How can I start DHCP3-server later, so that it waits for a bridge interface to initialise before loading?

I have Ubuntu 10.04 server currently setup with dhcp3-server as well as a bridged interface (br0) for use with virtual machines. The problem I have is that when the server reboots, dhcp3-server fails to load because of the extra delay caused by bringing up the bridged interface.

Essentially br0 doesn't have an IP address for use with DHCP3-Server until late in the boot cycle, well after DHCP3-server has attempted to load.

Once the server has booted I can run '/etc/init.d/dhcp3-server start' without any issue.

Is there any way I can either: - Force dhcp3-server to wait until the interface has loaded before attempting to load? - Start dhcp3-server after everything else has loaded up?

Solution 1:

One solution is to tell the dhcp-server not to start automatically and then add the following two lines to you /etc/network/interfaces file for you bridge definition

post-up /etc/init.d/dhcp3-server start
pre-down /etc/init.d/dhcp3-server stop

So it will end up looking like this

iface br0 inet static
    bridge_ports eth0 eth1
    address 192.168.1.2
    broadcast 192.168.1.255
    netmask 255.255.255.0
    gateway 192.168.1.1
    post-up /etc/init.d/dhcp3-server start
    pre-down /etc/init.d/dhcp3-server stop

This way the network management (ifup/ifdown, NOT network-manager) will start the DHCP server after bringing up the bridge, and shut it down before removing the bridge.

Solution 2:

You could modify the /etc/init.d/dhcp3-server startup script to wait for an IP address to be available on br0. For instance: (Warning: untested code!)

# wait 5 secs between br0-ready tests
wait_time_between_probes=5
# maximum number of attempts (i.e., timeout)
max_attempts=10

log_progress_msg "Waiting for br0 to get an IP address"
for n in $(seq 1 $max_attempts); do
  if /sbin/ifconfig br0 | egrep -q "inet addr:" ; then
    # IP address ready on br0, exit loop
    break
  else
    sleep $wait_time_between_probes
  fi
done
if [ "$n" = "$max_attempts" ]; then
    log_warning_msg "Maximum number of attempts reached, but br0 has no IP address yet" 
    log_warning_msg "Continuing anyway but DHCP3 server might not start correctly"
fi 

The snippet should go into the startup script, within the case ... start) part, before startup of the DHCP3 daemon. Of course, you should tune the wait time and number of attempts to match your environment (how long does it take maximum for br0 to get the IP address?)