[Dnsmasq-discuss] No IPv6 router advertisements for subnets other than /64?

Quintus quintus at quintilianus.eu
Mon Sep 16 11:04:47 BST 2013


Am Mon, 16 Sep 2013 10:44:01 +0100
schrieb Simon Kelley <simon at thekelleys.org.uk>:

> SLAAC works by generating addresses by composing the prefix and the 
> so-called Interface Identifier.
> 
> RFC-4291 says: "For all unicast addresses, except those that start
> with the binary value 000, Interface IDs are required to be 64 bits
> long and to be constructed in Modified EUI-64 format."
> 
> and RFC-4862 says: "If the sum of the prefix length and interface 
> identifier length does not equal 128 bits, the Prefix Information
> option MUST be ignored."
> 
> from which I deduce that the prefix length MUST be 64 when using
> SLAAC.

Hm, ok. Seems I have been wrong, the RFCs are indeed pretty clear about
that.

> If you read the RFCs in detail, it is (as usual) rather more
> complicated than that, but I think that for all practical purposes in
> the situations where dnsmasq is used, it's true.

Probably you’re right.

> Looking at the code, simply removing the test in src/option.c is all 
> that's required to remove the restriction, the code that generates 
> router advertisements doesn't seem to depend on prefix-length==64.
> It's not been tested though, so no promises.

Given the RFCs seem to enforce the 64-bit prefix length, it’s unlikely
clients will get another prefix length right, even if dnsmasq would
generate the RAs as expected.

Thanks for the information anyway. Btw. dnsmasq is great :-)

Greetings,
Marvin

-- 
Blog: http://www.quintilianus.eu

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