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+// Copyright (C) 2016 Internet Systems Consortium, Inc. ("ISC")
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+//
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+// This Source Code Form is subject to the terms of the Mozilla Public
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+// License, v. 2.0. If a copy of the MPL was not distributed with this
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+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
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+
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+/**
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+ @page dhcpv4o6Dhcp4 DHCPv4-over-DHCPv6 DHCPv4 Server Side
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+
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+Kea supports DHCPv4-over-DHCPv6 using cooperating DHCPv6 and DHCPv4
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+servers. This section describes the DHCPv4 server side.
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+
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+@section Dhcp4to6Ipc DHCPv6-to-DHCPv4 Inter Process Communication
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+
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+The @c Dhcp4to6Ipc class is derived from the base @c Dhcp4o6IpcBase as
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+a singleton class (by the static @ref isc::dhcp::Dhcp4to6Ipc::instance
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+function). @ref isc::dhcp::Dhcp4to6Ipc::open is called to open IPC
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+sockets and to register @ref isc::dhcp::Dhcp4to6Ipc::handler on
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+external sockets on the @c IfaceMgr.
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+
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+@section dhcp4to6Receive DHCPv4-over-DHCPv6 Packet Processing
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+
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+DHCPv6 DHCPv4-QUERY messages are forwarded by the DHCPv6 server on the IPC.
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+The interface manager (@c IfaceMgr) using the external socket mechanism
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+invokes @ref isc::dhcp::Dhcp4to6Ipc::handler, the packet is received using
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+(inherited) @ref isc::dhcp::Dhcp4o6IpcBase::receive which decodes and strips
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+the ISC Vendor option.
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+
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+The handler verifies there is one and only one DHCPv4-Message option
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+and extracts it. @ref isc::dhcp::Dhcpv4Srv::processPacket processes
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+the DHCPv4 query as a @c Pkt4o6 instance and builds the
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+DHCPv4-over-DHCPv6 response.
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+Registered callouts for "buffer4_send" are called (@ref
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+dhcpv4HooksBuffer4Send, note all the other DHCPv4 hook points are
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+served during the standard processing). The response is sent back
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+to the DHCPv6 server on the IPC.
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+
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+@section dhcp4to6Specific Modified DHCPv4 Routines
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+
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+For a @c Pkt4o6 query the @ref isc::dhcp::Dhcpv4Exchange::initResponse
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+performs some extra steps (@ref isc::dhcp::Dhcpv4Exchange::initResponse4o6):
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+the DHCPv6 response is built and the response member is reset to
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+a @c Pkt4o6 instance with DHCPv4 and DHCPv6 parts.
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+
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+The subnet selection (@ref isc::dhcp::Dhcpv4Srv::selectSubnet) is specialized
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+for @c Pkt4o6 queries (@ref isc::dhcp::Dhcpv4Srv::selectSubnet4o6):
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+the subnet selector class (@c SubnetSelector) is filled using DHCPv4 and
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+DHCPv6 information and the @ref isc::dhcp::CfgSubnets4::selectSubnet4o6
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+function is called (instead of @ref isc::dhcp::CfgSubnets4::selectSubnet).
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+
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+In @ref isc::dhcp::Dhcpv4Srv::adjustIfaceData for @c Pkt4o6 queries
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+the local address is set to the incoming interface assigned address
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+(same case than for a broadcast local address).
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+
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+In @ref isc::dhcp::Dhcpv4Srv::adjustRemoteAddr for @c Pkt4o6 queries the
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+remote address is set to the query one (which is in fact an IPv6 address).
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+
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+In @ref isc::dhcp::Dhcpv4Srv::acceptDirectRequest @c Pkt4o6 queries are
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+accepted (they are considered as being relayed).
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+
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+@section dhcp4SubnetSelect DHCPv4 Subnet Selection
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+
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+@todo Move this to libdhcpsrv
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+
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+Selectors (i.e., members of @c SubnetSelector class) are:
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+- incoming interface name
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+- gateway address - giaddr field
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+- client address - ciaddr field
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+- local address
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+- remote address
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+- option select - from the first Relay Agent Link Selection suboption or
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+ from a Subnet Selection option
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+- client classes - used to reject a matching rule and try next rules
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+
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+First use the option select, next if the message was relayed (not undefined
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+gateway address) use the gateway address as a subnet relay address.
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+
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+If a subnet was not already selected choose an address between:
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+
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+- if the gateway address is not undefined the gateway address
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+- if the client address is not undefined and the local address not the
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+ broadcast address (i.e., renew or rebind) the client address
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+- if the remote address is not undefined and the local address not the
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+ broadcast address (i.e., renew or rebind) the remote address
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+- at this point try the interface name as a subnet interface
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+- if the interface name does not select a subnet choose the interface address
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+ (last resort)
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+
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+Match the chosen address in a subnet address range.
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+
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+@section dhcp4o6SubnetSelect DHCPv4-over-DHCPv6 Subnet Selection
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+
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+@todo Move this to libdhcpsrv
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+
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+Selectors (i.e., members of @c SubnetSelector class) are:
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+- incoming interface name
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+- gateway address - giaddr field (should be always undefined)
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+- client address - ciaddr field
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+- local address - set to the interface address
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+- remote address - IPv6 address
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+- option select - from a Subnet Selection option
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+- first relay link address - (IPv6) undefined or the first relay link
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+ address which is not undefined or link local (i.e., usable)
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+- interface ID - (IPv6) when a relay message includes an interface ID
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+ relay option
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+- client classes - used to reject a matching rule and try next rules
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+
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+Check if DHCPv4-over-DHCPv6 is enabled for the subnet and continue
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+with the next one is when it is not.
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+
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+First the remote address is matched in a subnet IPv6 range, second
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+the interface ID if it is set is matched, third the interface name.
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+
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+These rules are applied for each subnet configuration so if two
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+subnets match the first one is returned (vs. the first matching rule).
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+
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+@todo better DHCPv4-over-DHCPv6 selection, e.g., handle relayed
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+messages and return best (vs first) match.
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+
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+*/
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Francis Dupont