02-dhcp.dox 3.28 KB
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/**
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 * @page dhcpv4 DHCPv4 Server Component
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 *
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 * BIND10 offers DHCPv4 server implementation. It is implemented as
 * b10-dhcp4 component.  Its primary code is located in
 * isc::dhcp::Dhcpv4Srv class. It uses \ref libdhcp extensively,
 * especially isc::dhcp::Pkt4, isc::dhcp::Option and
 * isc::dhcp::IfaceMgr classes. Currently this code offers skeleton
 * functionality, i.e. it is able to receive and process incoming
 * requests and trasmit responses. However, it does not have database
 * management, so it returns only one, hardcoded lease to whoever asks
 * for it.
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 *
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 * DHCPv4 server component does not support direct traffic (relayed
 * only), as support for transmission to hosts without IPv4 address
 * assigned is not implemented in IfaceMgr yet.
 *
 * DHCPv4 server component does not use BIND10 logging yet.
 *
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 * @section dhcpv4Session BIND10 message queue integration
 *
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 * DHCPv4 server component is now integrated with the BIND10 message queue.
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 * The integration is performed by establishSession() and disconnectSession()
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 * functions in isc::dhcp::ControlledDhcpv4Srv class. main() method defined
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 * in the src/bin/dhcp4/main.cc file instantiates isc::dhcp::ControlledDhcpv4Srv
 * class that establishes connection with msgq and install necessary handlers
 * for receiving commands and configuration updates. It is derived from
 * a base isc::dhcp::Dhcpv4Srv class that implements DHCPv4 server functionality,
 * without any controlling mechanisms.
 *
 * ControlledDhcpv4Srv instantiates several components to make management
 * session possible. In particular, isc::cc::Session cc_session
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 * object uses ASIO for establishing connection. It registers its socket
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 * in isc::asiolink::IOService io_service object. Typically, other components
 * (e.g. auth or resolver) that use ASIO for their communication, register their
 * other sockets in the
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 * same io_service and then just call io_service.run() method that does
 * not return, until one of the callback decides that it is time to shut down
 * the whole component cal calls io_service.stop(). DHCPv4 works in a
 * different way. It does receive messages using select()
 * (see isc::dhcp::IfaceMgr::receive4()), which is incompatible with ASIO.
 * To solve this problem, socket descriptor is extracted from cc_session
 * object and is passed to IfaceMgr by using isc::dhcp::IfaceMgr::set_session_socket().
 * IfaceMgr then uses this socket in its select() call. If there is some
 * data to be read, it calls registered callback that is supposed to
 * read and process incoming data.
 *
 * This somewhat complicated approach is needed for a simple reason. In
 * embedded deployments there will be no message queue. Not referring directly
 * to anything related to message queue in isc::dhcp::Dhcpv4Srv and
 * isc::dhcp::IfaceMgr classes brings in two benefits. First, the can
 * be used with and without message queue. Second benefit is related to the
 * first one: \ref libdhcp is supposed to be simple and robust and not require
 * many dependencies. One notable example of a use case that benefits from
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 * this approach is a perfdhcp tool. Finally, the idea is that it should be
 * possible to instantiate Dhcpv4Srv object directly, thus getting a server
 * that does not support msgq. That is useful for embedded environments.
 * It may also be useful in validation.
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 *
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 */