[systemd-devel] systemd-networkd, IPv6PrivacyExtensions=kernel, sysctl and devicenames

[Ede Wolf]

Am 22.05.20 um 17:58 schrieb Andrei Borzenkov:
>>
>> The problem is, that sysctl.conf is being executed before the interfaces
>> get their eventual names.
>>
> 
> That sounds like actual bug. What systemd version do you use?

At least it is, what the journal suggest, as seen below. However, 
generally it sounds like a good idea to read sysctl.conf as early as 
possible


As for my version:

$  pacman -Qs systemd
local/systemd 245.5-2
     system and service manager
local/systemd-libs 245.5-2
     systemd client libraries
local/systemd-sysvcompat 245.5-2
     sysvinit compat for systemd

$ uname -a
Linux willy 5.6.13-arch1-1 #1 SMP PREEMPT Thu, 14 May 2020 06:52:53 
+0000 x86_64 GNU/Linux

Same issues with the arch lts-kernel, this behaviour is consistent and 
easily reproducable.


>>>> And the logs read:
>>>>
>>>> journalctl -b0 | grep -E 'sysctl|ens3|eth0'
>>>> 08:56:46 systemd[263]: systemd-sysctl.service: Executing:
>>>> /usr/lib/systemd/systemd-sysctl
>>>> 08:56:46 systemd-sysctl[263]: Couldn't write '2' to
>>>> 'net/ipv6/conf/ens3/addr_gen_mode', ignoring: No such file or directory
>>>> 08:56:46 systemd-sysctl[263]: Couldn't write '2' to
>>>> 'net/ipv6/conf/ens3/use_tempaddr', ignoring: No such file or directory
>>>> 08:56:47 kernel: virtio_net virtio0 ens3: renamed from eth0
>>>> 08:56:47 systemd[1]: sys-subsystem-net-devices-ens3.device: Changed dead
>>>> -> plugged
>>>> 08:56:47 systemd[1]:
>>>> sys-devices-pci0000:00-0000:00:03.0-virtio0-net-ens3.device: Changed
>>>> dead -> plugged
>>>> 08:56:51 systemd-networkd[459]: ens3: Interface name change detected,
>>>> ens3 has been renamed to eth0.
> 
> I wonder where this comes from.
> 
>>>> 08:56:51 systemd-networkd[459]: eth0: Interface name change detected,
>>>> eth0 has been renamed to ens3.
>>>> 08:56:51 systemd-networkd[459]: ens3: IPv6 successfully enabled
>>>> 08:56:51 systemd-networkd[459]: ens3: Link UP
>>>> 08:56:51 systemd-networkd[459]: ens3: Gained carrier
>>>> ...

Same wonders here, but, despite the mysterial double renaming, 
systemd-sysctl is still being called before even the first rename. A 
good second here. So I have no chance to set any values for ens3 at all.


> 
> udev applies sysctl just once - when interface first appears. 

Any deeper insight on this? I was under the impression, sysctl is read 
by systemd-sysctl only and not by udev at all? But admittedly, I am not 
very knowledgable here. So I'd like to improve.


> using final name (that can be different from initial kernel name). So if
> device appeared under eth0 and got name ens3 during event procesing,
> udev applies sysctl to ens3 and never to eth0. If udev applied settings
> to eth0 before rename, these settings would be preserved after rename.

That does at least not correspond to my observation.

It seems, the kernel creates

net/ipv6/conf/eth0/use_tempaddr

upon detection and after renaming,

net/ipv6/conf/ens3/use_tempaddr

does not keep the values.

Is there a way to make systemd-sysctl a bit more verbose, so we can see 
wether the eth0 setting are actually being applied?

So far we only see the error from ens3, but no positives from the eth0 
settings.

Nonetheless, this would still impose a problem. As the eth* names are 
not reliable, faking a setting intended for ens3 to eth0 in sysctl.conf 
is a bit risky and most likely not the idea behind the whole renaming 
thingy.