要使用网络通讯，所以不可避免的要用到dhcp。理想的网络通讯方式是下面3种都要支持:
1,接入已有网络。这便要求可以作为dhcp客户端。
2,作为DHCP服务器，动态分配IP。
简单说下前2种情况。
使用步骤：
（1）在内核的网络项里面把DHCP配置上；
（2）在busybox里面把[*]udhcp server(udhcpd)
[*]udhcp client(udhcpc)都选上。
udhcpd就是终端设备作为DHCP服务器
udhcpc就是终端设备作为DHCP客户端
busybox里面对dhcp都已经给出例子了，
[zhh@localhost busybox-1.14.1]$ ls ./examples/udhcp/
sample.bound sample.deconfig sample.nak sample.renew sample.script simple.script udhcpd.conf
比如使用udhcpc时
就可以直接把simple.script拿来使用，改不改名字都可以，busybox里面默认的目录文件是/usr/share/udhcpc/default.script
可以查看下帮助
# udhcpc --help
BusyBox v1.14.1 (2010-01-22 10:35:16 CST) multi-call binary
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
#
这样就很好理解了。
下面就说说使用udhcpd，同样可以直接把busybox自带的udhcpd.conf作为参考，怎么设置该文件就不详述了，去参考相关文档即可。看看udhcpd怎么使用
# udhcpd --help
BusyBox v1.14.1 (2010-01-22 10:35:16 CST) multi-call binary
Usage: udhcpd [-fS] [-P N] [configfile]
DHCP server
-f Run in foreground
-S Log to syslog too
-P N Use port N instead of default 67
帮助一目了然。
我使用的时候直接
#udhcpd /etc/udhcpd.conf
即可。
还有一个租约文件要简要说说，使用udhcpd时会要求建立一个租约文件，按照提示创建一个就行了，它起什么作用呢？就是为了记录客户端所获得的IP，如果没有租约文件，则会重新给客户端分配一个新IP，如果有，则使用原先分配的IP。
例如：udhcpc -i wlan0 是获取wlan0的ip地址，如果想获取之后直接退出，添加 -q参数，即：udhcpc -q -i wlan0
udhcpc切换到后台：
它自带参数可以实现该功能。如下：
udhcpc -b -i eth0 -p /var/run/udhcpc.pid -R
解释一下，-b就是切换到后台指令，-i是指定使用哪个网络接口，双网卡的时候一定要使用它来指定eth0 or eth1。
对于 -R参数，其实也很重要，在关闭udhcpc时，可以向dhcpserver发送release取消租约。
udhcpc scripts
Here's some debian scripts I wrote for udhcpc 0.9.6. I wanted to make it easy to run local scripts for DHCP changes so I created /etc/udhcpc.d. All the /usr/share/udhcpc/default.* scripts have been merged into /etc/udhcpc.d/S50default. S99local is an example script that patches resolv.conf and calls a DNS update script.
/etc/init.d/udhcpc /etc/udhcpc.script /etc/udhcpc.d/S50default /etc/udhcpc.d/S99local
http://udhcp.busybox.net/README.udhcpc
udhcp client (udhcpc) -------------------- The udhcp client negotiates a lease with the DHCP server and notifies a set
of scripts when a leases is obtained or lost. command line options ------------------- The command line options for
the udhcp client are: -c, --clientid=CLIENTID Client identifier -H, --hostname=HOSTNAME Client hostname -h, Alias for -H -f,
--foreground Do not fork after getting lease -b, --background Fork to background if lease cannot be immediately negotiated. -i,
--interface=INTERFACE Interface to use (default: eth0) -n, --now Exit with failure if lease cannot be immediately negotiated. -p,
--pidfile=file Store process ID of daemon in file -q, --quit Quit after obtaining lease -r, --request=IP IP address to request
(default: none) -s, --script=file Run file at dhcp events (default: /usr/share/udhcpc/default.script) -v, --version Display version
If the requested IP address cannot be obtained, the client accepts the address that the server offers. udhcp client scripts
------------------- When an event occurs, udhcpc calls the action script. The script by default is /usr/share/udhcpc/default.script
but this can be changed via the command line arguments. The three possible arguments to the script are: deconfig: This argument
is used when udhcpc starts, and when a leases is lost. The script should put the interface in an up, but deconfigured state, ie:
ifconfig $interface 0.0.0.0. bound: This argument is used when udhcpc moves from an unbound, to a bound state. All of the paramaters
are set in enviromental variables, The script should configure the interface, and set any other relavent parameters (default gateway,
dns server, etc). renew: This argument is used when a DHCP lease is renewed. All of the paramaters are set in enviromental variables.
This argument is used when the interface is already configured, so the IP address, will not change, however, the other DHCP paramaters,
such as the default gateway, subnet mask, and dns server may change. nak: This argument is used with udhcpc receives a NAK message.
The script with the deconfig argument will be called directly afterwards, so no changes to the network interface are neccessary.
This hook is provided for purely informational purposes (the message option may contain a reason for the NAK). The paramaters for
enviromental variables are as follows: $HOME - The set $HOME env or "/" $PATH - the set $PATH env or "/bin:/usr/bin:/sbin:/usr/sbin"
$1 - What action the script should perform interface - The interface this was obtained on ip - The obtained IP siaddr - The bootp next
server option sname - The bootp server name option boot_file - The bootp boot file option subnet - The assigend subnet mask timezone -
Offset in seconds from UTC router - A list of routers timesvr - A list of time servers namesvr - A list of IEN 116 name servers dns -
A list of DNS server logsvr - A list of MIT-LCS UDP log servers cookiesvr - A list of RFC 865 cookie servers lprsvr - A list of LPR
servers hostname - The assigned hostname bootsize - The length in 512 octect blocks of the bootfile domain - The domain name of the
network swapsvr - The IP address of the client's swap server rootpath - The path name of the client's root disk ipttl - The TTL to use
for this network mtu - The MTU to use for this network broadcast - The broadcast address for this network ntpsrv - A list of NTP
servers wins - A list of WINS servers lease - The lease time, in seconds dhcptype - DHCP message type (safely ignored) serverid -
The IP of the server message - Reason for a DHCPNAK tftp - The TFTP server name bootfile - The bootfile name additional options are
easily added in options.c. note on udhcpc's random seed --------------------------- udhcpc will seed its random number generator
(used for generating xid's) by reading /dev/urandom. If you have a lot of embedded systems on the same network, with no entropy, you can
either seed /dev/urandom by a method of your own, or doing the following on startup: ifconfig eth0 > /dev/urandom in order to seed
/dev/urandom with some data (mac address) unique to your system. If reading /dev/urandom fails, udhcpc will fall back to its old behavior
of seeding with time(0). signals accepted by udhcpc ------------------------- udhcpc also responds to SIGUSR1 and SIGUSR2.
SIGUSR1 will force a renew state, and SIGUSR2 will force a release of the current lease, and cause udhcpc to go into an inactive state
(until it is killed, or receives a SIGUSR1). You do not need to sleep between sending signals, as signals received are processed
sequencially in the order they are received. compile time options ------------------- options.c contains a set of dhcp options
for the client: name[10]: The name of the option as it will appear in scripts flags: The type of option, as well as if it will be
requested by the client (OPTION_REQ) code: The DHCP code for this option
使用busybox下的simple.script
进入busybox目录：busybox1.19.2/example/udhcp/，拷贝文件simple.script到根文件系统的/usr/share/udhcpc/目录下，
更名为default.script，再将default.script中resolv_conf=”/etc/resolv.conf”更改为resolv_conf=”/tmp/resolv.conf”。
(说明：由于测试使用cramfs文件系统，etc目录不可写，运行udhcpc无法在/etc目录下创建resolv.conf文件。)