/*********************************************
* dhcpc
* dhcpc是dhcp的客户端，在busybox中实现。今天正好了解一下。
* Tony, 2016-5-20, Shenzhen
********************************************/
udhcpc -b -i eth0 -p /var/run/udhcpc.pid -R &
-b:后台运行
-i:指定接口
-p:pid文件
-R:关闭udhcpc时，可以向dhcpserver发送release取消租约
Usage: udhcpc [-Cfbnqtvo] [-c CID] [-V VCLS] [-H HOSTNAME] [-i INTERFACE]
[-p pidfile] [-r IP] [-s script] [-O dhcp-option]... [-P N]
-V CLASSID Vendor class identifier
-i INTERFACE Interface to use (default: eth0)
-H,-h HOSTNAME Client hostname
-c CLIENTID Client identifier
-C Suppress default client identifier
-p file Create pidfile
-r IP IP address to request
-s file Run file at DHCP events (default /usr/share/udhcpc/default.script)
-t N Send up to N request packets
-T N Try to get a lease for N seconds (default 3)
-A N Wait N seconds (default 20) after failure
-O OPT Request DHCP option OPT (cumulative)
-o Do not request any options (unless -O is also given)
-f Run in foreground
-b Background if lease is not immediately obtained
-S Log to syslog too
-n Exit with failure if lease is not immediately obtained
-q Quit after obtaining lease
-R Release IP on quit
-P N Use port N instead of default 68
-a Use arping to validate offered address
