The following article explains how to use the linux network configuration scripts to automatically select the correct network/settings according to the location (i.e. avilable essid).

This is the /etc/network/interfaces file :

auto ath1

iface jaist inet dhcp
     wireless-essid JAISTALL

iface home inet dhcp
     wireless-essid MyPlace pre-up /sbin/wpa_supplicant_5.4 -Bw -dd -Dwext -iath1 -c/etc/fonera.conf 
     post-down killall -q wpa_supplicant_5.4

 
# For temporary locations using wep
#     iface temp inet dhcp
#     wireless-essid TMP
#     wireless-key1 s:password
#     wireless-defaultkey 1
#     wireless-keymode open


mapping ath1
     script /mapper
     map MyPlace                   ,, home
     map JAISTALL 00:0F:23:A4:A4:00,, jaist
     map TMP                       ,, temp

# Add MAC addresses after the SSID, if the wireless network does not broadcast
# essid, at least one MAC address of one accesspoint must be defined. On
# networks that boradcast the essid, there is no need to configure MAC
# addresses.


iface ath0 inet dhcp
     wireless-essid MyPlace
     pre-up /sbin/wpa_supplicant_5.4 -Bw -dd -Dwext -iath1 -c/etc/fonera.conf 
     post-down killall -q wpa_supplicant_5.4

The “MyPlace” is the essid that I use at home and “JAISTALL” is the essid at school. At home I use WPA (wpa_supplicant) while in JAIST its MAC based access control.

The mapper file selects the virtual interface according to which essid is available

#! /bin/bash

ifname=$1

ifconfig $ifname up

function log { 

  time=$(date +%d%m%y-%H:%M:%S)
  echo $time   $@ >> /tmp/res

}

  function TestMac {
  
  ifname=$1
  vifname=$2
  mac=$3
  
  
  
if [ "$mac" != "" ]

then

     log "Looking for MAC address $mac using $ifname"
     res=$(iwlist $1 ap | grep $mac)
     if [ "$res" != "" ]
     then
             log "MAC address found, mapping $ifname to $vifname"
             echo $vifname
     else 
             log "Not found"
     fi
fi

}
  
while read ssid mac vifname 
do
     log "Mapping $ifname to $vifname {$ssid}"

     if [ "$(iwlist $ifname scanning | grep $ssid)"  != "" ]
     then
             log "$ssid found mapping $ifname to $vifname !"
             echo $vifname
             exit
     fi

     # process mac addresses if they exist

     MAC1=$(echo $mac | cut -d"," -f1)
     MAC2=$(echo $mac | cut -d"," -f2)
     MAC3=$(echo $mac | cut -d"," -f3)

     TestMac $ifname $vifname $MAC1
     TestMac $ifname $vifname $MAC2
     TestMac $ifname $vifname $MAC3

done

When your laptop boots, the interface ath1 will be configured according to your location. At anytime, you can run “ifup ath1” to setup your interface by hand. You can enforce some location for example by issuing “ifup ath1=jaist” in order to connect to essid JAISTALL when both essids (JAISTALL and MyPlace) are available …

If you are planning to use these files, be sure to change the following items accorsing to your own setup.

• ath1 : the wireless interface name
• MyPlace : The essid at your home, or location 1
• JAISTALL : The essid of your second location
• /sbin/wpa_supplicant_5.4 : Path to the wpa_supplicant program
• /etc/fonera.conf : path to the configuration file for the wpa_supplicant program