eth0:1 aliases in NM, manual setup

[Thomas Haller]

On Mon, 2024-07-22 at 12:45 +0300, Andrei Borzenkov wrote:
> On Mon, Jul 22, 2024 at 11:17 AM Íñigo Huguet <ihuguet at redhat.com>
> wrote:
> 
> 
> AFAIU kernel considers the very first interface address for a given
> subnet/scope as primary and all addresses for the same combination of
> subnet/scope added later as secondary. So, to keep the specific
> address as primary it has to be configured first. I suspect legacy
> scripts implicitly processed ifcfg-ethX before ifcfg-ethX:N, so
> address in ifcfg-ethX became primary.
> 
> Does NetworkManager process ipv4.addresses in deterministic order?
> Can
> we rely that for "ipv4.addresses A1 A2 A3" it will always add A1
> first, then A2, then A3? Especially with DHCP where there could be
> delay waiting for completion of the DHCP transaction.
> 

Hi,

It works.

Addresses can become ready for configuration in any order. For example,
if you do DAD/ACD. Or if you do `nmcli device modify DEV ipv4.addresses
ADDRS`.

When NetworkManager notices that the order is not as required, it will
delete and re-add them in the right order.


The complexity of getting that right, is considerable. It's here:
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/777418dfd786722d4dc16d4e60d2ca724f93b55c/src/libnm-platform/nm-platform.c#L4327


Note that for IPv4, a conflict only exists (and a meaningful concept of
"primary") for addresses in the same subnet. For IPv6, it's only
meaningful between addresses in the same scope. That corresponds to how
kernel treats it.


Thomas