IPv6 router advertisement
Ted Lemon
Ted.Lemon at nominum.com
Tue May 5 20:09:35 UTC 2009
On May 5, 2009, at 2:58 PM, Paul Selkirk wrote:
> I've confirmed the RA behavior. radvd is perfectly happy to advertise
> a /96 prefix with the A bit set, but the client won't auto-configure
> an address. RFC 4862 doesn't put a limit on the prefix length, but
> dmesg reports "IPv6 autoconf: prefix with wrong length 96" (Linux
> kernel 2.6.25.17). Do you feel like hacking the kernel?
You can't do stateless autoconf in a /96. Generally speaking, using
a /96 is going to create all kinds of problems - stateless won't work,
cga won't work, etc. Here's what RFC2462 says:
If the sum of the prefix length and interface identifier length does
not equal 128 bits, the Prefix Information option MUST be ignored. An
implementation MAY wish to log a system management error in this case.
It is the responsibility of the system administrator to insure that
the lengths of prefixes contained in Router Advertisements are
consistent with the length of interface identifiers for that link
type. Note that interface identifiers will typically be 64-bits long
and based on EUI-64 identifiers as described in [ADDR-ARCH].
This same text appears in RFC4862; I included the text from 2462
because it's shorter, but both documents say the same thing. So
Linux is doing the right thing here. If you want to use a /96, you
have to either use stateful address configuration (DHCPv6) or manual
configuration.
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