Network Backend Detection — Linux (Universal)

If you like this project, consider supporting me on Buy Me a Coffee ☕️

Network Backend Detection — Linux (Universal)

This cheatsheet provides a universal workflow to determine which network management backend (NetworkManager, systemd-networkd, Netplan, or legacy scripts) is controlling your network interfaces. Essential for troubleshooting network configuration conflicts on any Linux distribution (Ubuntu, Debian, RHEL, CentOS, Fedora).

Quick Detection Workflow / Быстрая проверка

Production Runbook: Identify Network Backend / Определение сетевого бэкенда

Run these commands in sequence to identify your active network backend on any Linux system:

# Step 1: Check active services / Проверить активные сервисы
systemctl is-active NetworkManager systemd-networkd networking network

# Step 2: Check active listening processes / Проверить процессы, слушающие сеть
sudo netstat -tulpn | grep -E 'NetworkManager|systemd-networkd' 

# Step 3: Check interface ownership (Universal) / Проверить владение интерфейсами
networkctl list      # systemd-networkd check / Проверка systemd-networkd
nmcli device status  # NetworkManager check / Проверка NetworkManager

# Step 4: Check routing table source / Проверить источник таблицы маршрутизации
ip route show default

Universal Backend Check / Универсальная проверка бэкенда

Service Status Matrix / Матрица статусов сервисов

Check which service is actually running and enabled.

systemctl status NetworkManager systemd-networkd networking network --no-pager

Interpretation:

• NetworkManager: Active on most Desktops (Gnome/KDE) and RHEL/CentOS 7+.
• systemd-networkd: Active on modern Servers (Ubuntu Server 18.04+, Arch, CoreOS).
• networking: Active on legacy Debian/Ubuntu (ifupdown).
• network: Active on legacy RHEL/CentOS (initscripts).

NetworkManager (NM)

Common on: Ubuntu Desktop, Fedora, RHEL 7/8/9, CentOS.

Check Status & Managed Devices / Проверка статуса и устройств

systemctl status NetworkManager  # Check service / Проверить сервис
nmcli general status             # minimal status / Краткий статус
nmcli device status              # List devices / Список устройств

Sample Output:

DEVICE       TYPE      STATE      CONNECTION 
enp3s0       ethernet  connected  Wired connection 1 
wlan0        wifi      connected  MyWiFi 
lo           loopback  unmanaged  --

• connected: Managed by NM.
• unmanaged: Ignored by NM (likely managed by another backend).

Configuration Locations / Расположение конфигурации

• Main Config: /etc/NetworkManager/NetworkManager.conf
• Connections: /etc/NetworkManager/system-connections/ (Keyfiles)
• Legacy Configs: /etc/sysconfig/network-scripts/ifcfg-* (RHEL/CentOS)

systemd-networkd

Common on: Ubuntu Server, Arch Linux, Container OS, Embedded.

Check Status & Managed Devices / Проверка статуса и устройств

systemctl status systemd-networkd  # Check service / Проверить сервис
networkctl list                    # List interfaces / Список интерфейсов
networkctl status <INTERFACE>      # Detail status / Детальный статус

Sample Output:

IDX LINK    TYPE     OPERATIONAL SETUP     
  1 lo      loopback carrier     unmanaged 
  2 enp3s0  ether    routable    configured

• configured: Managed by systemd-networkd.
• unmanaged: Ignored (likely managed by NM or legacy scripts).

Configuration Locations / Расположение конфигурации

• Global: /etc/systemd/networkd.conf
• Profiles: /etc/systemd/network/*.network, /lib/systemd/network/*.network

Distro-Specific Layers / Дистрибутив-специфичные слои

Netplan (Ubuntu/Debian)

Abstract renderer generator. Runs on top of NM or networkd.

Check Renderer / Проверить рендерер

sudo netplan get
# OR look at config files / ИЛИ посмотреть файлы конфигурации
cat /etc/netplan/*.yaml

Config Interpretation:

network:
  version: 2
  renderer: networkd  # OR NetworkManager

• if renderer is missing, default is networkd on Server, NetworkManager on Desktop.

Legacy: Ifupdown (Debian/Old Ubuntu)

Traditional Debian-style networking.

Check Status / Проверка статуса

systemctl status networking
cat /etc/network/interfaces
ls /etc/network/interfaces.d/

Active Check: If /run/network/ifstate exists and interacts with active interfaces.

Legacy: Sysconfig (RHEL/CentOS/Fedora)

Traditional Red Hat-style networking (initscripts).

Check Status / Проверка статуса

systemctl status network
ls /etc/sysconfig/network-scripts/ifcfg-*

[!NOTE] On modern RHEL 8/9, ifcfg files are often read by NetworkManager via the nm-settings-ifcfg-rh plugin, even if the legacy network service is gone.

Interface Ownership / Принадлежность интерфейса

Use this script block to determine the "owner" of a specific interface (replace <INTERFACE>).

INTF="<INTERFACE>" # e.g., eth0

echo "Checking owner for $INTF..."

# 1. Check NetworkManager
if nmcli device status | grep -q "$INTF.*connected"; then
    echo "[X] NetworkManager is managing $INTF"
fi

# 2. Check systemd-networkd
if networkctl status "$INTF" | grep -q "configured"; then
    echo "[X] systemd-networkd is managing $INTF"
fi

# 3. Check legacy ifupdown (Debian)
if grep -q "$INTF" /etc/network/interfaces; then
    echo "[X] defined in /etc/network/interfaces (ifupdown)"
fi

# 4. Check legacy sysconfig (RHEL)
if [ -f "/etc/sysconfig/network-scripts/ifcfg-$INTF" ]; then
    echo "[X] defined in /etc/sysconfig/network-scripts/ifcfg-$INTF"
fi

Comparison Tables / Таблицы сравнения

Network Management Backends / Сетевые бэкенды

Interface States / Состояния интерфейсов

Troubleshooting / Устранение неполадок

Conflict Resolution / Разрешение конфликтов

[!CAUTION] Never run two network managers managing the same interface simultaneously. This causes route flapping and connection drops.

Scenario 1: Switch from NetworkManager to networkd

# 1. Stop NM / Остановить NM
sudo systemctl disable --now NetworkManager

# 2. Enable networkd / Включить networkd
sudo systemctl enable --now systemd-networkd

# 3. (Ubuntu only) Apply Netplan / (Только Ubuntu) Применить Netplan
# Edit /etc/netplan/01-config.yaml -> set renderer: networkd
sudo netplan apply

Scenario 2: Ignored Interface (Unmanaged)

If an interface shows unmanaged in both nmcli and networkctl:

1. Check /etc/network/interfaces: If listed here, it might be locked by legacy ifupdown.
2. Check NetworkManager.conf:

   [keyfile]
   unmanaged-devices=interface-name:<INTERFACE>

Logs & Debugging / Логи и отладка

# NetworkManager logs / Логи NM
journalctl -u NetworkManager -f

# systemd-networkd logs / Логи networkd
journalctl -u systemd-networkd -f

# Kernel network events / События ядра сети
dmesg | grep -i <INTERFACE>