DHCPv6 Overview

Dynamic Host Configuration Protocol (DHCP) is an extension of BOOTP that defines a protocol for passing configuration information to hosts on a network. DHCPv6 supports IPv6, the next-generation Internet protocol. DHCPv6 enables DHCP servers to transmit configuration parameters using extensions to IPv6 nodes. It automatically allocates reusable network addresses and reduces the cost of managing IPv6 nodes in environments where administrators require more control over the allocation of IP addresses. DHCPv6 manages network resources such as IP addresses, Network Time Protocol (NTP), Domain Name System (DNS), and other server addresses at a centralized location.




	NOTE: All references to DHCPv6 in this document refers to DHCPv6 2.001

This section discusses the following topics:

DHCPv6 Components

Table 1-17 describes the components that the DHCPv6 framework includes.

Table 1-17 DHCPv6 Components

Component	Description
DHCP client	The host that needs to be configured.
DHCP server	The server that caters to the DHCP client’s configuration requirements.
DHCP relay or agent	The host that facilitates initial communication between the DHCP server and the client when they are on different links.

Autoconfiguration

Autoconfiguration is a mechanism that does not require any manual intervention to configure a host in a network environment.

Following are the types of autoconfiguration methods:

Stateless address autoconfiguration
Stateless autoconfiguration does not require a manually configured server but enables IPv6 hosts to configure their own addresses using a local IPv6 router. This method is easy to use. However, this autoconfiguration method lacks network access control capabilities and manages only addresses. It does not make the optimum use of the available address space.
Stateful address autoconfiguration
Stateful autoconfiguration involves a client and a server. An unconfigured node checks for router advertisements on the network. If there are no router advertisements on the network, the node determines that it needs to use DHCPv6 to configure an interface.
DHCPv6 does not require a DHCPv6 server on each link. It works across relays.

Ports

DHCPv6 uses the following Internet User Datagram Protocol (UDP) ports:

546
DHCP servers use this client port as the destination port to send messages to clients and relays. In addition, the relays or agents use this port as the destination port for messages sent to the clients.
547
DHCP clients use this agent port as the destination port to send messages to agents or relays. In addition, relays use this port as the destination port for messages sent to the servers.

For more information on port reference, refer to the following link:

http://www.iana.org/assignments/port-numbers

Multicast Addresses

DHCPv6 uses the following multicast addresses:

All DHCP Agents address - FF02::1:2
DHCP clients use this link-scoped multicast address to communicate with the on-link agent. All agents within the same DHCP domain belong to this multicast group.
All DHCP Servers address - FF05::1:3
DHCP clients or relays use this site-scoped multicast address to communicate with servers in one of the following scenarios:
- When they need to send messages to all servers.
- When they do not know the server’s unicast address.
All servers within the site belong to this multicast group.
See HP-UX IPv6 Porting Guide, available at http://www.docs.hp.com/en/netcom.html#IPv6, for more information about link-local and site-local addresses.

How DHCPv6 Works

This section describes how the DHCPv6 software works.

It discusses the following topics:

Client/Server Operation

This section describes how the DHCPv6 server and the client communicate with each other.

Servers and clients on the same link communicate directly with each other.

The DHCPv6 client initiates the message interaction by sending a SOLICIT message. It uses the SOLICIT message to locate DHCPv6 servers, which are able to provide an IPv6 address and other configuration parameters. Multiple servers respond to the client’s message with a DHCP ADVERTISE message. The DHCP ADVERTISE message contains a preference value, which indicates the server’s ability to offer services to the client. You need to set this preference value for a server. See “The dhcpv6d Server Daemon” for more information.

The server preference variable is an unsigned single octet value, with the lowest preference value being 0 and the highest 255. Clients choose higher preference servers over lower preference servers to configure their interfaces.

The client then sends the DHCP REQUEST message to the server, which is able to provide the required configuration parameters.

Figure 1-4 depicts how the client and the server, on the same link, communicate with each other.

Figure 1-4 When Client and Server Are on the Same Link

The client and the server communicate with each other by exchanging packets as follows (see the numbers 1,2,3,4, and 5 in Figure 1-4):

The client sends a DHCP SOLICIT message to the ALL DHCP Agents address (FF02::1:2) to locate suitable servers.
Multiple servers respond to the SOLICIT message by sending a DHCP ADVERTISE message to the client.
The client sends a DHCP REQUEST message to the DHCPv6 server that has the highest preference value.
The server responds to the client’s message by sending a DHCP REPLY message. The DHCP REPLY message contains the IPv6 address and configuration parameters required by the client.
The server sends the DHCP REPLY message to respond to the client’s REQUEST, RENEW, REBIND, RELEASE, RECONFIGURE, CONFIRM, and INFORMATION REQUEST messages.
The client sends a DHCP RELEASE message to return one or more IPv6 addresses to the server when it has completed using an IPv6 address.

See “Message Types” for more information about message types.

DHCPv6 requires relays to be set up on the client’s link when the client and the server are on different links. Relays receive messages from the client and forward them verbatim to a set of servers that the DHCP domain includes. Relays forward the client’s messages to servers using any of the following addresses:

All DHCP Servers Site-local multicast address
Other site-local multicast addresses set up within the DHCP domain to include the servers in that domain
A list of unicast addresses for servers

Figure 1-5 depicts how the client and the server, on different links, communicate with each other.

Figure 1-5 When Client and Server Are on Different Links

The client and the server exchange messages with each other via a relay as follows (see numbers 1,2,3,4, and 5 in Figure 1-5):

The client sends a DHCP SOLICIT message to the ALL DHCP Agents address to locate servers that are able to offer the required services.
The relay forwards the client’s message to servers by sending RELAY-FORW messages. The client message is encapsulated in the RELAY-FORW message.
Servers respond to the client’s message by sending a RELAY-RELAY message. The server message is encapsulated in the RELAY-RELAY message.
The relay forwards the server message to the client’s message by sending ADVERTISE messages.
NOTE: All other message interactions are identical to those explained in the previous scenario.

Identifying and Managing IPv6 Addresses

Servers and clients manage IPv6 addresses in groups called Identity Associations (IAs). A unique identifier identifies each IA. DHCPv6 servers allocate IPv6 addresses to clients as IAs. Clients use the addresses in an IA to configure interfaces. Each address in an IA has its own preferred and valid lifetimes.

DHCPv6 Features

This section describes the features that the DHCPv6 software supports. It discusses the following topics:

Message Types

Table 1-18 describes the messages exchanged between the client and the server through a relay.

Table 1-18 DHCPv6 Message Types

Message Type	Description
`SOLICIT`	A client uses this message to locate servers.
`ADVERTISE`	A server uses this message to respond to `SOLICIT` messages.
`REQUEST`	A client uses this message to request addresses and configuration parameters from servers.
`CONFIRM`	A client sends a `CONFIRM` message to any available server to determine whether the addresses it was assigned are still appropriate to the link to which the client is connected.
`RENEW`	A client uses this message to renew addresses from the server when the lease on an identify association (IA) is about to expire.
`REBIND`	A client uses this message to renew addresses from all available DHCPv6 servers when the lease on an IA is about to expire.
`REPLY`	A server sends a REPLY message containing assigned addresses and configuration parameters in response to a `SOLICIT`, `REQUEST`, `RENEW`, `REBIND` message received from a client. A server sends a `REPLY` message containing configuration parameters in response to an `INFORMATION-REQUEST` message. A server sends a `REPLY` message in response to a CONFIRM message confirming or denying that the addresses assigned to the client are appropriate to the link to which the client is connected. A server sends a `REPLY` message to acknowledge receipt of a `RELEASE` or `DECLINE` message.
`RELEASE`	Clients use this message to return one or more IP addresses to servers.
`DECLINE`	A client sends a `DECLINE` message to a server to indicate that the client has determined that one or more addresses assigned by the server are already in use on the link to which the client is connected.
`RECONFIGURE`	Servers send his message to inform clients that the server has new or updated configuration parameters, and that the clients need to initiate a Request/Reply transaction with the servers in order to receive the updated information.
`INFORMATION-REQUEST`	A client sends an `INFORMATION-REQUEST` message to a server to request configuration parameters without the assignment of any IP addresses to the client.
`RELAY-FORW`	A relay agent uses this to forward client messages to servers.
`SERVER-FORWARD`	A server sends a `RELAY-REPL` message to a relay agent containing a message that the relay agent delivers to a client. The `RELAY-REPL` message can be relayed by other relay agents for delivery to the destination relay agent. The server encapsulates the client message as an option in the `RELAY-REPL` message, which the relay agent extracts and relays to the client.

Multiple IP Address Request

DHCPv6 allows clients to request multiple IP addresses to configure an interface.

Configuration Parameters from a DHCPv6 Server

DHCPv6 clients do not need to obtain all the configuration parameters for an interface from the same DHCPv6 server. They can communicate with different servers within the same DHCP domain to obtain this information.

Reconfiguration Messages

The DHCPv6 server sends reconfiguration messages to the client to inform the client that the server has new or updated configuration parameters. It also indicates that the client can initiate a Request or Reply transaction with the server in order to obtain the new information. The clients listen on a specific port to receive the DHCP Reconfigure messages from the server.

DHCPv6 Files

Table 1-19 describes the files and daemons that the DHCPv6 software contains.

Table 1-19 DHCPv6 Files

Name and Location	Server	Client	Use
`/etc/dhcpv6tab`	X		You can use this file to specify the default client, pool group, and relay settings.
`/etc/dhcpv6db`	X		Files in this directory contain the client lease information. The DHCPv6 server reads these files to build its internal database and to maintain the leases.
`/etc/dhcpv6client.data`		X	This file contains the configuration parameters obtained from the server. The client daemon reads this file to build its internal database and to maintain the leases.