Archlinux says, try dhcpcd in case your installer can't connect automatically. This worked on the spot for me AFTER installation. Just type "dhcpcd" as a command, wait a few seconds, and then "ip route" or "ping 8.8.8.8" to check if it works. I even used "watch ip address" to see how fast these inet and inet6 addresses pop up.

Instead of using dhcpcd, I can also connect from zero with:

e1000e is a "Network Driver", says modinfo. By loading this module I get a eth0 link (no, don't look for it in /dev). The "ip" commands activate the link, choose an address and define a local (subnet) and then a default gateway. It took me quite some time to test this out (ip link help, ip address help, ip route help...).

So this is instructive, but "dhcpcd" is faster to type, and probably more robust. And you can stop it cleanly with "dhcpcd -x".

With a working ping and the mirrorlist in /etc/pacman.d/ you have all you need for pacman or pacstrap.

And yes, I saw a kernel message renaming eth0 to enoxxx, so you have to check first with "ip link" what interfaces are already set up by systemd. It happens here:

The .16 in my address is just any (free) number, and 192.168.0.1 should always be the gateway. Maybe I was lucky, but it is good news that you can hack around under systemd and inet6. I will try netctl, though.

And right now I have booted with systemd and have dhcpcd started. I got address .15 on "eno1" and three inet6 addresses, but apart from that the result is quite the same as with above "ip"-hack.