How to know if the network is configured through /etc/network/interfaces, NetworkManager, Systemd or Netplan?

Question

I recently installed Debian 12.7 on a fresh storage, this is a headless server with no desktop environment.

I checked the NetworkConfiguration documentation of Debian and I saw this:

4 ways to configure the network

• The interfaces configuration file at /etc/network/interfaces (this page): for basic or simple configurations (e.g. workstation)
• NetworkManager: This is the default for Laptop configuration
• Systemd: Debian reference Doc Chapter 5
• Netplan: Debian reference Doc Chapter 5

And now I'm confused because I don't understand what part apply to my installation.

The system uses the default configuration so the answer may be straightforward, but let's assume that I connect to a machine that I don't know and I want to identify the network configuration.

How to identify the way that a Debian (or another Linux distro) system use to configure its network stack?

My attempts

/etc/network/interfaces

The file exists:

root@serveur:~# cat /etc/network/interfaces
# 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 enp2s0
iface enp2s0 inet dhcp
# This is an autoconfigured IPv6 interface
iface enp2s0 inet6 auto

NetworkManager

The service doesn't exist:

root@serveur:~# service network-manager status
Unit network-manager.service could not be found.

Update: as shown in this answer from Z0OM, it should be service NetworkManager status instead, but the result is the same.

Systemd

One file exists but its content is commented:

root@serveur:~# ls -lrth /etc/systemd/network/
total 0

root@serveur:~# cat /etc/systemd/networkd.conf 
#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it under the
#  terms of the GNU Lesser General Public License as published by the Free
#  Software Foundation; either version 2.1 of the License, or (at your option)
#  any later version.
#
# Entries in this file show the compile time defaults. Local configuration
# should be created by either modifying this file, or by creating "drop-ins" in
# the networkd.conf.d/ subdirectory. The latter is generally recommended.
# Defaults can be restored by simply deleting this file and all drop-ins.
#
# See networkd.conf(5) for details.

[Network]
#SpeedMeter=no
#SpeedMeterIntervalSec=10sec
#ManageForeignRoutingPolicyRules=yes
#ManageForeignRoutes=yes
#RouteTable=

[DHCPv4]
#DUIDType=vendor
#DUIDRawData=

[DHCPv6]
#DUIDType=vendor
#DUIDRawData=

Netplan

There is no conf:

root@serveur:~# ls -lrth /etc/netplan/
ls: cannot access '/etc/netplan/': No such file or directory

Observations

This system uses /etc/network/interfaces and Systemd to configure the networking?

Answer 1

It depends on what you choose during the installation / packages / environment / standard.

Personally, I always leave the network unconfigured during installation, whether it's a client, server, desktop/tower, or laptop, and I configure everything in /etc/network/interfaces.

/etc/network/interfaces and networking.service

This file is used for configuring network interfaces in Debian and other systems

Check /etc/network/interfaces for basic configurations.

For @Bob comment and Answer:

/etc/network/interfaces is actually managed/read by ifupdown which is invoked by networking.service in a modern install. So that's the better way to check if it's enabled/in-use, as it's possible to have stray /etc/network/interfaces files left over even if the system has been configured to use a different method.

Check the status of networking.service with:

systemctl status networking.service

Check if is enable:

systemctl is-enabled networking.service

NetworkManager

If your system uses NetworkManager, it manages the connections over its own tools.

Check if NetworkManager is running systemctl status NetworkManager

systemd-networkd

If the system is using systemd-networkd, configuration will be located in /etc/systemd/network/ check for the status

Check for systemd-networkd configuration

Current state of the network interfaces managed by systemd-networkd:

networkctl
networkctl status
networkctl status --all

Status of the systemd-networkd service with:

• Active or inactive
• Logs and error messages
• Additional status information

systemctl status systemd-networkd

Netplan

Netplan is often used on systems that are configured for cloud environments

From @Jörg W Mittag comment:

Also note that Netplan is a frontend which allows you to describe the desired network configuration in an abstract manner. It does not configure network devices itself. It renders a concrete configuration for either or both of NetworkManager and systemd-networkd

Check for Netplan configurations in /etc/netplan/

About Netplan

Network configuration on Ubuntu is handled through Netplan, which provides a high-level, distribution-agnostic way to define how the network on your system should be set up via a YAML configuration file.

It does not directly configure network devices, it generates specific configurations for backends like NetworkManager and systemd-networkd based on the defined settings in YAML files. This allows users to define their desired network configuration in a more user-friendly manner, which is then translated into the appropriate configuration for the underlying networking system

Your system uses /etc/network/interfaces and networking.service for network, and systemd doesn’t seem to be managing the network.

NetworkManager and Netplan are not in use.

Sea also

• Old post with some useful information regarding interfaces/devices as well: What are these unexpected network devices which appear in kernel log?

Answer 2

If the command line tools were installed you could try the control app for each of them.

For Network Manager it would be:

nmcli general status

For systemd-networkd it would be:

networkctl status

For Netplan it would be:

netplan status

Answer 3

As an addendum to Z0OM's answer, note that the simple existence of /etc/network/interfaces does not necessarily mean anything. Rather, you want to check for the existence of the ifupdown package, or the networking.service systemd service that invokes ifup in a standard install.

Otherwise, it's possible that ifupdown has been removed or disabled but the config files still exist.

Answer 4

I discovered this question while searching for something else, but had just developed a solution for my own use in Debian. I think it answers the OP's question:

How to identify the way that a Debian (or another Linux distro) system use to configure its network stack?

This is a rather simple-minded answer I think. It assumes that the software being used to manage the network runs as a service under systemd, and so amounts to listing all systemd units, and applying grep filters:

$ string1=$(systemctl list-units --all | grep -Ei 'networking\.|NetworkManager\.|networkd\.' | grep 'service' | grep -E 'running')  
$ string2=$(systemctl list-units --all | grep -Ei 'networking\.|NetworkManager\.|networkd\.' | grep 'service' | grep -Ev 'dead')  
$ if [ -n "$string1" ]; then echo ${string1::24}; else echo ${string2::20}; fi 

As it turns out - for the versions of Debian 'bookworm' that I use at least, there are only 3 network services to choose from :

^{Note that Netplan isn't installed in my default installation, but could probably be accommodated under this scheme.}

• NetworkManager.service
• systemd-networkd.service
• networking.service

If you list those services, you see the following as the default setup (using NetworkManager) :

$ systemctl list-units --all | grep -Ei 'networking\.|NetworkManager\.|networkd\.' | grep 'service'  

  networking.service                     loaded    active   exited    Raise network interfaces
  NetworkManager.service                 loaded    active   running   Network Manager
  systemd-networkd.service               loaded    inactive dead      Network Configuration

However, if you disable & mask NetworkManager in favor of ifupdown/networking.service, you will see the following. Note that the networking.service status does not change. :

$ systemctl list-units --all | grep -Ei 'networking\.|NetworkManager\.|networkd\.' | grep 'service' 

  networking.service                     loaded    active   exited    Raise network interfaces
  NetworkManager.service                 masked    inactive dead      NetworkManager.service
  systemd-networkd.service               loaded    inactive dead      Network Configuration