Non-Local IPv6 Router Advertisement

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Revision as of 22:41, 15 January 2012 by MacFreek (Talk | contribs) (Use global scope IPv6 address in the router advertisement)

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IPv6 uses router advertisements (part of the Neighbour Discovery protocol of ICMPv6) to announce the default gateway. Hosts send a router solicitation message, and routers respond with a router advertisement message. This is unlike IPv4 where gateways are typically specified as part of the DHCP messages. Another difference is that IPv6 router advertisements use multicast (IPv6 ff02::1 and MAC address 33:33:00:00:00:01 for router advertisements; IPv6 ff02::2 and MAC address 33:33:00:00:00:02 for router solicitations).

Link-local addresses

Another striking feature of IPv6 is that is uses link-local IP addresses for local communication. Since neighbour discovery is local in scope, this means that the router sends the router solicitation with a link-local IPv6 ad the source address.

By default, a host stores the source IP address and source MAC address in the routing table. Hence, you will see IPv6 link local addresses in the IPv6 routing table:

% netstat -rn -f inet6
Destination        Gateway                         Flags       Netif
default            fe80::200:24ff:fece:69ef%en0    UGcI        eth0
::1                link#1                          UHL         lo0

This is unlike what most users expect. For example, if the host has IPv6 address 2001:0db8:637:2a:200:26ff:fea3:817d, most users would expect 2001:0db8:637:2a::1 as the gateway:

% netstat -rn -f inet6
Destination        Gateway                         Flags       Netif
default            2001:0db8:637:2a::1%eth0        UGcI        eth0
::1                link#1                          UHL         lo0

In reality, the IPv6 address of the gateway is irrelevant, as a packet on the wire would only contain the MAC destination address of the gateway. The IPv6 destination address contains the address of the final destination, not of the gateway.

Specify Global IPv6 Address for the Gateway

For easier debugging, network administrators may prefer to see a global IPv6 address as the gateway in hosts, instead of a link-local IPv6 address. Technically, it is possible to accomplish this in two ways.

  • Disable link-local IPv6 addresses on the gateway
  • Use the global scope IPv6 address in the router advertisement

Disable link-local IPv6 addresses on the gateway

An interface typically has one IPv4 address, but multiple IPv6 addresses. Consider the following example:

    ether 00:23:df:97:90:38
    inet netmask 0xffffff00 broadcast
    inet6 2001:db8:637:2a::3 prefixlen 64
    inet6 2001:db8:637:2a:223:dfff:fe97:9038 prefixlen 64 autoconf
    inet6 2001:db8:637:2a:acfe:e43b:2c35:d387 prefixlen 64 autoconf temporary
    inet6 fe80::223:dfff:fe97:9038%en0 prefixlen 64 scopeid 0x4

This host has four IPv6 addresses:

  • A manually configured or DHCPv6 assigned address (2001:db8:637:2a::3)
  • A SLAAC (Stateless Address Auto Configuration) address based on the router advertisement and the MAC address of the interface (this can be identified by the ff:fe part of the IPv6 address: 2001:db8:637:2a:223:dfff:fe97:9038 ). Note that SLAAC is enabled in the kernel by default. Unlike IPv4 DHCP which need to be specifically enabled, SLAAC is always configured if the server receives an router advertisement.
  • An temporary address, automatically configured based on the router advertisement, but not based on the MAC address for enhanced privacy (2001:db8:637:2a:acfe:e43b:2c35:d387)
  • A link-local address based on the MAC address and interface name (recognisable by the fe80 prefix and the interface appendix after the % sign: fe80::223:dfff:fe97:9038%en0)

You can disable respectively SLAAC and link-local address with the following kernel parameters.

For BSD kernels:

sysctl net.inet6.ip6.accept_rtadv=0
sysctl net.inet6.ip6.auto_linklocal=0

For Linux kernels:

sysctl net.ipv6.conf.all.accept_ra_defrtr=0
sysctl net.ipv6.conf.default.accept_ra_defrtr=0
sysctl net.ipv6.conf.all.autoconf=0
sysctl net.ipv6.conf.default.autoconf=0

Use global scope IPv6 address in the router advertisement

Let's dive into the actual router advertisement message for a minute.

 0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F 
Ethernet header 0 preamble
8 destination MAC
14 source MAC
20 type
IP header 22 version traffic class flow ...
24 .. label
26 payload length
28 next header hop limit
30 source IP
46 destination IP
ICMPv6 Router Advertisement header 62 Type (134 = RA) Code
64 Checksum
66 hop limit flags
68 router lifetime
70 reachable time
74 retrans timer
Link-layer option 78 type (1=link layer) length (1 = 8 bytes)
80 source MAC
Prefix option 86 type (3=prefix) length (4 = 32 bytes)
88 prefix length L A R reserved
90 valid lifetime
94 reserved
98 prefix IP
Ethernet footer 112 Checksum

As you can see, a router advertisement contains two options: one to specify the router MAC address and one to specify the IP prefix of the subnet.

The original specification (RFC 4861) of the Prefix Information only specified two flags:

on-link flag.
autonomous address-configuration flag. Specifies that the prefix can be used for stateless auto-configuration.

This original specification did not contain the IP address of the router. That had to be derived from the source IP address in the IPv6 header. This IP address is usually the link-local IPv6 address, since the IPv6 specifies that in these (link-local) communications, a local scope IP address should be used.

The specification was later changed (in RFC 6275) to include a third flag:

router address flag. Signals that the prefix IP contains the complete IP address assigned to the sending router.

If this is set, the host can use the global IP address of the router instead of the link-local IP address.

The radvd software, distributed with Linux, can set the R flag using the AdvRouterAddr on. Be sure to set the global IP address of the router in the prefix field:

% cat /etc/radvd.conf 
interface eth0
     AdvSendAdvert on;
     AdvManagedFlag off;
     AdvOtherConfigFlag off;
     prefix 2001:0db8:637:2a::1/64
         AdvOnLink on;
         AdvAutonomous on;
         AdvRouterAddr on;

The rtadvd software, distributed with BSD, is not able to set the R flag, even though it is able to set the prefix to the router IP.

The following two configuration settings do not work. rtadvd specified the IP prefix flags in the pinfoflags field. Note that this field does not understand hexadecimal, though it does understand letter flags (L + A + R = 0x80 + 0x40 + 0x20 = 0xE0 = 224).

% cat /etc/rtadvd.conf 
% cat /etc/rtadvd.conf 

(in both cases, the L and A flag are set, but the R flag is unset on FreeBSD 9.0)

There is also rtadvd software written by HP, which is unrelated to the rtadvd software above (which was written in the KAME project). I have not tested the HP software.