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The last point that needs to be explained about ICMPv6 is the `Redirect` message. This message is used when there is more than one router on a subnet as shown in the figure below.
In this network, ``router1`` is the default router for all hosts. The second router, ``router2`` provides connectivity to a specific IPv6 subnet, e.g. ``2001:db8:abcd::/48``. These two routers attached to the same subnet can be used in different ways. First, it is possible to manually configure the routing tables on all hosts to add a route towards ``2001:db8:abcd::/48`` via ``router2``. Unfortunately, forcing such manual configuration boils down all the benefits of using address auto-configuration in IPv6. The second approach is to automatically configure a default route via ``router1`` on all hosts. With such route, when a host needs to send a packet to any address within ``2001:db8:abcd::/48``, it will send it to ``router1``. ``router1`` would consult its routing table and find that the packet needs to be sent again on the subnet to reach ``router2``. This is a waste of time. A better approach would be to enable the hosts to automatically learn the new route. This is possible thanks to the ICMPv6 `Redirect` message. When ``router1`` receives a packet that needs to be forwarded back on the same interface, it replies with a `Redirect` message that indicates that the packet should have been sent via ``router2``. Upon reception of a `Redirect` message, the host updates it forwarding table to include a new transient entry for the destination reported in the message. A timeout is usually associated with this transient entry to automatically delete it after some time.
An alternative is the Dynamic Host Configuration Protocol (DHCP) defined in :rfc:`2131` and :rfc:`3315`. DHCP allows a host to automatically retrieve its assigned IPv6 address, but relies on server. A DHCP server is associated to each subnet [#fdhcpserver]_. Each DHCP server manages a pool of IPv6 addresses assigned to the subnet. When a host is first attached to the subnet, it sends a DHCP request message in a UDP segment (the DHCP server listens on port 67). As the host knows neither its IPv6 address nor the IPv6 address of the DHCP server, this UDP segment is sent inside a multicast packet target at the DHCP servers. The DHCP request may contain various options such as the name of the host, its datalink layer address, etc. The server captures the DHCP request and selects an unassigned address in its address pool. It then sends the assigned IPv6 address in a DHCP reply message which contains the datalink layer address of the host and additional information such as the subnet mask, the address of the default router or the address of the DNS resolver. The DHCP reply also specifies the lifetime of the address allocation. This forces the host to renew its address allocation once it expires. Thanks to the limited lease time, IP addresses are automatically returned to the pool of addresses when hosts are powered off.
Both SLAAC and DHCPv6 can be extended to provide additional information beyond the IPv6 prefix/address. For example, :rfc:`6106` defines options for the ICMPv6 ND message that can carry the IPv6 address of the recursive DNS resolver and a list of default domain search suffixes. It is also possible to combine SLAAC with DHCPv6. :rfc:`3736` defines a stateless variant of DHCPv6 that can be used to distribute DNS information while SLAAC is used to distribute the prefixes.
Footnotes
The full list of allocated IPv6 multicast addresses is available at http://www.iana.org/assignments/ipv6-multicast-addresses
The IANA_ maintains the list of all allocated Next Header types at http://www.iana.org/assignments/protocol-numbers/
When IPv4 was designed, the situation was different. The IPv4 header includes a checksum that only covers the network header. This checksum is computed by the source and updated by all intermediate routers that decrement the TTL, which is the IPv4 equivalent of the `HopLimit` used by IPv6.
Until a few years ago, all hosts replied to `Echo request` ICMP messages. However, due to the security problems that have affected TCP/IP implementations, many of these implementations can now be configured to disable answering `Echo request` ICMP messages.
For a discussion of the issues with the router alert IP option, see http://tools.ietf.org/html/draft-rahman-rtg-router-alert-dangerous-00 or http://tools.ietf.org/html/draft-rahman-rtg-router-alert-considerations-03
For simplicity, you assume that each datalink layer interface is assigned a 64 bits MAC address. As we will see later, today's datalink layer technologies mainly use 48 bits MAC addresses, but the smaller addresses can easily be converted into 64 bits addresses.
:rfc:`4291` and :rfc:`4861` explain in more details how the IPv6 multicast address is determined from the target IPv6 unicast address. These details are outside the scope of this book, but may matter if you try to understand a packet trace.
The DAD algorithm is also used with `link-local` addresses.
Using a datalink layer address to derive a 64 bits identifier for each host raises privacy concerns as the host will always use the same identifier. Attackers could use this to track hosts on the Internet. An extension to the Stateless Address Configuration mechanism that does not raise privacy concerns is defined in :rfc:`4941`. These privacy extensions allow a host to generate its 64 bits identifier randomly every time it attaches to a subnet. It then becomes impossible for an attacker to use the 64-bits identifier to track a host.
Using a `Hop Limit` of ``255`` prevents one family of attacks against ICMPv6, but other attacks still remain possible. A detailed discussion of the security issues with IPv6 is outside the scope of this book. It is possible to secure NDP by using the `Cryptographically Generated IPv6 Addresses` (CGA) defined in :rfc:`3972`. The Secure Neighbor Discovery Protocol is defined in :rfc:`3971`. A detailed discussion of the security of IPv6 may be found in [HV2008]_.
In practice, there is usually one DHCP server per group of subnets and the routers capture on each subnet the DHCP messages and forward them to the DHCP server.

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../../protocols/ipv6.rst:859
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locale/pot/protocols/ipv6.pot, string 233