Problem with ifupdown script

[Josef Wolf]

Hello,

I want to configure my laptop to automatically use eth0 or wlan0 depending
on whether the network cable is plugged or not.

I use traditional network configuration instead of NetworkManager because
I configure my systems automatically with a system similar to cfengine,
and networkmanager's interactive configuration doesn't play well with
automatic configuration.

So I have put this into /etc/network/interfaces

  auto lo eth0 wlan0
  iface lo inet loopback
  
  iface eth0 inet dhcp

  iface wlan0 inet dhcp
  wpa-conf /var/cfsvn/conf/wpa_supplicant.conf

and installed ifplugd to configure/deconfigure eth0 when the cable is
plugged/unplugged. Then, to configure/deconfigure wlan0, I decided to
use the ifupdown mechanism:

  #! /bin/sh
  # this is /etc/network/if-up.d/myifup
  [ x$IFACE = xeth0 ] && ifdown --force wlan0
  exit 0


  #! /bin/sh
  # this is /etc/network/if-down.d/myifup
  [ x$IFACE = xeth0 ] && ifup --force wlan0
  exit 0

The problem is that this mechanism works only every other time. When I
unplug the cable and plug it again, it works. When I unplug/plug again,
it doesn't work (wlan0 don't come up). Then I unplug/plug again and it
works. Again unplug/plug and it don't work. and so forth.

Any ideas why wlan0 come up only every second time? Any hints?