I can confirm the same issue with Ubuntu 18.04 (used the DVD). SSSD cannot resolve the the backend and then says it's offline. Restarting SSSD after a reboot always resolves the issue.
I've found two workarounds in 18.04:
1) Use your own resolv.conf and not rely on systemd-resolved.service.
"unlink /etc/resolv.conf" and then create your own resolv.conf that doesn't rely on systemd-resolved.
2) Make SSSD start after systemd-resolved.service. I prefer this method because setting up the nameservers is still automatic (in case they change).
cp -p /lib/systemd/system/sssd.service /etc/systemd/system/
sed -i 's/Wants.*$/Wants &\nAfter=systemd-resolved.service/g' /etc/systemd/system/sssd.service
I can confirm the same issue with Ubuntu 18.04 (used the DVD). SSSD cannot resolve the the backend and then says it's offline. Restarting SSSD after a reboot always resolves the issue.
I've found two workarounds in 18.04: resolved. service.
1) Use your own resolv.conf and not rely on systemd-
"unlink /etc/resolv.conf" and then create your own resolv.conf that doesn't rely on systemd-resolved.
2) Make SSSD start after systemd- resolved. service. I prefer this method because setting up the nameservers is still automatic (in case they change). system/ sssd.service /etc/systemd/ system/ systemd- resolved. service/ g' /etc/systemd/ system/ sssd.service
cp -p /lib/systemd/
sed -i 's/Wants.*$/Wants &\nAfter=
Hope this helps someone.