I seem to be stuck in a systemd boot loop. Fedora 27 with most recent updates (today) on ASUS ZenBook UX430UQ. Disabled rhgb quiet in grub2. Getting the following lines on boot:

wlp2s0: associated
IPv6: ADDRCONF(NETDEV_CHANGE): wlp2s0: link becomes ready
IN=wlp2s0 OUT= MAC=ff:ff:ff:ff:ff:ff:xx:xx:xx:xx:xx:08:00 SRC=192.68.0.106 DST192.168.0.255 LEN=184 TOS=0x00 PREC=0x00 TTL=128 ID=5591 PROTO=UDP SPT=17500 DPT=17500 LEN=164

That last line (IN=wlp2s0...) just keeps repeating every 15ish seconds (with minor deviations). I had disabled ipv6 (via ip6tables and /etc/sysctl.conf) but left ipv6 loopback open.

(/etc/sysctl.conf)
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

If I ultimately need to fresh install, so be it. Would appreciate any guidance to get the boot sequence to complete or recover a few files before starting fresh.

That line is from your firewall and indicates that port 17500 is blocked for your IPv4
address. Something is broadcasting on your IPv4 network (192.168.1.255) and
your firewall is blocking it. Check google for port 17500 and see what service uses
it, then open the port on the firewall or suppress the message in (r)syslog.

IPv6 is usually autoconfigured in virgin Network Manager installs and always\
comes up later than the IPv4 address. I don't think it's a problem unless your IPv6 addr
doesn't come up.

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Good info, however, I still can't boot to runlevel 5. Even switching from runlevel 3 to 5 hangs to display dropped connections...

I've also tried using yum to downgrade NetworkManager to an older version with no success. Also tried switching selinux back to permissive mode, relabeling (touch /.autorelabel && restart) the filesystem. No luck there either.

Part of my project is to create a Fedora security config script for fresh installs and I was able to recover it from the problem system. I was hoping to run this to ground so I wouldn't have the same issue later, but I'm going to reinstall, follow the script, and reboot often. Maybe I'll have the same issue, maybe I won't. One change I will make, is NOT disabling IPv6 from NetworkManager config. Instead I'm going to do it at the kernel level and set ip6tables to allow all but log traffic to confirm that IPv6 is disabled. If anyone finds this thread with a similar issue and I forget to reply back, consider that as a possible solution. However, I haven't been able to get those changes to fix the problem once the problem is encountered... which is why I'm blowing it away and starting fresh.

There's a general lack of good info on the interwebs regarding this issue. I'll report back if I'm able to get closer to the root of the problem. Wish me luck.

Narrowed it down. Restarted after each state change.

The issue can be reproduced when the accounts-daemon.service is stopped/disabled/masked. A forum claimed that running this service poses security risks and masking it would mitigate with no system impact.

Don't believe everything you read online. Obviously, I would be more diligent on a prod system. This is a brand new install on my free time. Besides, you're not learning things if you're not breaking things.

source:
"accounts-daemon.service is a potential security risk. It is part of AccountsService, which allows programs to get and manipulate user account information. I can't think of a good reason to allow this kind of behind-my-back operations, so I mask it." -https://www.linux.com/learn/cleaning-your-linux-startup-process

OK, I knew I had seen and used that port 17500 before: it's the dropbox LAN synch service. You can configure dropbox to not do LAN synch, or open the port on your firewall if you would like it done. It (dropbox) does cause a lot of disk activity depending on how many files / directories are being synchronized, and heavy disk activity is often perceived as loop / hang. Try and disable dropbox altogether (~/.config/autostart/dropbox.desktop I believe) using a rescue boot stick or other live boot media, then give the boot another shot.

I just installed Fedora 27 myself and it should work for. Curious about the accounts-daemon service, I have it enabled and it doesn't even listen on any ports, I don't what it does, and it is always silent after the systemd "Starting / Started" messages. I like the line about not learning if not breaking - I've been breaking things for 15 years lol