[libvirt] dnsmasq didn't work as except

[taget at linux.vnet.ibm.com]

hi all
   The vm created by libvirtd can not acquire a ip from dnsmasq.

nobody   14203     1  0 21:31 ?        00:00:00 /usr/sbin/dnsmasq  
--strict-order --bind-interfaces  
--pid-file=/var/run/libvirt/network/default.pid --conf-file=  
--except-interface lo --listen-address 192.168.122.1 --dhcp-range  
192.168.122.2,192.168.122.254  
--dhcp-leasefile=/var/lib/libvirt/dnsmasq/default.leases  
--dhcp-lease-max=253 --dhcp-no-override

note that --conf-file= is empty ,is there any thing I missing?
thanks Eli.