Restarting DNS service causes isc-dhcp-server to stop handing out leases

[Andrew Martin]

Hello,

I am running the isc-dhcp-server (4.1.ESV-R4-0ubuntu5.6) for DHCP along with
dnsmasq (2.59-4ubuntu0.1) for DNS on a pair of Ubuntu 12.04 servers:
server1 - 192.168.0.101
   - isc-dhcp-server
   - dnsmasq

server2 - 192.168.0.102
   - isc-dhcp-server
   - dnsmasq

I have configured isc-dhcp-server's peer failover mode. I have several VLAN
interfaces configured on both servers, e.g eth0.<vlan>, and have configured
isc-dhcp-server to listen on all of them and hand out leases: 
/usr/sbin/dhcpd -f -q -4 -pf /run/dhcp-server/dhcpd.pid -cf /etc/dhcp/dhcpd.conf eth0.2 eth0.3 eth0.4 eth0.5 eth0.6 eth0.7

The isc-dhcp-server has the following config: 
http://pastebin.com/RM6EPshy

Last night, I applied some changes to the dnsmasq config (changing one of the
upstream DNS servers) and then restarted networking and the dnsmasq service:
/etc/init.d/networking restart 
service dnsmasq restart 
Shortly thereafter, the isc-dhcp-server restarted via a cron job that restarts 
it every 15 minutes. Here are these syslog messages from these 3 events: 
http://pastebin.com/33NTnMbY

At this point, the isc-dhcp-server seems to have stopped handing out leases. The
cron job continued to restart it every 15 minutes, so it was restarted a number
of times during the night, but none of those restarts appear to have corrected
the problem. All of the restarts logged one of these 3 variations in the syslog:
http://pastebin.com/3x3fWPHW

This morning, I restarted dnsmasq again and then restarted isc-dhcp-server after
doing so, and the problem appears to have been resolved:
http://pastebin.com/wr9iabAj

Is there a way to obtain more debug information about what caused this, or how
to prevent it in the future? Note that while isc-dhcp-server was in this bad
state, I tried running it in the foreground with debug mode (-f and -d) but did
not receive any additional information.

Thanks,

Andrew Martin