Problem with acd on upgrade to 1.52

[Beniamino Galvani]

On Fri, Jul 11, 2025 at 10:11:09AM +0200, Christian Schaubschläger wrote:
> Hi,
> 
> thanks for the analysis!
> 
> > First, the creation of the ACD probe fails. This shouldn't
> > happen. Perhaps are you building NM with eBPF support? If so, it's
> > possible that the process doesn't have the necessary capabilities to
> > use the eBPF program. Anyway, the logging needs to be improved to
> > indicate the exact cause for the failure.
> 
> Yes, I did compile it with eBPF support (though not entirely intentional ;-)
> And I just checked my kernel: I do not have CONFIG_BPF_SYSCALL enabled. Maybe that's the problem?
> 
> However, I compiled 1.48 also with eBPF support, and I don't see the problem there...

I don't know, maybe the timeout mechanism changed.

> > Second, there is a mechanism that retries the ACD probe creation and
> > gives up after some seconds. Apparently this is not working and NM
> > never recovers, as the log shows:
> >
> > Jul 10 09:08:23 acd[192.168.0.54, probing]: probing currently  stillnot possible (timeout 200 msec; failure to create nacd instance, retry probing on timeout)
> > Jul 10 09:08:25 acd[192.168.0.54, probing]: probing currently  stillnot possible (timeout 200 msec; failure to create nacd instance, retry probing on timeout)
> > Jul 10 09:08:29 acd[192.168.0.54, probing]: probing currently  stillnot possible (timeout 200 msec; failure to create nacd instance, retry probing on timeout)
> > ...
> > Jul 10 09:08:34 acd[192.168.0.54, probing]: probing currently  stillnot possible (timeout 200 msec; failure to create nacd instance, retry probing on timeout)
> 
> When NM is in the above state and I do
> 
> ip link set dev eth0 arp off
> 
> it immediately resumes and the client gets an ip address:
> 
> http://datenaustausch.s3.eu-west-1.amazonaws.com/Logs/daemon-noarp-resume.txt
> 
> (logs are from today from a different subnet).

Yes, if you disable ARP, the probe creation gets skipped. But ARP is necessary to
contact other hosts, I guess ;)

> > I will investigate this and prepare a patch. In the meantime, you can
> > try disabling ACD for the connection:
> >
> >  nmcli connection modify "Wired connection 1" ipv4.dad-timeout 0
> Can I disable ACD in general via configuration?

cat << EOF > /etc/NetworkManager/conf.d/20-ipv4-dad.conf
[connection-dad-default]
ipv4.dad-timeout=0
EOF

Beniamino
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