Re: Problem with IPv6 privacy addresses in 7.0

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 28 May 2026 18:01:27 +0200
Beniamino Galvani <bgalvani@redhat.com> wrote:

> On Thu, May 28, 2026 at 05:24:28PM +0200, Íñigo Huguet wrote:
> > On Thu, May 28, 2026 at 4:53 PM Stefano Brivio <sbrivio@redhat.com> wrote:  
> > > I think what's considered UAPI in this case isn't so clear-cut. I guess
> > > it would have been pretty hard for anybody not familiar with
> > > NetworkManager's codebase to imagine that an application would rely on
> > > IPv6 addresses (and only IPv6) to be returned in the reverse order.  
> > 
> > IMHO it's not because an application would rely on the order. It's
> > because the order matters, as the first one is considered the primary.

Yes, that's also the case, but:

> > This is not NetworkManager specific. Or I am mistaken? I'm speaking by
> > memory

...from what I understood of the issue at hand there's a part that's
specific to NetworkManager here, because NetworkManager replaces /
deletes the Privacy Extensions addresses, instead of different
addresses, because it relies on that specific (reversed) order.

Anyway, yes, I see your point about UAPI now.

> Yes, exactly. If you do:
> 
>  ip addr add dev X 3fff::1/64
>  ip addr dev dev X 3fff::2/64
> 
> then the old kernel chooses the address that was added last as source
> address for outgoing communications. With the new kernel it chooses
> the address that was added first.
> 
> I think that any script or network management tool that cares about
> the source address selection is impacted. Indeed, the commit had
> effects on one of the selftests, which had to be modified to change
> the order of iproute2 invocations.
>  
> > > > If the fix must be in NetworkManager, we only need to parse them in
> > > > non-reverse order like IPv4, I guess.  
> > >
> > > But that would then require some form of detection, and, at least
> > > according to Fernando, isn't the most robust option anyway, as ideally
> > > NetworkManager shouldn't rely on the order at all.  
> > 
> > True  
> 
> Correct, if the new behavior is considered better, there should be a
> way to detect which order must be used. Otherwise userspace tools
> won't be able to maintain the same behavior with different kernels.

My remark here is about whether NetworkManager needs to detect this at
all. If it used timestamps to detect recent / older addresses, as
Fernando mentioned, then you wouldn't need any detection at all, right?
Or is there something else we're missing?

By the way, if really needed, one could detect that with the equivalent
of:

  ip a a ::2 dev lo && ip a a ::3 dev lo && [ $(ip -6 -j a | jq -rM '.[] .addr_info[0].local') = ::3 ] && echo wrong || echo right

...depending on the application, that's not necessarily practical, but
so far we have no evidence of any application needing to do that at all.

-- 
Stefano