[RADV] prefix deprecation

[Ondrej Zajicek]

On Thu, Nov 20, 2025 at 01:18:06PM +0100, Michael Saxl via Bird-users wrote:
> Hello,
> 
> I'm doing some experiments using radv and bird3.
> as expected it bird3 automatically sends prefix informations if not somehow disabled.
> 
> when I dynamically add a prefix to an interface (ip a a address/64 dev eno1) it immediatly blasts out that prefix with the default parameters (hardcoded in radv.c as radv_prefix_config default_prefix).
> 
> Now if that prefix is removed, this prefix is sent as valid_lifetime and preferred_lifetime = 0, but at least linux (NetworkManager) seems to ignore this deprecation, mainly because onlink and autonomous are not set.

Hello

Yes, this seems to be a bug. RFC 4861 section 6.3.4 says:

    Prefix Information options that have the "on-link" (L) flag set
    indicate a prefix identifying a range of addresses that should be
    considered on-link.  Note, however, that a Prefix Information option
    with the on-link flag set to zero conveys no information concerning
    on-link determination and MUST NOT be interpreted to mean that
    addresses covered by the prefix are off-link.  The only way to cancel
    a previous on-link indication is to advertise that prefix with the
    L-bit set and the Lifetime set to zero.

While RFC 4862 section 4 says:

    One Prefix Information option field, the "autonomous address-configuration
    flag", indicates whether or not the option even applies to stateless
    autoconfiguration. 

and section 5.5.3 says:

    a) If the Autonomous flag is not set, silently ignore the Prefix
    Information option.


That seems like a solid case that prefix withdraw should use the same
combination of flags like previous announcement.

Will check it.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
"To err is human -- to blame it on a computer is even more so."