[libvirt] dnsmasq didn't work as except

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.