嵌入式linux通过DHCP自动获取IP地址实现

前提：系统已经实现DNS（即使用ping www.baidu.com测试时能ping通）。
1．  在内核中添加以下选项：
Networking  --->
   [*] 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, [ * ] Network packet filtering (replaces ipchains)  --->选项，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
2．  Busybox中添加以下选项：
Networking Utilities
--->
   udhcp Server/Client  --->
        [] udhcp Server (udhcpd)       //在此不作服务端，故不选。生成udhcpd命令                             
        [*] 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。
我从 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-  
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
#!/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