Specifying IPv6 off-link for a subnet

[Mel Pilgrim]

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.