Comment 3 for bug 1395896

There is a hint in the bug report. The error message is:

  CannotConfigureDHCP: DHCPv4 server failed to restart: stop: Unknown instance:
  start: Job failed to start

The first error occurs when you attempt to stop a service which is already stopped:

  kiko@gasolinux:~/Dropbox/Download$ sudo initctl status dmesg
  dmesg stop/waiting
  kiko@gasolinux:~/Dropbox/Download$ sudo stop dmesg
  stop: Unknown instance:

What appears to have happened is that there was an error starting the DHCP server, which can happen when there is an error in the config file; here's what happens when you have a broken file:

  kiko@chorus:~$ sudo start isc-dhcp-server
  start: Job failed to start

The syslog output is invaluable in that situation:

kiko@chorus:~$ sudo tail /var/log/syslog
Feb 12 15:37:51 chorus dhcpd: key "rndc-key";
Feb 12 15:37:51 chorus dhcpd: ^
Feb 12 15:37:51 chorus dhcpd: /etc/dhcp/dhcpd.conf line 37: unknown key rndc-key
Feb 12 15:37:51 chorus dhcpd: key "rndc-key"
Feb 12 15:37:51 chorus dhcpd: ^
Feb 12 15:37:51 chorus dhcpd: /etc/dhcp/dhcpd.conf line 42: unknown key rndc-key
Feb 12 15:37:51 chorus dhcpd: key "rndc-key"
Feb 12 15:37:51 chorus dhcpd: ^
Feb 12 15:37:51 chorus dhcpd: Configuration file errors encountered -- exiting
Feb 12 15:37:51 chorus kernel: [376499.698220] init: isc-dhcp-server pre-start process (1011) terminated with status 1

I'm afraid without syslog output we can't easily help this one.