Ethernet fails to connect: "No IP config"

Question

After a few hours of troubleshooting I was finally able to dual boot the latest version of Kubuntu (21.04) alongside Windows. Completely fresh install, nothing changed, tampered with, etc. However, there's no internet. It tries connecting for a while, but then fails with the error "No IP configuration available". After following tutorials and really not understanding what was going on I found out that ifap wasn't even installed. My best guess is that some linux kernel headers are missing as one guy said who had the exact same issue. I don't know which headers to install, or even how to at this point. I checked and I still have the network folder with some files in it, but the interfaces file itself was missing. I also have netplan since 18.04 and up use that as far as I know. Either way, I'm missing some interfaces and I have no idea what to do.

sudo lshw -C network outputs:

*-network                 
       description: Ethernet interface
       product: RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller
       vendor: Realtek Semiconductor Co., Ltd.
       physical id: 0
       bus info: pci@0000:07:00.0
       logical name: enp7s0
       version: 0c
       serial: e0:d5:5e:6a:e9:11
       size: 100Mbit/s
       capacity: 1Gbit/s
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress msix vpd bus_master cap_list ethernet physical tp mii 10bt 10bt-fd 100bt 100bt-fd 1000bt-fd autonegotiation
       configuration: autonegotiation=on broadcast=yes driver=r8169 driverversion=5.11.0-25-generic duplex=full firmware=rtl8168g-2_0.0.1 02/06/13 ip=192.168.1.194 latency=0 link=yes multicast=yes port=twisted pair speed=100Mbit/s
       resources: irq:18 ioport:d000(size=256) memory:f7100000-f7100fff memory:f2200000-f2203fff

cat /etc/netplan/*.yaml outputs:

# Let NetworkManager manage all devices on this system
network:
  version: 2
  renderer: NetworkManager

dkms status shows nothing.

Answer 1

MSI/MSIX interrupts were enabled for certain ethernet cards in Ubuntu 20.xx. This can cause intermittent ethernet operation. Here's a patch to fix it. Follow the embedded instructions to install.

#!/bin/sh

# https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1779817
#
# Attached is a work-around for the in-kernel driver that is as unhacky as I can make it.

# filename: r8169_disable_msi

# Drop it in /etc/initramfs-tools/scripts/init-top and chmod a+x it. Add 'r8169_disable_msi'
# to your kernel command line (/etc/default/grub, GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
# usually.) 

# Remember to update-initramfs and update-grub as necessary.

# sudo update-initramfs -c -k $(uname -r)
# sudo update-grub
# reboot

# For the moment it disables MSI on everything with the ID 0x10ec:0x8168, as there seems to
# be no way to get the MAC version from userspace - and certainly not before the driver is
# loaded. Other PCI IDs may need adding..

# Still hoping we can cherry pick the in-driver workaround for bionic...?

PREREQ=""
prereqs()
{
    echo "$PREREQ"
}
case $1 in
# get pre-requisites
prereqs)
    prereqs
    exit 0
    ;;
esac

disable_msi () {
    for i in /sys/bus/pci/devices/*; do 
        if [ $(cat $i/vendor) = "0x10ec" -a $(cat $i/device) = "0x8168" ]; then
            echo 0 >$i/msi_bus
        fi
    done
}

for x in $(cat /proc/cmdline); do
        case ${x} in
        r8169_disable_msi)
        disable_msi
        break
                ;;
        esac
done