Hi,

Each time the system starts, the card's MAC address changes which I
never looked into seriously. I have been using eth1 as default network
interface for three years just to use this onboard ethernet card.

Yesterday out of nowhere I thought I should use eth0 as default network
interface and changed eth1 to eth0 in /etc/network/interfaces file. And
then I lost network connection, had spent hours to troubleshoot.

When the system boots, udev assigns eth0 to Realtek ethernet card whose
MAC address matches the value in the file.

$ more 70-persistent-net.rules
# This file was automatically generated by the /lib/udev/write_net_rules
# program, run by the persistent-net-generator.rules rules file.
#
# You can modify it, as long as you keep each rule on a single
# line, and change only the value of the NAME= key.

# lspci -nn | grep Eth
# Retrieve MAC address of each adapter
# udevadm info -a -p /sys/class/net/eth0 [eth1]
# ifconfig -a
# or
# ifconfig eth0 [eth1]

# Qualcomm Atheros AR8161 Gigabit Ethernet [1969:1091]
# Kernel module: alx
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*",
ATTR{address}=="a2:71:b7:12:2d:c
0", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"

# Realtek Semiconductor Co., Ltd. RTL-8100/8101L/8139 PCI Fast Ethernet
Adapter

# Kernel module: 8139too
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*",
ATTR{address}=="00:0e:e8:de:d2:e
9", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth1"

I installed liquorix 4.7-2 kernel a week ago, have been using it since.
This kernel contains systemd-udevd module which I wasn't aware. I'm
going back to Devuan kernel.

$ dmesg | grep eth
[    1.505817] 8139too 0000:03:05.0 eth0: RealTek RTL8139 at 
0xffffc90000c6ac00, 00:0e:e8:de:d2:e9, IRQ 20
[    1.513275] alx 0000:02:00.0 eth1: Qualcomm Atheros AR816x/AR817x 
Ethernet [5e:05:c2:ee:4f:d5]
[    4.340798] 8139too 0000:03:05.0 rename2: renamed from eth0
[    4.349560] systemd-udevd[425]: renamed network interface eth0 to 
rename2
[   97.238629] alx 0000:02:00.0 eth1: NIC Up: 100 Mbps Full