Open Source, Physics & Politics

After upgrading two systems running the ISC DHCP server from Ubuntu 14.04 LTS (Trusty) to Ubuntu 16.04 LTS (Xenial), I experienced some trouble with the DHCP server not starting on system boot. The log file contained the following message:

Internet Systems Consortium DHCP Server 4.3.3
Copyright 2004-2015 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Config file: /etc/dhcp/dhcpd.conf
Database file: /var/lib/dhcp/dhcpd.leases
PID file: /run/dhcp-server/dhcpd.pid
Wrote 0 class decls to leases file.
Wrote 0 deleted host decls to leases file.
Wrote 0 new dynamic host decls to leases file.
Wrote 389 leases to leases file.

No subnet declaration for ovsbr0p1 (no IPv4 addresses).
** Ignoring requests on ovsbr0p1.  If this is not what
   you want, please write a subnet declaration
   in your dhcpd.conf file for the network segment
   to which interface ovsbr0p1 is attached. **

 

Not configured to listen on any interfaces!

If you think you have received this message due to a bug rather
than a configuration issue please read the section on submitting
bugs on either our web page at www.isc.org or in the README file
before submitting a bug.  These pages explain the proper
process and the information we find helpful for debugging..

exiting.

Obviously the problem was that the DHCP server was started before the network interface ovsbr0p1 (the primary network interface of the host) was brought up. I guessed that this problem was somehow related to the migration from Upstart to Systemd. Still, it was strange because the isc-dhcp-server.service was configured to be started after the network-online.target.

I could not figure out why Systemd though that the network-online.target had been reached before the interface was configured completely, but I found a simple workaround: I added a file /etc/systemd/system/isc-dhcp-server.service.d/retry.conf with the following content:

[Service]
Restart=on-failure
RestartSec=15

This means that the first start of the DHCP server will still fail most of the time, but Systemd will retry 15 seconds later and typically the network interface will be online by then (if it is not, Systemd will try again another 15 seconds later).

After upgrading a virtual machine from Ubuntu 14.04 LTS to Ubuntu 16.04 LTS, I was getting weird problems with the network configuration. The network would still be brought up, but scripts that were specified in /etc/network/interfaces would not be run when the corresponding interface was brought up.

The logfile /var/log/syslog would contain messages like "ifup: interface eth0 already configured". On other VMs that had also been updated from Ubuntu 14.04 LTS and uses a similar network configuration, this problem would not appear.

I found the solution to this problem in the Ubuntu Forums: There was a file /etc/udev/rules.d/85-ifupdown.rules that caused problems with the network initialization. After deleting this file, the problems went away. I guess that this file was present in a rather old release of Ubuntu and thus the problem only appears when a system has previously been upgraded from that release of Ubuntu.