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Since I’m a lazy git, I want my laptop to automatically switch back & forth between my wired and wireless interfaces. Seems that stuff like Network Manager can do that for you, but it’s not really my thing. I don’t like stuff where you need a GUI to configure it, a duplicaton of network configuration, and it also tends to hang my machine. No idea why, though.

After an afternoon of fiddling around with several things, I came up with the recipe:
1 portion ifplugd, a good mix of ifupdown configuration with guessnet mappings, and some home-grown scripts. Mix well, and let simmer over a hot stove for half an hour. ;)

The details (tailored to Debian Sid):

1. Install ifplugd and guessnet: apt-get install ifplugd guessnet
2. Configure the interface you want ifplugd to monitor. For me, this is eth0 (wired ethernet). You can do this by editing /etc/default/ifplugd and adding eth0 in the INTERFACES field.
   Restart ifplugd (/etc/init.d/ifplugd restart)
3. Edit your /etc/network/interfaces file the way you like it. I’m using multiple wireless entries with guessnet:
   

   mapping ath0
           script guessnet-ifupdown
           map verbose: false
           map debug: false
           map autofilter: true
   
   iface ath0-work inet dhcp
           test wireless essid WORK
           wpa-ssid WORK
           wpa-key-mgmt WPA-PSK
           wpa-proto WPA
           wpa-psk "***"
           wpa-driver wext
   
   iface ath0-home inet dhcp
           test wireless essid HOME
           wpa-ssid HOME
           wpa-key-mgmt WPA-PSK
           wpa-proto WPA
           wpa-psk "***"
           wpa-driver wext
   

   For syntax info, see man guessnet

4. Replace the script in /etc/ifplugd/action.d with something more usable. The installed script only calls ifup or ifdown depending on what’s happening. What we want is to ifdown the interface, and ifup the other.

   Something like this:

   #!/bin/sh
   set -e
   
   WIRED_INTERFACE="eth0"
   WIFI_INTERFACE="ath0"
   WIFI_MODULE="ath_pci"
   IFUPDOWN_STATE="/etc/network/run/ifstate"
   
   if [ $# -ne 2 ]; then
           echo "Incorrect usage!"
           echo "$0:  "
           exit 1
   fi
   
   case "$2" in
   up)
           if [ "$1" = $WIRED_INTERFACE ]; then
                   # Wired interface is going up, bring wireless down
                   # if it is active.
                   WIFI_MODULE_LOADED=$(lsmod | grep ^$WIFI_MODULE | wc -l)
                   if [ $WIFI_MODULE_LOADED -eq 1 ]; then
                           /sbin/ifdown $WIFI_INTERFACE
                           /sbin/rmmod $WIFI_MODULE
                   fi
                   /sbin/ifup $WIRED_INTERFACE
           else
                   /sbin/ifup $1
           fi
           ;;
   down)
           if [ "$1" = $WIRED_INTERFACE ]; then
                   # Wired interface is going down, bring up the
                   # wireless one.
                   /sbin/ifdown $WIRED_INTERFACE
   
                   /sbin/modprobe $WIFI_MODULE
                   /sbin/ifconfig $WIFI_INTERFACE up
                   sleep 5
                   /sbin/ifup $WIFI_INTERFACE
   
                   WIFI_CONFIGURED=$(grep ^$WIFI_INTERFACE $IFUPDOWN_STATE | wc -l)
                   if [ $WIFI_CONFIGURED -eq 0 ]; then
                           # Interface was not configured, bring it back down
                           # to save power
                           /sbin/rmmod $WIFI_MODULE
                   fi
           else
                   /sbin/ifdown $1
           fi
           ;;
   esac
   

Now, every time ifplugd configures up eth0, ath0 is automatically deconfigured, and vice versa.
The actual configuration of the interfaces is still in /etc/network/interfaces, so you can still handle it by hand if you want to.

As always, it works fine for me, but YMMV, and TIMTOWTDI!

This entry was posted on Sunday, March 8th, 2009 at 19:50 and is filed under Linux / unix, Software-related. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.