Mac Mini running El Capitan Server unresponsive
I managed to get into the system while it was manifesting the above symptoms. configd itself was consuming around 140% CPU; and there were 10 (!) instances of smtpd, each consuming around 40% CPU. Killing configd restored sanity (including removal of all the smtpd processes).

To find out what configd was up to I executed sudo configd -v and the system seized up and had to be rebooted. So it seems to be a configd problem.

The call of configd -vdeposited the following in system.log:

Aug 22 09:29:53 s-mx-01 sudo[26480]: macowner : TTY=ttys000 ; PWD=/Users/macowner ; USER=root ; COMMAND=/usr/libexec/configd -v
Aug 22 09:29:53 s-mx-01 configd[26484]: pthread_set_qos_class_self_np() failed: Operation not permitted
Aug 22 09:29:54 s-mx-01 configd[26484]: SCDynamicStoreCreate() failed: Permission denied
Aug 22 09:29:54 s-mx-01 configd[26484]: kernel event monitor disabled
Aug 22 09:29:54 s-mx-01 configd[26484]: SCPreferencesScheduleWithRunLoop() failed: Permission denied
Aug 22 09:29:54 s-mx-01 configd[26484]: SCDynamicStoreCreate() failed: Permission denied
Aug 22 09:29:54 s-mx-01 configd[26484]: Network information server: Connection invalid
Aug 22 09:29:54 s-mx-01 configd[26484]: DNS configuration server: Connection invalid
Aug 22 09:29:54 s-mx-01 configd[26484]: Connection invalid
Aug 22 09:29:55 s-mx-01 configd[26484]: flush_routes: removing route for 169.254.101.43 failed: No such process
Aug 22 09:29:55 s-mx-01 configd[26484]: flush_routes: removing route for 169.254.173.101 failed: No such process
Aug 22 09:29:55 s-mx-01 configd[26484]: flush_routes: removing route for 169.254.221.184 failed: No such process
Aug 22 09:29:55 s-mx-01 configd[26484]: flush_routes: removing route for 169.254.224.132 failed: No such process
Aug 22 09:29:55 s-mx-01 configd[26484]: IPMonitor ip_plugin_init SCDynamicStoreCreate failed: Permission denied
Aug 22 09:29:55 s-mx-01 configd[26484]: SCDynamicStoreCreate() failed: Permission denied
Aug 22 09:29:55 s-mx-01 configd[26484]: SCNC Controller: notify_register_check failed, status = 1000000
Aug 22 09:29:55 s-mx-01 configd[26484]: EAPOLController: SCDynamicStoreCreate() failed, Permission denied
Aug 22 09:29:56 s-mx-01 configd[26484]: IPConfigurationSCPreferencesSetCallback failed: Permission denied`


My current theory is that there is a configuration problem in the mailing system which unhinges configd.