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I am learning about Linux networking on my Kubuntu 18.04 workstation, and I see there that both NetworkManager and networkd-dispatcher are running:

oleg@eclectic:~$ sudo ps -ef | grep -i net
root        56     2  0 Oct11 ?        00:00:00 [netns]
root      1097     1  0 Oct11 ?        00:00:02 /usr/sbin/NetworkManager --no-daemon
root      1098     1  0 Oct11 ?        00:00:00 /usr/bin/python3 /usr/bin/networkd-dispatcher --run-startup-triggers
root      1250     1  0 Oct11 ?        00:00:02 /usr/sbin/inetd
root      1593  1097  0 Oct11 ?        00:00:00 /sbin/dhclient -d -q -sf /usr/lib/NetworkManager/nm-dhcp-helper...

NetworkManager seems to be configured to handle "everything" by netplan:

oleg@eclectic:~$ cat /etc/netplan/01-network-manager-all.yaml 
# Let NetworkManager manage all devices on this system
network:
  version: 2
  renderer: NetworkManager

I believe that netplan would hand everything off to networkd if NetworkManager was not installed.

There are also a bunch of files in /etc/network/, and I am not sure what is handling those. A lot of packages seem to use this directory:

oleg@eclectic:~$ dpkg -S /etc/network
avahi-daemon, ifupdown2, wpasupplicant, openvpn, postfix, netbase, avahi-autoipd, wireless-tools, clamav-freshclam: /etc/network

My initial thoughts after some Googling are that NetworkManager and networkd are both running, but netplan generates configuration such that only one is actually doing something. But I do not know how I would verify this, figure out the chain of operations, or configure it to suit my nefarious plans for dummy0.

Question: How exactly are NetworkManager, networkd, and netplan working together? In other words, as the computer boots up, which processes hand what of to what other processes? How do tools from packages like net-tools, ifupdown2, and iproute2 and directories like /etc/network/ fit into all this? And finally, how can I figure this out myself and learn the details using the command line?

Update: I am not looking for a high-level overview. Rather, I want to know how these components are interacting, conflicting, or avoiding conflict on the kernel level or similar.

13

systemd-networkd when its enabled honors configurations in /etc/systemd/network

Whereas NetworkManager will follow some automatic rules like it will try to connect to a wired connection if it finds one.

Both are enabled and disabled as systemd services and systemd using udev creates the device files for the network adapters. Generally you probably shouldn't have both enabled at the same time but if you do you'd need to be careful that their configurations don't conflict.

netplan before either network manager starts creates configurations for either based on its own configuration. In this way netplan is really a configuration abstraction and wheather it uses systemd-networkd or NetworkManager is part of the configuration.

iproute2 is a package of tools for configuring network interfaces at the command line. It includes link configuration similar to network managers and the old ifconfig. While net-tools, ipupdown and ifupdown2 use network configurations in /etc/network/interfaces to configure and deconfigure interfaces.

ifupdown or its other versions can be used to manage networks but its not automatic but a systemd service can be used to start it at boot.

ifupdown2 uses dependency graphs similar to what systemd does but with network interfaces. Further it acts as an abstraction to other tools like iproute2

  • 1
    Thank you for the compare/ contrast, but I was looking for more about how these components interact with each other, on a much deeper level. For example, what exactly happens in the kernel when both systemd and NetworkManager try to control the same interface at the same time? What do the iproute2/ ifupdown tools do in the kernel that conflicts (or avoids conflict) with networkd/ NetworkManager? What commands can I use to see which tools did what to which interfaces (on the udev/ kernel level)? – Oleg Oct 19 '18 at 17:11
  • Best explanation I've seen so far as there is general confusion with many explanations claiming that netplan is either NetworkManager or systemd-networkd, but not both. Some links would have been helpful. E.g. manpages.ubuntu.com/manpages/cosmic/man5/netplan.5.html – Craig Hicks Apr 1 at 7:27

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