So I had a great idea. Buy two 2.5gbe ethernet adapters (like these) and connect them to get faster link to my NAS.
Imagine my surprise when my Debian 10 Buster did not detect the network card (it works fine in Windows).
04:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. Device 8125
Frustrated, I bought a 10gbe adapter (Intel X540-T1). Another suprise. It only speaks 100/1000/10000, and does NOT support 2.5gbe.
So now I have 2 network cards, neither of which allows 2.5gbe on my debian.
Almost giving up, I noticed that 5.4 kernel upstreamed the rtl8125 support…. So maybe I can use backports? [courtesy of oitbs]
echo "deb http://ftp.us.debian.org/debian/ buster-backports main" >> /etc/apt/sources.list
echo "deb-src http://ftp.us.debian.org/debian/ buster-backports main" >> /etc/apt/sources.list
apt update ; apt install -t buster-backports linux-image-amd64
It installed kernel: 5.6.14-2~bpo10+1
Reboot, and soon after the RTL8125 is working now!