I recently switched my Fedora 21 office workstation from Fedora's old /etc/init.d/network init script based method of network setup to using the (relatively new) systemd network setup functionality, for reasons that I covered yesterday. The systemd documentation is a little bit scant and not complete, so in the process I accumulated some notes that I'm going to write down.

First, I'm going to assume that you're having networkd take over everything from the ground up, possibly including giving your physical network devices stable names. If you were previously doing this through udev, you'll need to comment out bits of /etc/udev/rules.d/70-persistent-net.rules (or wherever your system put it).

To configure your networking you need to set up two files for each network connection. The first file will describe the underlying device, using .link files for physical devices and .netdev files for VLANs, bridges, and so on. For physical links, you can use various things to identify the device (I use just the MAC address, which matches what I doing in udev) and then set its name with 'Name=' in the '[Link]' section. Just to make you a bit confused, the VLANs set up on a physical device are not configured in its .link file.

The second file describes the actual networking on the device (physical or virtual), including virtual devices associated with it; this is done with .network files. Again you can use various things to identify which device you want to operate on; I used the name of the device (a [Match] section with Name=<whatever>). Most of the setup will be done in the [Network] section, including telling networkd what VLANs to create. If you want IP aliases on a give interface, specify multiple addresses. Although it's not documented, experimentally the last address specified becomes the primary (default) address of the interface, ie the default source address for traffic going out that interface.

(This is unfortunately reversed from what I expected, which was that the first address specified would be the primary. Hopefully the systemd people will not change this behavior but document it, and then provide a way of specifying primary versus secondary addresses.)

If you're setting up IP aliases for an interface, it's important to know that ifconfig will now be misleading. In the old approach, alias interfaces got created (eg 'em0:0') and showed the alias IP. In the networkd world those interfaces are not created and you need to turn to 'ip addr list' in order to see your IP aliases. Not knowing this can be very alarming, since in ifconfig it looks like your aliases disappeared. In general you can expect networkd to give you somewhat different ifconfig and ip output because it does stuff somewhat differently.

For setting up VLANs, the VLAN= name in your physical device's .network file is paired up with the [NetDev] Name= setting in your VLAN's .netdev file. You then create another .network file with a [Match] Name= setting of your VLAN's name to configure the VLAN interface's IP address and so on. Unfortunately this is a bit tedious, since your .netdev VLAN file basically exists to set a single value (the [VLAN] Id= setting); it would be more convenient (although less pure) if you could just put that information into a new [VLAN] section in the .network file that specified Name and Id together.

If you're uniquely specifying physical devices in .link files (eg with a MAC address for all of them, with no wildcards) and devices in .network files, I believe that the filenames of all of these files are arbitrary. I chose to give my VLANs filenames of eg 'em0.151.netdev' (where em0.151 is the interface name) just in case. As you can see, there seems to be relatively little constraint on the interface names and I was able to match the names required by my old Fedora ifcfg-* setup so that I didn't have to change any of my scripts et al.

You don't need to define a lo interface; networkd will set one up automatically and do the right thing.

Once you have everything set up in /etc/systemd/network, you need to enable this by (in my case) 'chkconfig --del network; systemctl enable systemd-networkd' and then rebooting. If you have systemd .service units that want to wait for networking to be up, you also want to enable the systemd-networkd-wait-online.service unit, which does what it says in its manpage, and then make your units depend on it in the usual way. Note that this is not quite the same as setting your SysV init script ordering so that your init scripts came after network, since this service waits for at least one interface to be plugged in to something (unfortunately there's no option to override this). While systemd still creates the 'sys-subsystem-net-devices-<name>.device' pseudo-devices, they will now appear faster and with less configured than they did with the old init scripts.

(I used to wait for the appearance of the em0.151 device as a sign that the underlying em0 device had been fully configured with IP addresses attached and so on. This is no longer the case in the networkd world, so this hack broke on me.)

In another unfortunate thing, there's no syntax checker for networkd files and it is somewhat hard to get warning messages. networkd will log complaints to the systemd journal, but it won't print them out on the console during boot or anything (at least not that I saw). However I believe that you can start or restart it while the system is live and then see if things complain.

(Why yes I did make a mistake the first time around. It turns out that the Label= setting in the [Address] section of .network files is not for a description of what the address is and does not like 'labels' that have spaces or other funny games in them.)

On the whole, systemd-networkd doesn't cover all of the cases but then neither did Fedora ifcfg-* files. I was able to transform all of my rather complex ifcfg-* setup into networkd control files with relatively little effort and hassle and the result came very close to working the first time. My networkd config files have a few more lines than my ifcfg-* files, but on the other hand I feel that I fully understand my networkd files and will in the future even after my current exposure to them fades.

(My ifcfg-* files also contain a certain amount of black magic and superstition, which I'm happy to not be carrying forward, and at least some settings that turn out to be mistakes now that I've actually looked them up.)