有一款设备需要使用静态ip，但是还要用到dns解析域名。我的想法是定时去查看/etc/resolve.conf文件，如果为空，则获取一次dns。
在嵌入式linux中，我们使用busybox的udhcpc来获取ip，netmask，网关和dns。 运行udhcpc，在获取到ip，dns等信息后并不会直接更细系统的相关信息，而是调用一个脚本去设置。如本次使用的/etc/udhcpc.d/50default #!/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"

# return 0 if root is mounted on a network filesystem
root_is_nfs() {
    sed -n 's/^[^ ]* ([^ ]*) ([^ ]*) .*$/1 2/p' /proc/mounts |
    grep -q "^/ (nfs|smbfs|ncp|coda)$"
}

have_bin_ip=0
if [ -x /bin/ip ]; then
  have_bin_ip=1
fi

case "$1" in
    deconfig)
        if [ -x /sbin/resolvconf ]; then
            /sbin/resolvconf -d "${interface}.udhcpc"
        fi
        if ! root_is_nfs ; then
                        if [ $have_bin_ip -eq 1 ]; then
                                ip addr flush dev $interface
                                ip link set dev $interface up
                        else
                                /sbin/ifconfig $interface 0.0.0.0
                        fi
        fi
        ;;

    renew|bound)
                if [ $have_bin_ip -eq 1 ]; then
                        ip addr add dev $interface local $ip/$mask $BROADCAST
                else
                        /sbin/ifconfig $interface $ip $BROADCAST $NETMASK
                fi

        if [ -n "$router" ] ; then
            if ! root_is_nfs ; then
                                if [ $have_bin_ip -eq 1 ]; then
                                        while ip route del default 2>/dev/null ; do
                                                :
                                        done
                                else
                                        while route del default gw 0.0.0.0 dev $interface 2>/dev/null ; do
                                                :
                                        done
                                fi
            fi

            metric=0
            for i in $router ; do
                                if [ $have_bin_ip -eq 1 ]; then
                                        ip route add default via $i metric $metric
                                else
                                        route add default gw $i dev $interface metric $metric 2>/dev/null
                                fi
                                metric=$(($metric + 1))
            done
        fi

        # Update resolver configuration file
        R=""
        [ -n "$domain" ] && R="domain $domain
"
        for i in $dns; do
            echo "$0: Adding DNS $i"
            R="${R}nameserver $i
"
        done

        if [ -x /sbin/resolvconf ]; then
            echo -n "$R" | /sbin/resolvconf -a "${interface}.udhcpc"
        else
            echo -n "$R" > "$RESOLV_CONF"
        fi
        ;;
esac

exit 0


新建一个脚本，复制/etc/udhcpc.d/50default的内容并将红 {MOD}字体部分去掉。执行下面的命令即可只更新dns。 udhcpc -s /dir/test.sh -q -s后面的脚本必须用绝对地址，否则只会去/etc/udhcpc.d/目录查找相关脚本。 -q是为了执行一次即退出，否则udhcpc会常驻后台。