-- eth0
ifconfig eth0 down	# this will stop default gw too
kill -TERM `cat /var/run/dhcpcd-eth0.pid`	# get rid of dhcp client
dhcpcd eth0	# dhcpclient up now
netstat -nr	# check default gw
                                                                              
ifconfig eth0 192.168.1.2 netmask 255.255.255.0                               
route add default gw 192.168.1.1                                           
 
-- wlan0
ifconfig wlan0 down
iwconfig

# ls /var/run/
dhcpcd-eth0.pid
dhcpcd-wlan0.pid  wpa_supplicant

# wpa_passphrase myssid secret
# cat /root/wpa_cpfong.conf 
ap_scan=1
network={
	ssid="myssid"
	 key_mgmt=WPA-PSK
	#psk="secret"
	psk=109f4ef079fdeff677b1d50ebe7e45ffea0f50e4532437a0c01e635fa230244d7
}

wpa_supplicant -B -Dwext -iwlan0 -c/root/wpa_cpfong.conf
dhcpcd wlan0
ping 8.8.8.8

rm /var/run/wpa_supplicant/*
# you will know how system do wireless
ps aux |grep wpa_sup

#!/usr/bin/perl
die "pls pass in wpa_config" unless (@ARGV == 1);

$_ = $ARGV[0];
chomp;
if (/eth0/){
	print "/sbin/ifconfig wlan0 down\n";
	print "/usr/sbin/dhcpcd eth0\n";
}
else {
	print "/sbin/ifconfig eth0 down\n";
	print "/usr/sbin/wpa_supplicant -Dwext -iwlan0 -c$_ &\n";
	print "/usr/sbin/dhcpcd wlan0\n";
}