Kea 2.7.5
The Hooks API for the DHCPv4 Server

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 Hooks Developer's Guide. 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 Guide to Hooks for the Kea Component Developer.

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:
    • in - the server passes values to the callout but ignored any data returned.
    • out - the callout is expected to set this value.
    • in/out - 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.

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.

dhcp4_srv_configured

  • Arguments:
  • 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.
  • Next step status: 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.

cb4_updated

  • Arguments:
  • 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.
  • Next step status: Status codes returned by the callouts installed on this hook point are ignored.

buffer4_receive

  • Arguments:
  • 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.
  • Next step status: 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 isc::dhcp::Option objects (or derived) and add them to the "query4" object using the 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.

pkt4_receive

  • Arguments:
  • 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.
  • Next step status: 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.

subnet4_select

  • Arguments:
  • 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.
  • Next step status: 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.

host4_identifier

  • Arguments:
  • 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.
  • Next step status: 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".

lease4_select

  • Arguments:
  • 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.

    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).
  • Next step status: 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.

lease4_renew

  • Arguments:
  • 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 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.
  • Next step status: 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.

lease4_release

  • Arguments:
  • 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 Lease4 object that contains the lease to be released. It doesn't make sense to modify it at this time.
  • Next step status: 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.

lease4_decline

  • Arguments:
  • 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 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.
  • Next step status: 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.

ddns4_update

  • Arguments:

  • 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 kea-dhcp-ddns. 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 hostname,fwd-update, and rev-update 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 hostname 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 fwd-update is true (and kea-dhcp-ddns connectivity is enabled)
    • Reverse DNS update(s) will be done if rev-update is true (and kea-dhcp-ddns connectivity is enabled)
  • Next step status: Not applicable, its value will be ignored.

leases4_committed

  • Arguments:
  • 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.
  • Next step status: 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.

lease4_offer

  • Arguments:
  • 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 "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.

  • Next step status: 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 statitics, and then invoke callouts installed for lease4_server_decline.

lease4_server_decline

  • Arguments:
  • 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.
  • Next step status: Not applicable, its value will be ignored.

pkt4_send

  • Arguments:
  • 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 Pkt4 object can be modified at this time, except 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.)

    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.
  • Next step action: 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.

buffer4_send

  • Arguments:
  • 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 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 buffer_out_ (see 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.
  • Next step status: 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.

lease4_expire

  • Arguments:
    • name: lease4, type: isc::dhcp::Lease4Ptr, direction: in/out
    • name: remove_lease, type: bool, direction: in
  • 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.
  • Next step status: 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.

lease4_recover

  • Arguments:
  • 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).
  • Next step status: 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.

command_processed

  • Arguments:
  • 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.
  • Next step status: Not applicable, its value will be ignored.

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 CfgMgr. Option instances are created once, during server configuration, and the 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, 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 CfgMgr. This mechanism is enabled for all instances of 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 isc::dhcp::Pkt4::setCopyRetrievedOptions (true) to force copying options retrieved by 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 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.