I hit this bug too with Ubuntu 18.04. On boot sssd fails to find AD servers from DNS and then continues to query 127.0.0.1 port 53, which will never answer. The log looks like this:
(Wed Oct 10 13:04:13 2018) [sssd[be[ad.helsinki.fi]]] [resolv_getsrv_send] (0x0100): Trying to resolve SRV record of '_ldap._tcp.helsinki._sites.ad.helsinki.fi'
(Wed Oct 10 13:04:13 2018) [sssd[be[ad.helsinki.fi]]] [resolv_discover_srv_done] (0x0040): SRV query failed [11]: Could not contact DNS servers
which does not help much, since it doesn't mention the non existing DNS server (127.0.0.1) in the logs (you need to run strace to see it).
I hit this bug too with Ubuntu 18.04. On boot sssd fails to find AD servers from DNS and then continues to query 127.0.0.1 port 53, which will never answer. The log looks like this:
(Wed Oct 10 13:04:13 2018) [sssd[be[ ad.helsinki. fi]]] [resolv_ getsrv_ send] (0x0100): Trying to resolve SRV record of '_ldap. _tcp.helsinki. _sites. ad.helsinki. fi' ad.helsinki. fi]]] [resolv_ discover_ srv_done] (0x0040): SRV query failed [11]: Could not contact DNS servers
(Wed Oct 10 13:04:13 2018) [sssd[be[
which does not help much, since it doesn't mention the non existing DNS server (127.0.0.1) in the logs (you need to run strace to see it).
Restarting sssd makes it go back online.