While investigating a kernel crash that always happens when the weekly backup runs, I noticed that the my home server was missing firmware for the ethernet adapter:
kernel: [43.776781] r8169 0000:02:00.0: firmware: failed to load rtl_nic/rtl8168g-2.fw (-2) kernel: [43.868563] firmware_class: See https://wiki.debian.org/Firmware for information about missing firmware kernel: [43.981142] r8169 0000:02:00.0: Direct firmware load for rtl_nic/rtl8168g-2.fw failed with error -2 kernel: [43.981151] r8169 0000:02:00.0 eth0: unable to load firmware patch rtl_nic/rtl8168g-2.fw (-2)
I made sure that /etc/apt/sources.list contained non-free:
deb https://deb.debian.org/debian buster-backports main contrib non-free
and then simply installed the firmware-realtek package and rebooted. Now the kernel log shows that the proprietary firmware gets loaded:
kernel: [29.010944] r8169 0000:02:00.0: firmware: direct-loading firmware rtl_nic/rtl8168g-2.fw