Just notice one of the DNS server has the error message
28-Jun-2010 16:28:24.283 general: max open files (3520) is smaller than max sockets (4096)
It’s something to do with kernel setting, on FreeBSD, configure the following
shell> sysctl kern.maxfiles=4096 shell> sysctl kern.maxfilesperproc=4096
Alternatively, put the above configuration on /etc/sysctl.conf in order to configure it at start up level.