[Bug 233754] Router Advertisement configuration and prefixlen

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=233754

            Bug ID: 233754
           Summary: Router Advertisement configuration and prefixlen
           Product: Documentation
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: Documentation
          Assignee: doc at FreeBSD.org
          Reporter: lapo at lapo.it

Hi,
    while trying to debug a SLAAC configuration problem of mine, I noticed
something strange in the documentation:

https://www.freebsd.org/doc/handbook/network-ipv6.html

chapter 31.9.4 states:

    Next, create the configuration file, /etc/rtadvd.conf as seen in this
example: […]
    For a dedicated /64 subnet, nothing else needs to be changed. Otherwise,
change the prefixlen# to the correct value.

while `man rtadvd.conf` states:

     As described in rtadvd(8), you do not have to set this configuration file
     up at all, unless you need some special configurations.  You may even
     omit the file as a whole.  In such cases, the rtadvd daemon will
     automatically configure itself using default values specified in the
     specification.

and indeed, I checked with Wireshark and it is broadcasting the correct prefix
length (in my case, /80) with no need to have created that file.

Then my problem is that prefixlen ≠ 64 cannot be used by accept_rtadv or, at
least, it is what I understand from RTFS nd6_rts.c line 1495, which reads:

    if (ifidlen + pr->ndpr_plen != 128)

(and ifidlen is always 64, as returned by in6_if2idlen())

I'm not sure what's the best solution for the Handbook, but I'd probably remove
the `rtadv.conf` example altogether, as it gives no advantage and only the risk
of writing it wrong; also, as far as `rtadvd` goes, any prefix length is fine,
but then any FreeBSD client won't accept it anyways, so I'm not so sure
suggesting what to do "when prefixlen is different" is very useful… a warning
about the lack of SLAAC for any other prefixlen could be useful, for people who
don't know it already.

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