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  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  

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