openbsd 5.5 dhcpd failed bpf

[openbsdsir008]

rl0(接外网) >>>>>>> openbsd <<<<<<<< fxp0 （接二层交换机的trunk口）
fxp0上 创建 了11个vlan 即 /etc/hostname.vlan2-12

如何配置dhcpd服务 ，给二层交换机上对应的vlan用户 分配ip地址 ？？



# cat /etc/rc.conf.local
dhcpd_flags="vlan2 vlan3 vlan4 vlan5 vlan6 vlan7 vlan8 vlan9 vlan10 vlan11 vlan12"
#



# /etc/rc.d/dhcpd stop
# /etc/rc.d/dhcpd start
dhcpd(failed)
# tail /var/log/messages
Jun 26 15:09:53 thinkpad dhcpd[30660]: Can't find free bpf: No such file or directory
Jun 26 15:09:53 thinkpad dhcpd[30660]: exiting.
Jun 26 15:09:57 thinkpad dhcpd[26864]: Can't find free bpf: No such file or directory
Jun 26 15:09:57 thinkpad dhcpd[26864]: exiting.
Jun 26 15:11:36 thinkpad dhcpd[1748]: Can't find free bpf: No such file or directory
Jun 26 15:11:36 thinkpad dhcpd[1748]: exiting.
Jun 26 15:16:41 thinkpad dhcpd[18141]: Can't find free bpf: No such file or directory
Jun 26 15:16:41 thinkpad dhcpd[18141]: exiting.
Jun 26 15:26:16 thinkpad dhcpd[4921]: Can't find free bpf: No such file or directory
Jun 26 15:26:16 thinkpad dhcpd[4921]: exiting.

经过反复实验我发现：
当dhcpd_flags="" 分号内的接口数量超过9个时，dhcpd就不能启动成功。

[openbsdsir008]

经过 艰苦 的google
我找到问题的答案 了，但不知道 原理 是什么，为什么这样做，请大家教教我？
http://openbsd.7691.n7.nabble.com/dhcre ... 21582.html

cd /dev
for i in $(jot 20 10); do ./MAKEDEV bpf${i} ; done
to make 20 more bpfs. Each tcpdump and dhcrelay will want one of their
own so you may need more dev-entries.