How to switch between network interfaces?

[Piers Kittel <edinfo-list@biased.org>]

Hello all,

I have a HP Omnibook 500 laptop, with Debian 3.0r2 testing - and to 
access the internet/intranet, I use a WLAN card, but occasionally I plug 
it into the wired network to transfer big sized files, but each time I 
do that, I edit the interfaces file, comment out the wlan0 enteries, 
remove the comments commenting out the eth0 enteries then restart 
networking - isn't there an automatic way of doing that?

Also as I have a dhcp server on the internal net, both interfaces are 
set to get an IP from the server - and the IP address is the same for 
both interfaces.  No problem in itself, but if I have the eth0 interface 
enabled in the /etc/networking/interfaces file - it will hang there 
trying to find the dhcp server, while the network cable is unplugged.

I also find even when I get eth0 enabled and on the network, the laptop 
still recieves files through the wlan card, until I physically remove 
the card then it'd switch over directly to the wired interface.

So:

1) Is there anyway for the computer to automatically enable and then 
switch over to eth0, and disable wlan0 when the network cable is plugged 
in?  If not, is there a quick way to do all this?  And same the other 
way round, i.e. automatically enable and switch over to wlan0 and 
disable eth0 when the network cable is unplugged?

2) Is there a way for the laptop to detect if the wired cable is 
unplugged then it'd bypass or "disable" the eth0 interface at bootup so 
preventing the dhcp client trying to find an IP via eth0?

Thanks very much for your help in advance

Cheers - Piers