Arch Linux

I've reported this upstream. It was suggested to me to also create an issue here in order to hold the package in [testing].
Original forum post: https://bbs.archlinux.org/viewtopic.php?pid=1841539
Reported upstream: https://github.com/systemd/systemd/issues/12315

After booting, I have an IP and internet access (through a home router), but no DNS resolving. I use Arch Linux systemd-networkd from the testing repo (version 242), and I have systemd-resolved set up in the stub mode. The network conf is set to use DHCP. I have an ethernet device (enp5s0) and a bridge (br0) set up in a simple bridge configuration through systemd. Restarting systemd-networkd makes everything work fine. This seemingly started happening with 242 with no system configuration changes.

The previous version I was using is 241.93. Downgrading to that version seems to fix this issue, with some caveats: if I log into a non-graphical console after boot, and ping google right away, it fails to resolve. But if I ping again right after, it works. If I just log in to a DE, no apparent issues are visible for a while. However, at some point a DNS lookup will immediately fail (e.g., clicking a link while web browsing), but works upon trying again. I've been noticing this for a while, which is potentially the period of time I was using 241.7 (updated to on Feb 20).

When broken, output of resolvectl shows "Current Scopes: none" for the bridge, and no DNS servers. After networkd restart, scopes are "DNS LLMNR/IPv4 LLMNR/IPv6".

Setting the networkd unit to use debug mode shows some seemingly relevant info and differences. This is the journalctl output during the initial boot: [systemd-networkd-broken.log](https://github.com/systemd/systemd/files/3077616/systemd-networkd-broken.log)

Note that enp5s0 gets renamed to eth0, then back to enp5s0 for some reason (not sure if this is normal). Afterward, there's a
`br0: Could not bring up interface: Invalid argument`
and
`enp5s0: Failed to add to bridge master's slave list: File exists`

And this is it when restarting it, which shows no such issues:
[systemd-networkd-good.log](https://github.com/systemd/systemd/files/3077617/systemd-networkd-good.log)

Here is resolved on boot (not working:
[resolved-bad.log](https://github.com/systemd/systemd/files/3077619/resolved-bad.log)

And this is after restarting networkd:
[resolved-good.log](https://github.com/systemd/systemd/files/3077620/resolved-good.log)

I'm using linux 5.0.7. Both r8169 and r8168 ethernet drivers give the same behavior. This did not start happening along with a kernel update.