re: question about ifup and ifconfig

[Ren Weili <renweili@csvw.com>]

i have my laptop so:
/etc/init.d/networkings:
  case "$1" in
    start)
	doopt spoofprotect yes
        doopt syncookies no
        doopt ip_forward no

        echo -n "Configuring network interfaces: "
	echo "please select your environment:"
	echo "[1]:home"
	echo "[2]:office"
	read home_office
	if [ $home_office -eq "1" ]; then
        	ifup -a -i /etc/network/interfaces.home	
		rm -f /etc/resolv.conf
		ln -s /etc/resolv.conf.home /etc/resolv.conf
		rm -f /etc/apt/apt.conf
	elif [ $home_office -eq "2" ]; then
        	ifup -a -i /etc/network/interfaces.office		
		rm -f /etc/resolv.conf
		ln -s /etc/resolv.conf.office /etc/resolv.conf
		if [ ! -f /etc/apt/apt.conf ]; then
			ln -s /etc/apt/apt.conf.office
/etc/apt/apt.conf	
		fi
	fi
	echo "done."
	;;

and it works good.
my question here is: 
	1) is ifup/ifdown written in c and realized a set function of
ifconfig and add route table?
	2) how can i use ifconfig to bring up eth0 with dhcp?


thanks