Multiple Network connections (eg fo Internet and Mesa/Ethercat)

I'm  a bit stuck here for some reason. I put a new PC on my desk for work running Debian 12 and added a second Network card so I can test ethercat and mesa devices. But for some reason as soon as I plug in an ethercat device to the second NIC, I loose my internet.
I seem to be able to ping other PC's so it seems its a DNS issue.

This is what I ended up in my /etc/network/interfaces file
enp2s0 = network and internet
eno1 = for mesa or ethercat devices

So I just need to get the routing set up so that all traffic goes through enp2s0
Any ideas on how to fix this?
Quite annoying. I've never had these issues before but usually the internet is via wifi. This time iboth NIC's are wired.
 

You weren't using a a kernel later than Bookworms's most current RT kernel and the realtek dkms package were you ? I had all sorts of issues the other day and went back to linux-image-rt-amd64 (6.1.38-4)

I'm pretty sure that can't be related to kernel version.

Try looking at output of "ip r l" (shows default route) and "resolvectl" before / after plugging in ethercat device.

I know for sure mine was Kernel related, went away when I went to an earlier kernel.

Cornholie: Maybe you have had driver issues, but that seems not to be the case here because both network interfaces seem to work.

I observed exactly the same behavior on my machine, hence my response to Rod's issue. It wasn't an issue I thought was worth pursuing at the time as I was just testing at the time.
Rebooting with a previous kernel got things working again.
Whilst just plugged into the home network I could get configure the LAN NIC via dhcp. If I plugged in the 2nd cable got no response from both NICs.

cornholio wrote:

You weren't using a a kernel later than Bookworms's most current RT kernel and the realtek dkms package were you ? I had all sorts of issues the other day and went back to linux-image-rt-amd64 (6.1.38-4)

Nah, this is my (new) daily driver which is bog standard Bookworm 12.1 with the latest (2-3 day old) security updates applied.
I think its a routing issue. I just don't know how to fix it. I had this years ago on Mint 17.3.

The good thing is the SSD with Windows 10 installed was not upgraded to my free Windows 11 and has never been used and relegated to an emergency dual boot SSD with Debian on a second SSD. So far so good....

tommylight wrote:

Use Network Manager, it has settings for routing stuff only on local network, required for Mesa, and everything else goes through the other NIC.
I never ever have any issues, i can plug and unplug at will, change WiFi networks etc, while the machine is working.
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If you insist on using /etc/network/... set only the Mesa side NIC, leave the other NIC to network manager, that should work, although i never tried it.

Tommy, thanks. I'll try tomorrow.

PS: This PC has 20 cores... crazy.... I have not looked  but I assume thats 20 threads over 10 cores.....

 

PS: This PC has 20 cores... crazy.... I have not looked  but I assume thats 20 threads over 10 cores.....

Nah... THIS is crazy