[radvd-devel-l] Router advertisement and forwarding on client side?

Pekka Savola pekkas at netcore.fi
Mon Mar 19 09:45:05 EST 2007

Combining the responses..

On Mon, 19 Mar 2007, vincent.trucmuche wrote:
> Why radvd doesn't start if forwarding is disabled?
>RFC2461 states
>   "A Lifetime of 0 indicates that the router is not a
>default router and SHOULD NOT appear on the default
>router list."
>It appears that a machine on the link may announce prefixes
>w/o being a router.

In most scenarios encountered to date, radvd is useful when it's 
forwarding packets.  Non-forwarding operation could be supported if 
there is sufficient case to be made to do so, and appropriate code to 
do so.

What are the specific scenarios (e.g., when Lifetime is 0) where this 
would be useful?  In particular, what radvd would be advertising if it 
should be used to forward packets?  (Note that 'being a default 
router' is a subset of 'forwarding packets')

> My question is not really a question about radvd but very
> linked to it. It concern client side into the kernel.
> Why linux Kernel drop RA packets if in forwarding mode?
> I saw nothing in RFCs prohibiting a router to configure its
> interface address or its default route using RA.

RFC2462 and its successor says at the start of the abstract (note, 
_host_ there):

    This document specifies the steps a host takes in deciding how to
    autoconfigure its interfaces in IP version 6. [...]

The specifications haven't been designed with a router autoconfiguring 
its addresses.  While router configuring an address from the prefix it 
advertises (in contrast to configuring address from prefixes it hears) 
could be better justified, even this has been frowned on.

Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings

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