Only one network interface is up (working) at time Debian

Question

I am using Debian as virtualbox guest OS. I configured and enabled three network interfaces from the network manager in virtualbox settings. But only one interface is up at a time, when i connect (up) a interface, using debian GUI, the other interface automatically becomes "off" (down).

This is my uname -a output 4.19.0-10-amd64 #1 SMP Debian 4.19.132-1 (2020-07-24) x86_64 GNU/Linux

This is my /etc/network/interface file:

# 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

This is ip a output

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 08:00:27:14:e5:d4 brd ff:ff:ff:ff:ff:ff
    inet 192.168.56.108/24 brd 192.168.56.255 scope global dynamic noprefixroute enp0s3
       valid_lft 431sec preferred_lft 431sec
    inet6 fe80::a00:27ff:fe14:e5d4/64 scope link noprefixroute
       valid_lft forever preferred_lft forever
3: enp0s8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 08:00:27:05:b9:31 brd ff:ff:ff:ff:ff:ff
4: enp0s9: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 08:00:27:88:9d:2e brd ff:ff:ff:ff:ff:ff

I also try assigning IP statically, but nothing worked. How can make all interfaces work simultaneously.

Answer 1

I have a comparable situation. What works for me is diabling NetwokManager:

$ sudo systemctl stop NetworkManager.service

For completeness sake, my netwok configuration in /etc/network/interfaces looks like this:

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

allow-hotplug enp0s3
iface enp0s3 inet dhcp

allow-hotplug enp0s8
iface enp0s8 inet dhcp

and /etc/network/interfaces.d/ is an empty directory. I this works, you may have to disable NetwokManager: sudo systemctl disable NetworkManager

which probably means you can't use the GUI to control your network anymore.