-server, server-hostname or boot-file-name, we need to set the
    /// siaddr, sname or filename fields in the outgoing packet. Also, those
    /// values can be defined for subnet or in reservations. The values
    /// defined in reservation takes precedence over class values, which
    /// in turn take precedence over subnet values.
    ///
    /// @param ex DHCPv4 exchange holding the client's message and the server's
    ///           response to be adjusted.
    void setFixedFields(Dhcpv4Exchange& ex);

    /// @brief Processes Client FQDN and Hostname Options sent by a client.
    ///
    /// Client may send Client FQDN or Hostname option to communicate its name
    /// to the server. Server may use this name to perform DNS update for the
    /// lease being assigned to a client. If server takes responsibility for
    /// updating DNS for a client it may communicate it by sending the Client
    /// FQDN or Hostname %Option back to the client. Server select a different
    /// name than requested by a client to update DNS. In such case, the server
    /// stores this different name in its response.
    ///
    /// Client should not send both Client FQDN and Hostname options. However,
    /// if client sends both options, server should prefer Client FQDN option
    /// and ignore the Hostname option. If Client FQDN option is not present,
    /// the Hostname option is processed.
    ///
    /// The Client FQDN %Option is processed by this function as described in
    /// RFC4702.
    ///
    /// In response to a Hostname %Option sent by a client, the server may send
    /// Hostname option with the same or different hostname. If different
    /// hostname is sent, it is an indication to the client that server has
    /// overridden the client's preferred name and will rather use this
    /// different name to update DNS. However, since Hostname option doesn't
    /// carry an information whether DNS update will be carried by the server
    /// or not, the client is responsible for checking whether DNS update
    /// has been performed.
    ///
    /// After successful processing options stored in the first parameter,
    /// this function may add Client FQDN or Hostname option to the response
    /// message. In some cases, server may cease to add any options to the
    /// response, i.e. when server doesn't support DNS updates.
    ///
    /// This function does not throw. It simply logs the debug message if the
    /// processing of the FQDN or Hostname failed.
    ///
    /// @param ex The exchange holding both the client's message and the
    /// server's response.
    void processClientName(Dhcpv4Exchange& ex);

    /// @brief This function sets statistics related to DHCPv4 packets processing
    /// to their initial values.
    ///
    /// All of the statistics observed by the DHCPv4 server and with the names
    /// like "pkt4-" are reset to 0. This function must be invoked in the class
    /// constructor.
    void setPacketStatisticsDefaults();

    /// @brief Sets value of the test_send_responses_to_source_ flag.
    ///
    /// @param value new value of the test_send_responses_to_source_ flag.
    void setSendResponsesToSource(bool value) {
        test_send_responses_to_source_ = value;
    }

public:

    /// @brief this is a prefix added to the content of vendor-class option
    ///
    /// If incoming packet has a vendor class option, its content is
    /// prepended with this prefix and then interpreted as a class.
    /// For example, a packet that sends vendor class with value of "FOO"
    /// will cause the packet to be assigned to class VENDOR_CLASS_FOO.
    static const std::string VENDOR_CLASS_PREFIX;

private:
    /// @brief Process Client FQDN %Option sent by a client.
    ///
    /// This function is called by the @c Dhcpv4Srv::processClientName when
    /// the client has sent the FQDN option in its message to the server.
    /// It comprises the actual logic to parse the FQDN option and prepare
    /// the FQDN option to be sent back to the client in the server's
    /// response.
    ///
    /// @param ex The exchange holding both the client's message and the
    /// server's response.
    void processClientFqdnOption(Dhcpv4Exchange& ex);

    /// @brief Process Hostname %Option sent by a client.
    ///
    /// This method is called by the @c Dhcpv4Srv::processClientName to
    /// create an instance of the Hostname option to be returned to the
    /// client. If this instance is created it is included in the response
    /// message within the @c Dhcpv4Exchange object passed as an argument.
    ///
    /// The Hostname option instance is created if the client has included
    /// Hostname option in its query to the server or if the client has
    /// included Hostname option code in the Parameter Request List option.
    /// In the former case, the server can use the Hostname supplied by the
    /// client or replace it with a new hostname, depending on the server's
    /// configuration. A reserved hostname takes precedence over a hostname
    /// supplied by the client or auto generated hostname.
    ///
    /// If the 'qualifying-suffix' parameter is specified, its value is used
    /// to qualify a hostname. For example, if the host reservation contains
    /// a hostname 'marcin-laptop', and the qualifying suffix is
    /// 'example.isc.org', the hostname returned to the client will be
    /// 'marcin-laptop.example.isc.org'. If the 'qualifying-suffix' is not
    /// specified (empty), the reserved hostname is returned to the client
    /// unqualified.
    ///
    /// The 'qualifying-suffix' value is also used to qualify the hostname
    /// supplied by the client, when this hostname is unqualified,
    /// e.g. 'laptop-x'. If the supplied hostname is qualified, e.g.
    /// 'laptop-x.example.org', the qualifying suffix will not be appended
    /// to it.
    ///
    /// @param ex The exchange holding both the client's message and the
    /// server's response.
    void processHostnameOption(Dhcpv4Exchange& ex);

    /// @public
    /// @brief Marks lease as declined.
    ///
    /// This method moves a lease to declined state with all the steps involved:
    /// - trigger DNS removal (if necessary)
    /// - disassociate the client information
    /// - update lease in the database (switch to DECLINED state)
    /// - increase necessary statistics
    /// - call lease4_decline hook
    ///
    /// @param lease lease to be declined
    /// @param decline client's message
    /// @param context reference to a client context
    void declineLease(const Lease4Ptr& lease, const Pkt4Ptr& decline,
                      AllocEngine::ClientContext4Ptr& context);

protected:

    /// @brief Creates NameChangeRequests which correspond to the lease
    /// which has been acquired.
    ///
    /// If this function is called when an existing lease is renewed, it
    /// may generate NameChangeRequest to remove existing DNS entries which
    /// correspond to the old lease instance. This function may cease to
    /// generate NameChangeRequests if the notion of the client's FQDN hasn't
    /// changed between an old and new lease.
    ///
    /// @param lease A pointer to the new lease which has been acquired.
    /// @param old_lease A pointer to the instance of the old lease which has
    /// @param ddns_params DDNS configuration parameters
    /// been replaced by the new lease passed in the first argument. The NULL
    /// value indicates that the new lease has been allocated, rather than
    /// lease being renewed.
    void createNameChangeRequests(const Lease4Ptr& lease,
                                  const Lease4Ptr& old_lease,
                                  const DdnsParams& ddns_params);

    /// @brief Attempts to renew received addresses
    ///
    /// Attempts to renew existing lease. This typically includes finding a lease that
    /// corresponds to the received address. If no such lease is found, a status code
    /// response is generated.
    ///
    /// @param renew client's message asking for renew
    /// @param reply server's response (ACK or NAK)
    void renewLease(const Pkt4Ptr& renew, Pkt4Ptr& reply);

    /// @brief Adds server identifier option to the server's response.
    ///
    /// This method adds a server identifier to the DHCPv4 message if it doesn't
    /// exist yet. This is set to the local address on which the client's query has
    /// been received with the exception of broadcast traffic and DHCPv4o6 query for
    /// which a socket on the particular interface is found and its address is used
    /// as server id.
    ///
    /// @note This method doesn't throw exceptions by itself but the underlying
    /// classes being used my throw. The reason for this method to not sanity
    /// check the specified message is that it is meant to be called internally
    /// by the @c Dhcpv4Srv class.
    ///
    /// @note This method is static because it is not dependent on the class
    /// state.
    ///
    /// @param ex The exchange holding both the client's message and the
    /// server's response.
    static void appendServerID(Dhcpv4Exchange& ex);

    /// @brief Check if the relay port RAI sub-option was set in the query.
    ///
    /// @param ex The exchange holding the client's message
    /// @return the port to use to join the relay or 0 for the default
    static uint16_t checkRelayPort(const Dhcpv4Exchange& ex);

    /// @brief Set IP/UDP and interface parameters for the DHCPv4 response.
    ///
    /// This method sets the following parameters for the DHCPv4 message being
    /// sent to a client:
    /// - client unicast or a broadcast address,
    /// - client or relay port,
    /// - server address,
    /// - server port,
    /// - name and index of the interface which is to be used to send the
    /// message.
    ///
    /// Internally it calls the @c Dhcpv4Srv::adjustRemoteAddr to figure
    /// out the destination address (client unicast address or broadcast
    /// address).
    ///
    /// The destination port is always DHCPv4 client (68) or relay (67) port,
    /// depending if the response will be sent directly to a client, unless
    /// a client port was enforced from the command line.
    ///
    /// The source port is always set to DHCPv4 server port (67).
    ///
    /// The interface selected for the response is always the same as the
    /// one through which the query has been received.
    ///
    /// The source address for the response is the IPv4 address assigned to
    /// the interface being used to send the response. This function uses
    /// @c IfaceMgr to get the socket bound to the IPv4 address on the
    /// particular interface.
    ///
    /// @note This method is static because it is not dependent on the class
    /// state.
    ///
    /// @param ex The exchange holding both the client's message and the
    /// server's response.
    void adjustIfaceData(Dhcpv4Exchange& ex);

    /// @brief Sets remote addresses for outgoing packet.
    ///
    /// This method sets the local and remote addresses on outgoing packet.
    /// The addresses being set depend on the following conditions:
    /// - has incoming packet been relayed,
    /// - is direct response to a client without address supported,
    /// - type of the outgoing packet,
    /// - broadcast flag set in the incoming packet.
    ///
    /// @warning This method does not check whether provided packet pointers
    /// are valid. Make sure that pointers are correct before calling this
    /// function.
    ///
    /// @param ex The exchange holding both the client's message and the
    /// server's response.
    void adjustRemoteAddr(Dhcpv4Exchange& ex);

    /// @brief converts server-id to text
    /// Converts content of server-id option to a text representation, e.g.
    /// "192.0.2.1"
    ///
    /// @param opt option that contains server-id
    /// @return string representation
    static std::string srvidToString(const OptionPtr& opt);

    /// @brief Selects a subnet for a given client's packet.
    ///
    /// If selectSubnet is called to simply do sanity checks (check if a
    /// subnet would be selected), then there is no need to call hooks,
    /// as this will happen later (when selectSubnet is called again).
    /// In such case the sanity_only should be set to true.
    ///
    /// @param query client's message
    /// @param drop if it is true the packet will be dropped
    /// @param sanity_only if it is true the callout won't be called
    /// @return selected subnet (or NULL if no suitable subnet was found)
    isc::dhcp::Subnet4Ptr selectSubnet(const Pkt4Ptr& query,
                                       bool& drop,
                                       bool sanity_only = false) const;

    /// @brief Selects a subnet for a given client's DHCP4o6 packet.
    ///
    /// If selectSubnet is called to simply do sanity checks (check if a
    /// subnet would be selected), then there is no need to call hooks,
    /// as this will happen later (when selectSubnet is called again).
    /// In such case the sanity_only should be set to true.
    ///
    /// @param query client's message
    /// @param drop if it is true the packet will be dropped
    /// @param sanity_only if it is true the callout won't be called
    /// @return selected subnet (or NULL if no suitable subnet was found)
    isc::dhcp::Subnet4Ptr selectSubnet4o6(const Pkt4Ptr& query,
                                          bool& drop,
                                          bool sanity_only = false) const;

    /// @brief dummy wrapper around IfaceMgr::receive4
    ///
    /// This method is useful for testing purposes, where its replacement
    /// simulates reception of a packet. For that purpose it is protected.
    virtual Pkt4Ptr receivePacket(int timeout);

    /// @brief dummy wrapper around IfaceMgr::send()
    ///
    /// This method is useful for testing purposes, where its replacement
    /// simulates transmission of a packet. For that purpose it is protected.
    virtual void sendPacket(const Pkt4Ptr& pkt);

    /// @brief Assigns incoming packet to zero or more classes.
    ///
    /// @note This is done in two phases: first the content of the
    /// vendor-class-identifier option is used as a class, by
    /// calling @ref classifyByVendor(). Second classification match
    /// expressions are evaluated. The resulting classes will be stored
    /// in the packet (see @ref isc::dhcp::Pkt4::classes_ and
    /// @ref isc::dhcp::Pkt4::inClass).
    ///
    /// @param pkt packet to be classified
    void classifyPacket(const Pkt4Ptr& pkt);

protected:

    /// @brief Assigns incoming packet to zero or more classes (required pass).
    ///
    /// @note This required classification evaluates all classes which
    /// were marked for required evaluation. Classes are collected so
    /// evaluated in the reversed order than output option processing.
    ///
    /// @note The only-if-required flag is related because it avoids
    /// double evaluation (which is not forbidden).
    ///
    /// @param ex The exchange holding needed information.
    void requiredClassify(Dhcpv4Exchange& ex);

    /// @brief Perform deferred option unpacking.
    ///
    /// @note Options 43 and 224-254 are processed after classification.
    /// If a class configures a definition it is applied, if none
    /// the global (user) definition is applied. For option 43
    /// a last resort definition (same definition as used in previous Kea
    /// versions) is applied when none is found.
    ///
    /// @param query Pointer to the client message.
    void deferredUnpack(Pkt4Ptr& query);

    /// @brief Executes pkt4_send callout.
    ///
    /// @param callout_handle pointer to the callout handle.
    /// @param query Pointer to a query.
    /// @param rsp Pointer to a response.
    void processPacketPktSend(hooks::CalloutHandlePtr& callout_handle,
                              Pkt4Ptr& query, Pkt4Ptr& rsp);

    /// @brief Executes buffer4_send callout and sends the response.
    ///
    /// @param callout_handle pointer to the callout handle.
    /// @param rsp pointer to a response.
    void processPacketBufferSend(hooks::CalloutHandlePtr& callout_handle,
                                 Pkt4Ptr& rsp);

private:

    /// @public
    /// @brief Assign class using vendor-class-identifier option
    ///
    /// @note This is the first part of @ref classifyPacket
    ///
    /// @param pkt packet to be classified
    void classifyByVendor(const Pkt4Ptr& pkt);

    /// @private
    /// @brief Constructs netmask option based on subnet4
    /// @param subnet subnet for which the netmask will be calculated
    ///
    /// @return Option that contains netmask information
    static OptionPtr getNetmaskOption(const Subnet4Ptr& subnet);

protected:

    /// UDP port number on which server listens.
    uint16_t server_port_;

    /// UDP port number to which server sends all responses.
    uint16_t client_port_;

    /// Indicates if shutdown is in progress. Setting it to true will
    /// initiate server shutdown procedure.
    volatile bool shutdown_;

    /// @brief Allocation Engine.
    /// Pointer to the allocation engine that we are currently using
    /// It must be a pointer, because we will support changing engines
    /// during normal operation (e.g. to use different allocators)
    boost::shared_ptr<AllocEngine> alloc_engine_;

    /// Should broadcast be enabled on sockets (if true).
    bool use_bcast_;

    /// @brief Holds information about disabled DHCP service and/or
    /// disabled subnet/network scopes.
    NetworkStatePtr network_state_;

    /// @brief Controls access to the configuration backends.
    CBControlDHCPv4Ptr cb_control_;

private:

    /// @brief store value that defines if kea will send responses
    /// to a source address of incoming packet. Only for testing.
    bool test_send_responses_to_source_;

public:

    /// Class methods for DHCPv4-over-DHCPv6 handler

    /// @brief Updates statistics for received packets
    /// @param query packet received
    static void processStatsReceived(const Pkt4Ptr& query);

    /// @brief Updates statistics for transmitted packets
    /// @param response packet transmitted
    static void processStatsSent(const Pkt4Ptr& response);

    /// @brief Returns the index for "buffer4_receive" hook point
    /// @return the index for "buffer4_receive" hook point
    static int getHookIndexBuffer4Receive();

    /// @brief Returns the index for "pkt4_receive" hook point
    /// @return the index for "pkt4_receive" hook point
    static int getHookIndexPkt4Receive();

    /// @brief Returns the index for "subnet4_select" hook point
    /// @return the index for "subnet4_select" hook point
    static int getHookIndexSubnet4Select();

    /// @brief Returns the index for "lease4_release" hook point
    /// @return the index for "lease4_release" hook point
    static int getHookIndexLease4Release();

    /// @brief Returns the index for "pkt4_send" hook point
    /// @return the index for "pkt4_send" hook point
    static int getHookIndexPkt4Send();

    /// @brief Returns the index for "buffer4_send" hook point
    /// @return the index for "buffer4_send" hook point
    static int getHookIndexBuffer4Send();

    /// @brief Returns the index for "lease4_decline" hook point
    /// @return the index for "lease4_decline" hook point
    static int getHookIndexLease4Decline();

    /// @brief Return a list of all paths that contain passwords or secrets for
    /// kea-dhcp4.
    ///
    /// @return the list of lists of sequential JSON map keys needed to reach
    /// the passwords and secrets.
    std::list<std::list<std::string>> jsonPathsToRedact() const final override;
};

}  // namespace dhcp
}  // namespace isc

#endif // DHCP4_SRV_H