[SOLVED] Strange transient connection with NetworkManager

Roken · 2025-09-20 15:24:26

I wasn't sure if this was a PiHole issue, or an Arch issue, but it seems to be Arch. Please let me know if I'm barking up the wrong tree.

Yesterday, I suddenly lost internet connection on my main machine. I also have a second Arch machine connected via ethernet, and a Windows laptop connected wirelessly, and all three are routed through PiHole. /etc/resolv.conf on both Arch machines points to the PiHole.

My second Arch machine has no problems.

When I lost the connection on my main machine, I noted that I no longer have a WAN address (LAN is fine).

Changing to a public DNS server (e.g. OpenDNS, Google, ISP etc.) returned my connection.

More searching today and checking with NetworkManager's applet, at boot I have a connection named eth0 (the same as the interface), and that is the active connection. There is also listed Wired Connection 1, which is not active. If I disconnect the eth0 connection, and connect Wired connection 1, everything works fine again. However, after doing that, the eth0 connection has vanished.

I check my second Arch box and it exhibits no such behaviour.

Any idea what this transient eth0 connection is, and how I get rid of it once and for?

See image for the nm-tray before and after the disconnect and connect cycle.

NB. This happened out of the blue. I had made no configuration updates, and hadn't updated my system.

Thank you.

Last edited by Roken (2025-09-21 16:29:36)

Roken · 2025-09-21 14:23:50

OK, I've done more playing, and I noted that the eth0 connection has a different UUID each time it connects, which leads me to think that it may be connected to a virtual NAT connection (can't be sure) provided by VirtualBox.

Problem is, I have no idea how it's being issued to the host as a viable connectio. nmcli reports that the connection is local only, which explains the lack of WAN.

EDIT: It does appear to be connected to VB. None of my VM's have internet access after changing the host to Wired connection 1, and mu host has no internet connection using the eth0 connection (not interface).

Last edited by Roken (2025-09-21 14:52:13)

-thc · 2025-09-21 15:35:40

VB itself doesn't create a local network interface by default.
AFAIK all network accesses are created via vboxnetflt and vboxnetadp modules and don't require a (host) network adapter.

Maybe a complete log of one boot will shine more light on that.

sudo journalctl -b | curl -F 'file=@-' 0x0.st

Roken · 2025-09-21 15:37:45

And more info. It seems like I may have had two problems simultaneously. I've fixed local connection problem. VB connections failed, but if I manually set the DNS servers in the virtual machines (the guests) to, say, opendns, then they work. This suggests a pi-hole problem, so I'll pop off there, unless someone has a clue.

-thc · 2025-09-21 16:00:52

Be aware that systemd-resolved sets DNSSEC active by default after the last update. If your VMs use resolved that may lead to those issues.

If not, verify inside your VMs the IP address for the DNS server obtained by DHCP (in my case "10.0.2.3") and try to query directly from the VM:

drill archlinux,org @10.0.2.3

If that works the DNS management inside the VM is probably to blame.

Roken · 2025-09-21 16:17:13

OK - I honestly don't know what I've done. I think that PiHole config was a part of it, but not all of it, but it seems that in my playing and fiddling, apart from elevating the priority of Wired Connection 1 (gonna try reversing that, now), I've fixed it. Give me a few minutes to reset the priority and test.

The question still remains, what the hell happened in the first place that resulted in all of this. I hadn't touched any network config, and it just stopped working out of the blue. Back in a mo to mark SOLVED (I hope).

Roken · 2025-09-21 16:29:12

I have no idea, but after 4 - 5 days of this problem, I've fixed it, but I have no idea how. #mod note. Since I can't offer any useful info, if you want to delete the thread, please do.

In the meantime, I'll mark as SOLVED. and report for review and removal.

Arch Linux

#1 2025-09-20 15:24:26

[SOLVED] Strange transient connection with NetworkManager

#2 2025-09-21 14:23:50

Re: [SOLVED] Strange transient connection with NetworkManager

#3 2025-09-21 15:35:40

Re: [SOLVED] Strange transient connection with NetworkManager

#4 2025-09-21 15:37:45

Re: [SOLVED] Strange transient connection with NetworkManager

#5 2025-09-21 16:00:52

Re: [SOLVED] Strange transient connection with NetworkManager

#6 2025-09-21 16:17:13

Re: [SOLVED] Strange transient connection with NetworkManager

#7 2025-09-21 16:29:12

Re: [SOLVED] Strange transient connection with NetworkManager

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