Specifying IPv6 off-link for a subnet

[Scott]

On Wed, Jul 07, 2021 at 07:43:48PM -0700, Mel Pilgrim wrote:
> On 2021-07-05 8:45, Scott wrote:
> > I have manually configured IPv6 on the interfaces:
> > ifconfig_vmx0_ipv6="inet6 xx::yy prefer_source accept_rtadv -autoconf no_radr"
> > 
> > The prefix appears in the prefix list as:
> > # ndp -p
> > xx::/64 if=vmx0
> > flags=LO vltime=infinity, pltime=infinity, expire=Never, ref=1
> >    No advertising router
> > 
> > Here's the received RA:
> > 01:26:48.355186 IP6 (class 0xe0, hlim 255, next-header ICMPv6 (58) payload length: 64) fe80::1 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 64
> >          hop limit 64, Flags [none], pref high, router lifetime 1800s, reachable time 0ms, retrans timer 0ms
> >            source link-address option (1), length 8 (1): 00:00:5e:00:02:02
> >            mtu option (5), length 8 (1):  9000
> > 	  prefix info option (3), length 32 (4): xx::/64, Flags [none], valid
> > 	  time 3600s, pref. time 60s
> > 
> > Note that there is no L flag set, so the prefix is off-link.
> 
> You're misinterpreting what the L flag in a PIO means.
> 
> If set, the L flag indicates the prefix can be used for on-link 
> determination. But if the L bit is unset, the PIO isn't making any 
> assertion about the prefix being on-link or off-link. More importantly, 
> a host must not use a PIO with L=0 to conclude a prefix is off-link.
> 
> IOW, it is correct for a host to do nothing when it receives an RA PIO 
> with no flags for an already-configured prefix.
> 
Thanks,

so for the case of manually addressed devices on a private vLAN/IPPL there 
must be a way to specify that the configured prefix is not on-Link.  From 
section 5 of https://tools.ietf.org/id/draft-nordmark-intarea-ippl-00.xml#n-ipv6-over-ippl:

"IPv6 Neighbor Discovery RFC4861 can be used to get all the hosts on the link 
to send all unicast packets except those send to link-local destination 
addresses to the routers. That is done by setting the L-flag (on-link) to 
zero for all of the Prefix Information options."

How does one achieve this on FreeBSD?