FreeBSD 8 as an IPv6 router

[Mattia Rossi]

On 14/12/2011 11:41, Hiroki Sato wrote:
> Mattia Rossi<mrossi at swin.edu.au>  wrote
>    in<4EE7CDBE.1090605 at swin.edu.au>:
>
> mr>  Ok, this is something I always get a bit confused with. I understand
> mr>  that it's the right clean thing to set up a /64 on the interface which
> mr>  sends router advertisements, but I also would expect by nature, that
> mr>  whatever prefixlength you chose on the interface, rtadvd would simply
> mr>  grab the lowest /64 prefix out of the configured one to send router
> mr>  advertisements out.
> mr>
> mr>  The idea there is, that you might use this router for multiple
> mr>  subnets, and have a single default route.
> mr>
> mr>  Now of course to do that you'd need to configure rtadvd.conf, so I
> mr>  guess the whole thing missing here is a bit of documentation which
> mr>  says, that if you don't configure rtadvd via rtadvd.conf you're not
> mr>  allowed to be lazy and configure any prefix on the interface and
> mr>  expect rtadvd to do the right thing.
> mr>
> mr>  It seems to me, that a lot of people (including me) would expect that,
> mr>  so maybe some info about that wouldn't be to bad.
>
>   I do not think it is a good idea that the rtadvd daemon automatically
>   splits prefixes shorter than 64 to ones with just 64.  "Which prefix
>   should be advertised" is one of things which a sysadmin must specify
>   explicitly when it receives prefixes shorter than 64 via IA-PD or
>   something, and it should match the actual subnet structure.  A simple
>   way to do so is to assign an address onto eth0, in his example, with
>   desired /64 subnet prefix from the delegated (shorter) prefix, and
>   run rtadvd with no configuration file.  This is the expected
>   scenario.  A /60 address assigned on eth0 does not work as a default
>   router address for multiple /64 subnets anyway...
>
>   This trouble is caused by misconfiguration of sla-len and non-/64
>   prefix is assigned unexpectedly to eth0.  If all of the configuration
>   were correct rtadvd.conf was not needed in the first place, and even
>   if split /64 prefixes were automatically advertised by rtadvd at that
>   time the situation would not got better.
>

As said, I agree, it's probably not a good idea to autoselect a /64 from 
a shorter prefix. But I can imagine that a lot of people out there would 
expect this behaviour, and might just sit there and wonder why the hell 
the RAs are not being sent, if they have an address configured on the 
interface.
Most people do not look at the prefix length, also because many people 
use tools which automatically set prefixes and addresses up and or 
copy&paste configs from the Internet, as we've just seen.

So my suggestion here is to add some information to the rtadvd and 
rtadvd.conf manual page which explicitly tells people to look at the 
configured prefix length, as rtadvd will only work without configuration 
if the prefix length is set to /64, and eventually point to the RFCs in 
that paragraph as well.
 From there people will figure out how to change their configs to get 
addresses with a prefixlength of /64 themselves (e.g. change the sla-len 
in the wide-dhcp client)

It's a simple addition to the manpage which helps educating people as 
well and allows to keep things clean.

Mat