Re: [PATCH net v2] Revert "ipv6: preserve insertion order for same-scope addresses"

On Mon, 1 Jun 2026 12:03:59 +1000
Matthieu Baerts [off-list ref] wrote:

Hi Stefano,

On 29/05/2026 21:41, Stefano Brivio wrote:

quoted

On Fri, 29 May 2026 13:23:57 +0200
Fernando Fernandez Mancera [off-list ref] wrote:
  

quoted

Chris Adams reported that preserving insertion order for same-scope
addresses is causing SSH connections to be dropped after stopping a VM
while running NetworkManager.

NetworkManager caches the IPv6 address configuration, when a RA arrives,
it determines the list of addresses to configure and checks if the
addresses are already in the right order in the kernel. If they aren't,
NetworkManager removes and re-adds them to achieve the desired order.

As the order changes, NetworkManager is confused and reconfigures the
addresses on every update. In addition, this would also affect to cloud
tooling that relies on IPv6 addresses order to identify primary and
secondaries addresses.  

By the way, I'm still looking into this part, trying to find
"problematic" examples.

And I couldn't find any, yet, because it looks like there's always a
_single_ IPv6 address being used as a secondary for a primary IPv4
address.  

FYI, the order change also affected some specific scripts, e.g. here
with MPTCP and packetdrill:

  https://github.com/multipath-tcp/packetdrill/commit/1b7cd4482ce8

Because the order was not "natural" before, and different from IPv4, a
workaround was needed to keep the same order. I was happy to remove it,
but now it looks like I need to re-apply it :)

Ouch. :) We were are also pondering about some kind of workaround like
that (https://bugs.passt.top/show_bug.cgi?id=175#c9) but fixing the
kernel looked simpler and the right thing to do... until last week.

It would be nice to get the "natural" order back without breaking the
userspace (or with a way to choose the order).

I was thinking that if we implement a label like NLM_F_INSERT_LAST
(David's proposal for the name), we could patch iproute2 to set it on
'ip address restore' at least, other than using it in pasta(1).

But that wouldn't be enough for your case. At the same time always
adding it for RTM_NEWADDR requests in iproute2 could break somebody
else's scripts. I guess a reasonable solution could be to add an
additional parameter for ip-address... 'insert_last'? 'last'?

-- 
Stefano