# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp

This is just bare debian no GUI install, this is a NAS device so no desktop. I haven't done anything weird, however I migrated from debian to devuan lately. This happened after I did it.
About DHCP, in router settings I do set static lease for this NAS. But since it changes mac every time, it won't work.

user@devuan-seagate-personal-cloud:~$ cat /usr/lib/NetworkManager/conf.d/no-mac-addr-change.conf 
cat: /usr/lib/NetworkManager/conf.d/no-mac-addr-change.conf: No such file or folder

At least provide context. Which internet connection is this? Which interface? What do you mean by DHCP (isc-dhcp-client? dhcpcd5? udhcpc? something else?)? Whatever "DHCP" you use, what is the configuration? What is in /etc/network/interfaces? Maybe there are some udev rules? Maybe there is indeed something like macchanger? Maybe you use a network manager and configured something there?

~$ cat /usr/lib/NetworkManager/conf.d/no-mac-addr-change.conf 
# Certain drivers are known not to support changing the MAC address.
# Disable touching the MAC address on such devices.
#
# See man NetworkManager.conf
#
# https://bugzilla.gnome.org/show_bug.cgi?id=777523
[device-31-mac-addr-change]
match-device=driver:eagle_sdio,driver:wl
wifi.scan-rand-mac-address=no

which results in part of NetworkManager configuration

~$ sudo NetworkManager --print-config

some of the different places NetworkManager components can be found/hidden(depending on your point-of-view)
/etc/NetworkManager/
/usr/lib/NetworkManager/
/var/lib/NetworkManager/

if you have time this bug report may also provide some context and/or insight:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=879484

keep us posted on your progress!

Create the a new udev rule:

-- Add a new file /etc/udev/rules.d/75-static-mac

-- write
ACTION=="add", SUBSYSTEM=="net", ATTR{dev_id}=="0x0", RUN+="/usr/bin/iplink set dev %k address XX:XX:XX:XX:XX:XX"
into the new file.

-- Replace XX:XX:XX:XX:XX:XX with your current mac address

Regards
Berni

Seagate Personal Cloud, armhf NAS device

Maybe this device has some internal setting that keeps it from changing the MAC.

DHCP vs Static IP: Which One Is Better?
https://community.fs.com/article/dhcp-v … ences.html

Linux Static IP Address Configuration
https://www.cyberciti.biz/faq/linux-con … -tutorial/

Network Configuration - Manually
https://wiki.debian.org/NetworkConfigur … e_manually