mediageek posted this as a comment. This may essentially be the answer, and it's the best we've got (so far), so here it is:
You can configure a script in
(it exists in ubuntu 11.10 at least). I think these scripts are
executed after ("post") a device is disconnected. I don't know the
exact details for such a script, nor if it works in 10.04, hence I
posted this as a comment.
– medigeek Apr 12 '12 at 8:57
To complete that, I believe it may be sufficient to do this:
Create a script called
/etc/network/if-post-down.d directory by running:
gksu gedit start-eth1
Give it these contents in the text editor (then save the file and quit the text editor):
[ "$IFACE" = eth0 ] && ifup eth1
I believe that will check to see if the interface being disconnected is
eth0, and if it is, then it will start
Make the script executable so it will be run:
sudo chmod +x start-eth1
Please understand that I'm not sure how well this will work (or even if it will work).
If anyone has more information about this please feel free to edit this to improve it, and/or post a new answer.