实现DHCP自动获取IP地址

实现DHCP自动获取IP地址
1． 在内核中添加以下选项：
Device Drivers --->
Networking support --->
<*>Networking support
              Networking options --->
                   <*>Packet socket            //添加.配置CONFIG_PACKET
                  [*] IP: DHCP support            //添加
                  [*] Network packet filtering (replaces ipchains) --->
//添加，后面子选项可不选，配置CONFIG_NETFILTER
说明：若没选 Packet socket，在执行udhcpc命令时出现如下错误：

~ # udhcpc
udhcpc (v0.9.9-pre) started
udhcpc[208]: udhcpc (v0.9.9-pre) started
FATAL: couldn't listen on socket, Address family not supported by protocol
udhcpc[208]: FATAL: couldn't listen on socket, Address family not supported by protocol
[*] Network packet filtering (replaces ipchains) --->本人选项已测试可以不选

2． Busybox中添加以下选项：
Networking Utilities --->
   udhcp Server/Client --->
[ ] udhcp Server (udhcpd)    //生成服务端命令udhcpd，在此不作服务端，故不选。
  udhcp Client (udhcpc)        //生成udhcpc命令
   [ ] Lease display utility (dumpleases)
   [ ] Log udhcp messages to syslog (instead of stdout)
   [ ] Compile udhcp with noisy debugging messages
若busybox没编译相应选项，也可从网上下载相应文件，用arm-linux交叉编译得到udhcpd,udhcpc命令copy到usr/sbin下就可以了。
从网上下的udhcp_0.9.8cvs20050303.orig.tar.gz文件，解压后修改Makefile文件
在19行添加CROSS_COMPILE=arm-linux-    （生成ARM版）
注释12行的COMBINED_BINARY=1        （否则不生成udhcpc命令）

3． 建相关配置文件
从busybox的examples/udhcp/下copy simple.script文件到开发板/usr/share/udhcpc/下，并重命名为default.script，udhcp_0.9.8cvs20050303.orig.tar.gz中也有这样的文件。
[root@localhost root]# vi usr/share/udhcpc/default.script
说明：此步不需要，防止examples/udhcp/simple.script出错
#!/bin/sh
# udhcpc script edited by Tim Riker
[ -z "$1" ] && echo "Error: should be called from udhcpc" && exit 1
RESOLV_CONF="/etc/resolv.conf"
[ -n "$broadcast" ] && BROADCAST="broadcast $broadcast"
[ -n "$subnet" ] && NETMASK="netmask $subnet"
case "$1" in
        deconfig)
                /sbin/ifconfig $interface 0.0.0.0
                ;;
        renew|bound)
                /sbin/ifconfig $interface $ip $BROADCAST $NETMASK
                if [ -n "$router" ] ; then
                        echo "deleting routers"
                        while route del default gw 0.0.0.0 dev $interface ; do
                                :
                        done
                        for i in $router ; do
                                route add default gw $i dev $interface
                        done
                fi
                echo -n > $RESOLV_CONF
                [ -n "$domain" ] && echo search $domain >> $RESOLV_CONF
                for i in $dns ; do
                        echo adding dns $i
                        echo nameserver $i >> $RESOLV_CONF
                done
                ;;
esac
exit 0

4． 重启开发板，执行udhcpc就可自动获取IP地址了，以下是执行udhcpc的输出信息：
~ # udhcpc
udhcpc (v0.9.9-pre) started
udhcpc[228]: udhcpc (v0.9.9-pre) started
Sending discover...
udhcpc[228]: Sending discover...
Sending select for 192.168.1.109...
udhcpc[228]: Sending select for 192.168.1.109...
Lease of 192.168.1.109 obtained, lease time 86400
udhcpc[228]: Lease of 192.168.1.109 obtained, lease time 86400
deleting routers
route: SIOC[ADD|DEL]RT: No such process
adding dns 192.168.0.1

~ # ping www.baidu.com
PING www.a.shifen.com (220.181.38.4): 56 data bytes
64 bytes from 220.181.38.4: icmp_seq=0 ttl=52 time=1219.0 ms
[1] + Stopped ping www.baidu.com

5． 如果是双网卡必须用参数指明
例：udhcpc -i eth1

6． 运行时的一些注意事项
       busybox中要使用DNS时，要保证/etc/nsswitch.conf[/etc/resolv.conf,/etc/hosts, /etc/host.conf,/etc/hostname]文件存在（其中[]内的内容是否需要没有经过验证，反正有了肯定不会错，呵呵）；同时要求 libnss_dns,libnss_files,libresolv库存在。
       busybox中使用udhcpc时，要设置配置文件/usr/share/udhcpc/default.script；把examples/udhcp/simple.script文件copy到目的文件就可以了。