This happened some hours after I upgraded to Debian Buster on my Bandwagonhost VPS. The IP timed out all over the world. Then I did a research in the root shell. It turned out to be networking.service failure:
ifup[478]: Failed to bring up eth1.
systemd[1]: networking.service: Failed with result 'exit-code'.
Searched and found a solution1:
# apt install network-manager
# systemctl enable systemd-networkd
# systemctl enable systemd-resolved
# systemctl start systemd-networkd
# systemctl start systemd-resolved
# apt install isc-dhcp-client
# dpkg-reconfigure resolvconf
Then the IP worked again, but networking.service still failed with the same error. I searched again and found it related to /etc/network/interfaces2.
My /etc/network/interfaces looked like this:
source /etc/network/interfaces.d/*
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp
auto eth1
iface eth1 inet dhcp
Meanwhile the output of networkctl looked like this:
IDX LINK TYPE OPERATIONAL SETUP
1 lo loopback carrier unmanaged
2 eth0 ether routable unmanaged
There's nothing to do with so-called eth1, so this is my edited /etc/network/interfaces now:
source /etc/network/interfaces.d/*
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp
#auto eth1
#iface eth1 inet dhcp
The reason why that happened might be due to my selection of using the config file from maintainer's package when I did the upgrade.