Wow, thanks for that! This is some great info!
In my situation, the connection does come back on its own. It’s a very brief blip due to the usb nic dropping:
Mar 14 06:56:55 secret kernel: usb 4-1: USB disconnect, device number 4
Mar 14 06:56:55 secret kernel: xhci_hcd 0000:00:14.0: WARN Set TR Deq Ptr cmd failed due to incorrect slot or ep state.
Mar 14 06:56:55 secret kernel: cdc_ncm 4-1:2.0 enp0s20u1c2: unregister ‘cdc_ncm’ usb-0000:00:14.0-1, CDC NCM (NO ZLP)
Mar 14 06:56:55 secret kernel: xhci_hcd 0000:00:14.0: WARN Set TR Deq Ptr cmd failed due to incorrect slot or ep state.
Mar 14 06:56:55 secret kernel: xhci_hcd 0000:00:14.0: WARN Set TR Deq Ptr cmd failed due to incorrect slot or ep state.
Mar 14 06:56:55 secret dhcpd[8649]: receive_packet failed on enp0s20u1c2: Network is down
Mar 14 06:56:55 secret named[979]: no longer listening on 192.168.1.1#53
Mar 14 06:56:55 secret NetworkManager[893]: [1678795015.1500] device (enp0s20u1c2): state change: activated → unmanaged (reason ‘removed’, sys-iface-state: ‘removed’)
Mar 14 06:56:55 secret NetworkManager[893]: [1678795015.1506] device (enp0s20u1c2): set-link: failure to reset link negotiation
Mar 14 06:56:55 secret systemd[1]: Starting NetworkManager-dispatcher.service - Network Manager Script Dispatcher Service…
Mar 14 06:56:55 secret systemd[1]: Started NetworkManager-dispatcher.service - Network Manager Script Dispatcher Service.
Mar 14 06:56:55 secret systemd[1]: Stopping sm-client.service - Sendmail Mail Transport Client…
Mar 14 06:56:55 secret systemd[1]: sm-client.service: Deactivated successfully.
Mar 14 06:56:55 secret systemd[1]: Stopped sm-client.service - Sendmail Mail Transport Client.
Mar 14 06:56:55 secret systemd[1]: Stopping sendmail.service - Sendmail Mail Transport Agent…
Mar 14 06:56:55 secret systemd[1]: sendmail.service: Deactivated successfully.
Mar 14 06:56:55 secret systemd[1]: Stopped sendmail.service - Sendmail Mail Transport Agent.
Mar 14 06:56:55 secret systemd[1]: sendmail.service: Consumed 1.920s CPU time.
Mar 14 06:56:55 secret systemd[1]: Starting sendmail.service - Sendmail Mail Transport Agent…
Mar 14 06:56:55 secret systemd[1]: sendmail.service: Can’t open PID file /run/sendmail.pid (yet?) after start: Operation not permitted
Mar 14 06:56:55 secret systemd[1]: Started sendmail.service - Sendmail Mail Transport Agent.
Mar 14 06:56:55 secret systemd[1]: Starting sm-client.service - Sendmail Mail Transport Client…
Mar 14 06:56:55 secret systemd[1]: sm-client.service: Failed to parse PID from file /run/sm-client.pid: Invalid argument
Mar 14 06:56:55 secret systemd[1]: Started sm-client.service - Sendmail Mail Transport Client.
Mar 14 06:56:55 secret kernel: usb 4-1: new SuperSpeed USB device number 5 using xhci_hcd
Mar 14 06:56:55 secret kernel: usb 4-1: New USB device found, idVendor=0bda, idProduct=8156, bcdDevice=31.00
Mar 14 06:56:55 secret kernel: usb 4-1: New USB device strings: Mfr=1, Product=2, SerialNumber=6
Mar 14 06:56:55 secret kernel: usb 4-1: Product: USB 10/100/1G/2.5G LAN
Mar 14 06:56:55 secret kernel: usb 4-1: Manufacturer: Realtek
Mar 14 06:56:55 secret kernel: usb 4-1: SerialNumber: 001000001
Mar 14 06:56:55 secret kernel: cdc_ncm 4-1:2.0: MAC-Address:
Mar 14 06:56:55 secret kernel: cdc_ncm 4-1:2.0: setting rx_max = 16384
Mar 14 06:56:55 secret kernel: cdc_ncm 4-1:2.0: setting tx_max = 16384
Mar 14 06:56:55 secret kernel: cdc_ncm 4-1:2.0 eth0: register ‘cdc_ncm’ at usb-0000:00:14.0-1, CDC NCM (NO ZLP), 8c:ae:4c:dd:47:11
Mar 14 06:56:55 secret NetworkManager[893]: [1678795015.4109] manager: (eth0): new Ethernet device (/org/freedesktop/NetworkManager/Devices/8)
Mar 14 06:56:55 secret kernel: cdc_ncm 4-1:2.0 enp0s20u1c2: renamed from eth0
Mar 14 06:56:55 secret NetworkManager[893]: [1678795015.4379] device (eth0): interface index 8 renamed iface from ‘eth0’ to ‘enp0s20u1c2’
Mar 14 06:56:55 secret NetworkManager[893]: [1678795015.4461] device (enp0s20u1c2): state change: unmanaged → unavailable (reason ‘managed’, sys-iface-state: ‘external’)
Mar 14 06:56:57 secret ModemManager[994]: [base-manager] couldn’t check support for device ‘/sys/devices/pci0000:00/0000:00:14.0/usb4/4-1’: not supported by any plugin
Mar 14 06:56:58 secret kernel: IPv6: ADDRCONF(NETDEV_CHANGE): enp0s20u1c2: link becomes ready
Mar 14 06:56:58 secret NetworkManager[893]: [1678795018.9987] device (enp0s20u1c2): carrier: link connected
As you can see, the time from the USB disconnect to reconect is less than a second. The problem is the dhcpd service does not recover and has to be restarted. The script that ChatGPT generated for me actually worked this morning when it blipped again. I also see there is another kernel update so maybe that will fix it.
(Also interesting that named and sendmail both recover)
For now, I’m going to use this:
/bin/bash -c ‘tail -n0 -F /var/log/messages | grep --line-buffered “Network is down” | while read line; do echo “Network is down detected at $(date)” && sleep 5 && systemctl restart dhcpd.service && echo -e “Subject: DHCP service restarted\n\nDHCP service restarted at $(date).\n\nLog output:\n$line\n” | /usr/sbin/sendmail user@example.com; done’
Oh, and I will still look at nmcli monitor 
Thanks again for your time!