[ltp] Disabling WLAN when it's not used

[Edi Weitz]

My T23 has Ethernet as well as built-in WLAN and I use both. At home
I'm mostly using WLAN, when I'm visiting a customer where I'm very
often I use Ethernet. I have Linux (Debian) configured to try both
interfaces on startup so I can just boot the machine and will be
online, i.e. in /etc/network/interfaces I have

  auto eth0
  iface eth0 inet dhcp

  auto eth1
  iface eth1 inet dhcp
    wireless-mode Managed
    wireless-key restricted blablablablablablabla
    wireless-essid AirPort

and in /etc/dhcp3/dhclient.conf I have

  timeout 5;

so I don't have to wait too long.

But sometimes I'm on the road without network access at all. These are
usually the same times when I'm on battery. Even if both interfaces
don't find a suitable DHCP server I see two instances of dhclient in
the output of ps all the time and ifconfig says that both eth0 and
eth1 are UP.

Does that mean that the WLAN card will consume battery power all the
time? And if so, is there an automatic way (as opposed to issuing
ifdown manually) to disable the card once it hasn't found a DHCP
server?

Thanks,
Edi.